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2010-04-26
|
Chemical Abundances for the Outer Halo Cluster Pal 4 from Co-added High-Resolution Spectroscopy
|
Context: Chemical element abundances for distant Galactic globular clusters
(GCs) hold important clues to the origin of the Milky Way halo and its
substructures. Aims: We study the chemical composition of red giant stars in
Pal 4 - one of the most remote GCs in the Milky Way - and compare our abundance
measurements to those for both low surface brightness dwarf galaxies, and GCs
in the inner and the outer halo. Methods: By co-adding high-resolution, low-S/N
Keck/HIRES spectra of 19 stars along the red giant branch, we estimate chemical
abundance ratios of 20 alpha-, iron peak-, and neutron-capture elements. Our
method gives total uncertainties on most element-to-iron ratios of typically
0.2 dex. Results: We measure [Fe/H] = -1.41 +- 0.04 (statistical) +- 0.17
(systematic) and an alpha-enhancement of [alpha/Fe] = +0.38 +- 0.11 dex, which
is consistent with the canonical value of +0.4 dex found for Galactic halo
field stars and most halo GCs at this metallicity. Although Pal 4 has higher
enhancements in the heavier elements with respect to the halo, the majority of
the element ratios are, within the measurement errors, consistent with those
for local halo field stars. We find, however, evidence for a lower [Mg/Ca]
ratio than in other halo clusters. Conclusions: Based on the available
evidence, we conclude that the material from which Pal 4 and the Galactic halo
formed experienced similar enrichment processes, despite the apparently younger
age of this cluster. Within the limitations of our methodology we find no
significant indication of an iron spread, as is typical of genuine GCs of the
Milky Way. However, abundance ratios for individual stars in Pal 4 and other
distant satellites are urgently needed to understand the relationship, if any,
between remote GCs and other halo substructures (i.e., luminous and ultra-faint
dwarf spheroidal galaxies).
|
1004.4625v1
|
2010-06-05
|
Variations in the X-ray eclipse transitions of Cen X-3
|
We report here an investigation of the X-ray eclipse transitions of the high
mass X-ray binary pulsar Cen X-3 in different intensity states. Long term light
curve of Cen X-3 obtained with RXTE-ASM spanning for more than 5000 days shows
strong aperiodic flux variations with low and high states. We have investigated
the eclipse transitions of Cen X-3 in different intensity states with data
obtained from pointed observations with the more sensitive instruments on board
ASCA, BeppoSAX, XMM-Newton, Chandra and RXTE. We found a very clear trend of
sharp eclipse transitions in the high state and longer transitions in the low
state. This is a confirmation of this feature first observed with the RXTE-ASM
but now with much better clarity. From the light curves obtained from several
missions, it is seen that the eclipse egress in the low state starts earlier by
an orbital phase of 0.02 indicating that the observed X-rays originate from a
much larger region. We have also performed spectral analysis of the
post-eclipse part of each observations. From BeppoSAX observations, the
out-of-eclipse X-ray fluxes is found to differ by a factor of ~ 26 during the
high and low intensity states while the eclipse count rates differ by a factor
of only ~ 4.7. This indicates that in the low state, there is an additional
scattering medium which scatters some of the source photons towards the
observer even when the neutron star is completely eclipsed. We could also
resolve the three iron line components using XMM-Newton observation in the low
state. By comparing the iron line equivalent width during the high and low
states, it is seen that the width of iron line is relatively large during the
low state which supports the fact that significant reprocessing and scattering
of X-rays takes place in the low state.
|
1006.1035v1
|
2010-06-17
|
The supernova delay time distribution in galaxy clusters and implications for Type-Ia progenitors and metal enrichment
|
Knowledge of the supernova (SN) delay time distribution (DTD) - the SN rate
versus time that would follow a hypothetical brief burst of star formation -
can shed light on SN progenitors and physics. We compile recent measurements of
the Type-Ia SN (SN Ia) rate in galaxy clusters at redshifts z=0-1.45. Together
with the observed iron-to-stellar mass ratio in clusters, which constrains the
time-integrated number of SN Ia events in clusters, we recover the DTD of SNe
Ia in cluster environments. The DTD peaks at the shortest time-delay interval
we probe, 0<t<2.2 Gyr, with a low tail out to delays of ~10 Gyr, and is
remarkably consistent with several recent DTD reconstructions based on
different methods, in different environments. We test DTD models from the
literature, requiring that they simultaneously reproduce the observed cluster
SN rates and the observed iron-to-stellar mass ratios. A power-law DTD of the
form t^{-1.2+/-0.3}, extending to a Hubble time, can satisfy both constraints.
Shallower power laws, such as t^{-1/2} cannot, assuming a single DTD, and a
single star-formation burst (either brief or extended) at high z. This implies
50-85% of SNe Ia explode within 1 Gyr of star formation. DTDs from
double-degenerate (DD) models, which generically have ~t^{-1} shapes over a
wide range of timescales, match the data, but only if their predictions are
scaled up by factors of 5-10. Single degenerate (SD) DTDs always give poor fits
to the data, due to a lack of delayed SNe and overall low numbers of SNe. The
observations also permit a combination of two SN Ia populations - prompt (e.g.
SD) SNe Ia that explode within a few Gyr of star formation, and produce about
60% of the iron mass in clusters, and a DD population that contributes the
events seen at z<1.4. Our results support the existence of a DD progenitor
channel for SNe Ia, if the overall predicted numbers can be suitably increased.
|
1006.3576v3
|
2010-08-01
|
Chemical Enrichment in the Faintest Galaxies: the Carbon and Iron Abundance Spreads in the Boötes I Dwarf Spheroidal Galaxy and the Segue 1 System
|
We present an AAOmega spectroscopic study of red giant stars in Bootes I,
which is an ultra-faint dwarf galaxy, and Segue 1, suggested to be either an
extremely low-luminosity dwarf galaxy or a star cluster. Our focus is
quantifying the mean abundance and abundance dispersion in iron and carbon, and
searching for distant radial-velocity members, in these systems.
The primary conclusion of our investigation is that the spread of carbon
abundance in both Bootes I and Segue 1 is large. For Bootes I, 4 of our 16
velocity members have [C/H] < ~-3.1, while 2 have [C/H] > ~-2.3, suggesting a
range of Delta[C/H] ~ 0.8. For Segue 1 there exists a range Delta[C/H] ~ 1.0,
including our discovery of a star with [Fe/H] = -3.5 and [C/Fe] = +2.3, which
is a radial velocity member at a distance of 4 half-light radii from the system
center. The accompanying ranges in iron abundance are Delta[Fe/H] ~ 1.6 for
both Bootes I and Segue 1. For [Fe/H] < -3.0, the Galaxy's dwarf galaxy
satellites exhibit a dependence of [C/Fe] on [Fe/H] which is very similar to
that observed in its halo populations. We find [C/Fe] ~ 0.3 for stars in the
dwarf systems that we believe are the counterpart of the Spite et al. (2005)
``unmixed'' giants of the Galactic halo and for which they report [C/Fe] ~ 0.2,
and which presumably represents the natal relative abundance of carbon for
material with [Fe/H] = -3.0 to -4.0.
We confirm the correlation between luminosity and both mean metallicity and
abundance dispersion in the Galaxy's dwarf satellites, which extends to at
least as faint as Mv = -5. The very low mean metallicity of Segue 1, and the
high carbon dispersion in Bootes I, consistent with inhomogeneous chemical
evolution in near zero-abundance gas, suggest these ultra-faint systems could
be surviving examples of the very first bound systems.
|
1008.0137v1
|
2010-11-29
|
New Atomic Data for Trans-Iron Elements and Their Application to Abundance Determinations in Planetary Nebulae
|
[Abridged] Investigations of neutron(n)-capture element nucleosynthesis and
chemical evolution have largely been based on stellar spectroscopy. However,
the recent detection of these elements in several planetary nebulae (PNe)
indicates that nebular spectroscopy is a promising new tool for such studies.
In PNe, n-capture element abundance determinations reveal details of s-process
nucleosynthesis and convective mixing in evolved low-mass stars, as well as the
chemical evolution of elements that cannot be detected in stellar spectra. Only
one or two ions of a given trans-iron element can typically be detected in
individual nebulae. Elemental abundance determinations thus require corrections
for the abundances of unobserved ions. Such corrections rely on the
availability of atomic data for processes that control the ionization
equilibrium of nebulae. Until recently, these data were unknown for virtually
all n-capture element ions. For the first five ions of Se, Kr, and Xe -- the
three most widely detected n-capture elements in PNe -- we are calculating
photoionization cross sections and radiative and dielectronic recombination
rate coefficients using the multi-configuration Breit-Pauli atomic structure
code AUTOSTRUCTURE. Charge transfer rate coefficients are being determined with
a multichannel Landau-Zener code. To calibrate these calculations, we have
measured absolute photoionization cross sections of Se and Xe ions at the
Advanced Light Source synchrotron radiation facility. These atomic data can be
incorporated into photoionization codes, which we will use to derive ionization
corrections (hence abundances) for Se, Kr, and Xe in ionized nebulae. These
results are critical for honing nebular spectroscopy into a more effective tool
for investigating the production and chemical evolution of trans-iron elements
in the Universe.
|
1011.6311v1
|
2011-03-31
|
The metal contents of two groups of galaxies
|
The hot gas in clusters and groups of galaxies is continuously being enriched
with metals from supernovae and stars. It is well established that the
enrichment of the gas with elements from oxygen to iron is mainly caused by
supernova explosions. The origins of nitrogen and carbon are still being
debated. Possible candidates include massive, metal-rich stars, early
generations of massive stars, intermediate or low mass stars and Asymptotic
Giant Branch (AGB) stars. In this paper we accurately determine the metal
abundances of the gas in the groups of galaxies NGC 5044 and NGC 5813, and
discuss the nature of the objects that create these metals.
We mainly focus on carbon and nitrogen. We use spatially-resolved
high-resolution X-ray spectroscopy from XMM-Newton. For the spectral fitting,
multi-temperature hot gas models are used. The abundance ratios of carbon over
oxygen and nitrogen over oxygen that we find are high compared to the ratios in
the stars in the disk of our Galaxy. The oxygen and nitrogen abundances we
derive are similar to what was found in earlier work on other giant ellipticals
in comparable environments. We show that the iron abundances in both our
sources have a gradient along the cross-dispersion direction of the Reflection
Grating Spectrometer (RGS). We conclude that it is unlikely that the creation
of nitrogen and carbon takes place in massive stars, which end their lives as
core-collapse supernovae, enriching the medium with oxygen because oxygen
should then also be enhanced. Therefore we favour low-and intermediate mass
stars as sources of these elements. The abundances in the hot gas can best be
explained by a 30-40% contribution of type Ia supernovae based on the measured
oxygen and iron abundances and under the assumption of a Salpeter Initial Mass
Function (IMF).
|
1104.0001v1
|
2011-04-11
|
Self-consistent spin-wave theory for a frustrated Heisenberg model with biquadratic exchange in the columnar phase and its application to iron pnictides
|
Recent neutron scattering studies revealed the three dimensional character of
the magnetism in the iron pnictides and a strong anisotropy between the
exchange perpendicular and parallel to the spin stripes. We extend studies of
the J1-J2-Jc Heisenberg model with S = 1 using self-consistent spin-wave
theory. A discussion of two scenarios for the instability of the columnar phase
is provided. The relevance of a biquadratic exchange term between in-plane
nearest neighbors is discussed. We introduce mean-field decouplings for
biquadratic terms using the Dyson-Maleev and the Schwinger boson
representation. Remarkably their respective mean-field theories do not lead to
the same results, even at zero temperature. They are gauged in the N'eel phase
in comparison to exact diagonalization and series expansion. The J1-J2-Jc model
is analyzed under the influence of the biquadratic exchange Jbq and a detailed
description of the staggered magnetization and of the magnetic excitations is
given. The biquadratic exchange increases the renormalization of the in-plane
exchange constants which enhances the anisotropy between the exchange parallel
and perpendicular to the spin stripes. Applying the model to iron pnictides, it
is possible to reproduce the spin-wave dispersion for CaFe2As2 in the direction
perpendicular to the spin stripes and perpendicular to the planes.
Discrepancies remain in the direction parallel to the spin stripes which can be
resolved by passing from S = 1 to S = 2. In addition, results for the dynamical
structure factor within the self-consistent spin-wave theory are provided.
|
1104.1954v2
|
2011-04-22
|
K-shell Emission of Neutral Iron Line from Sgr B2 Excited by Subrelativistic Protons
|
We investigated the emission of K$\alpha$ iron line from the massive
molecular clouds in the Galactic center (GC). We assume that at present the
total flux of this emission consists of time variable component generated by
primary X-ray photons ejected by Sagittarius A$^\ast$ (Sgr A$^\ast$) in the
past and a relatively weak quasi-stationary component excited by impact of
protons which were generated by star accretion onto the central black hole. The
level of background emission was estimated from a rise of the 6.4 keV line
intensity in the direction of several molecular clouds, that we interpreted as
a stage when the X-ray front ejected by Sgr A$^\ast$ entered into these clouds.
The 6.4 keV emission before this intensity jump we interpreted as emission
generated by subrelativistic cosmic rays there. The cross-section of K$\alpha$
vacancies produced by protons differs from that of electrons or X-rays.
Therefore, we expect that this processes can be distinguished from the analysis
of the equivalent width of the iron line and time variations of the width can
be predicted. The line intensity from the clouds depends on their distance from
Sgr A$^\ast$ and the coefficient of spacial diffusion near the Galactic center.
We expect that in a few years the line intensity for the cloud G\,0.11$-$0.11
which is relatively close to Sgr A$^\ast$ will decreases to the level
$\lesssim$ 10% from its present value. For the cloud Sagittarius B2 (Sgr B2)
the situation is more intricate. If the diffusion coefficient $D\gtrsim
10^{27}$ cm$^2$ s$^{-1}$ then the expected stationary flux should be about 10%
of its level in 2000. In the opposite case the line intensity from Sgr B2
should drop down to zero because the protons do not reach the cloud.
|
1104.4484v1
|
2011-07-05
|
A giant planet in the triple system HD132563
|
As part of our radial velocity planet-search survey performed with SARG at
TNG, we monitored the components of HD 132563 for ten years. It is a binary
system formed by two rather similar solar type stars with a projected
separation of 4.1 arcsec, which corresponds to 400 AU at the distance of 96 pc.
The two components are moderately metal-poor and the age of the system is about
5 Gyr. We detected RV variations of HD 132563B with period of 1544 days and
semi-amplitude of 26 m/s. From the star characteristics and line profile
measurements, we infer their Keplerian origin. Therefore HD 132563B turns out
to host a planet with a projected mass msini=1.49 MJup at 2.6 AU with a
moderately eccentric orbit (e=0.22). The planet around HD 132563B is one of the
few that are known in triple stellar systems, as we found that the primary HD
132563A is itself a spectroscopic binary with a period longer than 15 years and
an eccentricity higher than 0.65. The spectroscopic component was not detected
in adaptive-optics images taken with AdOpt@TNG, since it expected at a
projected separation that was smaller than 0.2 arcsec at the time of our
observations. A small excess in K band difference between the components with
respect to the difference in V band is compatible with a companion of about
0.55 Msun. A preliminary statistical analysis of the occurrence of planets in
triple systems indicate a similar frequency of planets around the isolated
component in a triple system, components of wide binaries and single stars.
There is no significant iron abundance difference between the components. The
lack of stars in binary systems and open clusters showing strong enhancements
of iron abundance, comparable to the typical metallicity difference between
stars with and without giant planets, agrees with the idea that accretion of
planetary material producing iron abundance anomalies larger than 0.1 dex is
rare.
|
1107.0918v1
|
2011-11-21
|
Iron and alpha-element Production in the First One Billion Years after the Big Bang
|
We present measurements of carbon, oxygen, silicon, and iron in quasar
absorption systems existing when the universe was roughly one billion years
old. We measure column densities in nine low-ionization systems at 4.7 < z <
6.3 using Keck, Magellan, and VLT optical and near-infrared spectra with
moderate to high resolution. The column density ratios among C II, O I, Si II,
and Fe II are nearly identical to sub-DLAs and metal-poor ([M/H] < -1) DLAs at
lower redshifts, with no significant evolution over 2 < z < 6. The estimated
intrinsic scatter in the ratio of any two elements is also small, with a
typical r.m.s. deviation of <0.1 dex. These facts suggest that dust depletion
and ionization effects are minimal in our z > 4.7 systems, as in the
lower-redshift DLAs, and that the column density ratios are close to the
intrinsic relative element abundances. The abundances in our z > 4.7 systems
are therefore likely to represent the typical integrated yields from stellar
populations within the first gigayear of cosmic history. Due to the time limit
imposed by the age of the universe at these redshifts, our measurements thus
place direct constraints on the metal production of massive stars, including
iron yields of prompt supernovae. The lack of redshift evolution further
suggests that the metal inventories of most metal-poor absorption systems at z
> 2 are also dominated by massive stars, with minimal contributions from
delayed Type Ia supernovae or AGB winds. The relative abundances in our systems
broadly agree with those in very metal-poor, non-carbon-enhanced Galactic halo
stars. This is consistent with the picture in which present-day metal-poor
stars were potentially formed as early as one billion years after the Big Bang.
|
1111.4843v1
|
2012-01-04
|
Synthesis and physical properties of the new potassium iron selenide superconductor K0.80Fe1.76Se2
|
In this article we review our studies of the K0.80Fe1.76Se2 superconductor,
with an attempt to elucidate the crystal growth details and basic physical
properties over a wide range of temperatures and applied magnetic field,
including anisotropic magnetic and electrical transport properties,
thermodynamic, London penetration depth, magneto-optical imaging and Mossbauer
measurements. We find that: (i) Single crystals of similar stoichiometry can be
grown both by furnace-cooled and decanted methods; (ii) Single crystalline
K0.80Fe1.76Se2 shows moderate anisotropy in both magnetic susceptibility and
electrical resistivity and a small modulation of stoichiometry of the crystal,
which gives rise to broadened transitions; (iii) The upper critical field,
Hc2(T) is ~ 55 T at 2 K for H||c, manifesting a temperature dependent
anisotropy that peaks near 3.6 at 27 K and drops to 2.5 by 18 K; (iv) Mossbauer
measurements reveal that the iron sublattice in K0.80Fe1.76Se2 clearly exhibits
magnetic order, probably of the first order, from well below Tc to its Neel
temperature of Tn = 532 +/- 2 K. It is very important to note that, although,
at first glance there is an apparent dilemma posed by these data: high Tc
superconductivity in a near insulating, large ordered moment material, analysis
indicates that the sample may well consist of two phases with the minority
superconducting phase (that does not exhibit magnetic order) being finely
distributed, but connected with in an antiferromagnetic, poorly conducting,
matrix, essentially making a superconducting aerogel.
|
1201.0953v2
|
2012-02-13
|
A Variable Partial Covering Model for the Seyfert 1 Galaxy MCG-6-30-15
|
We propose a simple spectral model for the Seyfert 1 Galaxy MCG-6-30-15 that
can explain most of the 1 - 40 keV spectral variation by change of the partial
covering fraction, similar to the one proposed by Miller et al. (2008). Our
spectral model is composed of three continuum components; (1) a direct
power-law component, (2) a heavily absorbed power-law component by mildly
ionized intervening matter, and (3) a cold disk reflection component far from
the black hole with moderate solid-angle ({\Omega}/2{\pi} \approx 0.3)
accompanying a narrow fluorescent iron line. The first two components are
affected by the surrounding highly ionized thin absorber with N_H \approx
10^{23.4}cm-2 and log {\xi} \approx 3.4. The heavy absorber in the second
component is fragmented into many clouds, each of which is composed of radial
zones with different ionization states and column densities, the main body (N_H
\approx 10^24.2cm-2, log {\xi} \approx 1.6), the envelope (N_H \approx
10^22.1cm-2, log {\xi} \approx 1.9) and presumably a completely opaque core.
These parameters of the ionized absorbers, as well as the intrinsic spectral
shape of the X-ray source, are unchanged at all. The central X-ray source is
moderately extended, and its luminosity is not significantly variable. The
observed flux and spectral variations are mostly explained by variation of the
geometrical partial covering fraction of the central source from 0 (uncovered)
to \sim0.63 by the intervening ionized clouds in the line of sight. The ionized
iron K-edge of the heavily absorbed component explains most of the seemingly
broad line-like feature, a well-known spectral characteristic of MCG-6-30-15.
The direct component and the absorbed component anti-correlate, cancelling
their variations each other, so that the fractional spectral variation becomes
the minimum at the iron energy band; another observational characteristic of
MCG-6-30-15 is thus explained.
|
1202.2797v3
|
2012-02-23
|
The Similarity of Broad Iron Lines in X-ray Binaries and Active Galactic Nuclei
|
We have compared the 2001 XMM-Newton spectra of the stellar mass black hole
binary XTE J1650-500 and the active galaxy MGC-6-30-15, focusing on the broad,
excess emission features at ~4--7 keV displayed by both sources. Such features
are frequently observed in both low mass X-ray binaries and active galactic
nuclei. For the former case it is generally accepted that the excess arises due
to iron emission, but there is some controversy over whether their width is
partially enhanced by instrumental processes, and hence also over the intrinsic
broadening mechanism. Meanwhile, in the latter case, the origin of this feature
is still subject to debate; physically motivated reflection and absorption
interpretations are both able to reproduce the observed spectra. In this work
we make use of the contemporaneous BeppoSAX data to demonstrate that the
breadth of the excess observed in XTE J1650-500 is astrophysical rather than
instrumental, and proceed to highlight the similarity of the excesses present
in this source and MGC-6-30-15. Both optically thick accretion discs and
optically thin coronae, which in combination naturally give rise to
relativistically-broadened iron lines when the disc extends close to the black
hole, are commonly observed in both class of object. The simplest solution is
that the broad emission features present arise from a common process, which we
argue must be reflection from the inner regions of an accretion disc around a
rapidly rotating black hole; for XTE J1650-500 we find spin constraints of 0.84
< a* < 0.98 at the 90 per cent confidence level. Other interpretations proposed
for AGN add potentially unnecessary complexities to the theoretical framework
of accretion in strong gravity.
|
1202.5193v1
|
2012-03-02
|
Testing the effects of opacity and the chemical mixture on the excitation of pulsations in B stars of the Magellanic Clouds
|
The B-type pulsators known as \beta Cephei and Slowly Pulsating B (SPB) stars
present pulsations driven by the \kappa mechanism, which operates thanks to an
opacity bump due to the iron group elements. In low-metallicity environments
such as the Magellanic Clouds, \beta Cep and SPB pulsations are not expected.
Nevertheless, recent observations show evidence for the presence of B-type
pulsator candidates in both galaxies. We seek an explanation for the excitation
of \beta Cep and SPB modes in those galaxies by examining basic input physics
in stellar modelling: i) the specific metal mixture of B-type stars in the
Magellanic Clouds; ii) the role of a potential underestimation of stellar
opacities. We first derive the present-day chemical mixtures of B-type stars in
the Magellanic Clouds. Then, we compute stellar models for that metal mixture
and perform a non-adiabatic analysis of these models. In a second approach, we
simulate parametric enhancements of stellar opacities due to different iron
group elements. We then study their effects in models of B stars and their
stability. We find that adopting a representative chemical mixture of B stars
in the Small Magellanic Cloud cannot explain the presence of B-type pulsators
there. An increase of the opacity in the region of the iron-group bump could
drive B-type pulsations, but only if this increase occurs at the temperature
corresponding to the maximum contribution of Ni to this opacity bump. We
recommend an accurate computation of Ni opacity to understand B-type pulsators
in the Small Magellanic Cloud, as well as the frequency domain observed in some
Galactic hybrid \beta Cep-SPB stars.
|
1203.0527v1
|
2012-03-19
|
Hot Gas in Galaxy Groups: Recent Observations
|
Galaxy groups are the least massive systems where the bulk of baryons begin
to be accounted for. Not simply the scaled-down versions of rich clusters
following self-similar relations, galaxy groups are ideal systems to study
baryon physics, which is important for both cluster cosmology and galaxy
formation. We review the recent observational results on the hot gas in galaxy
groups. The first part of the paper is on the scaling relations, including
X-ray luminosity, entropy, gas fraction, baryon fraction and metal abundance.
Compared to clusters, groups have a lower fraction of hot gas around the center
(e.g., r < r_2500), but may have a comparable gas fraction at large radii
(e.g., r_2500 < r < r_500). Better constraints on the group gas and baryon
fractions require sample studies with different selection functions and deep
observations at r > r_500 regions. The hot gas in groups is also iron poor at
large radii (0.3 r_500 - 0.7 r_500). The iron content of the hot gas within the
central regions (r < 0.3 r_500) correlates with the group mass, in contrast to
the trend of the stellar mass fraction. It remains to be seen where the missing
iron in low-mass groups is. In the second part, we discuss several aspects of
X-ray cool cores in galaxy groups, including their difference from cluster cool
cores, radio AGN heating in groups and the cold gas in group cool cores.
Because of the vulnerability of the group cool cores to radio AGN heating and
the weak heat conduction in groups, group cool cores are important systems to
test the AGN feedback models and the multiphase cool core models. At the end of
the paper, some outstanding questions are listed.
|
1203.4228v1
|
2012-03-28
|
Dependence of the structural and physical properties of Tl1-yFe2-z(Se1-xSx)2 with isovalent substitution of Se by S: decrease of TNeel with S content
|
The effect of selenium substitution by sulfur or tellurium in the
Tl1-yFe2-zSe2 antiferromagnet was studied by x-ray and electron diffraction,
magnetization and transport measurements. Tl0.8Fe1.5(Se1-xXx)2 (nominal
composition) solid solutions were synthesized in the full x range (0<x<1) for
X=S and up to x=0.5 for X=Te, using the sealed tube technique. No
superconductivity was found down to 4.2K in the case of sulfur despite the fact
that the optimal crystallographic parameters, determined by Rietveld
refinements, are reached in the series (i.e. the Fe-(Se,S) interplane height
and (Se,S)-Fe-(Se,S) angle for which the critical superconducting transition
T$_{c}$ is usually maximal in pnictides). Quasi full Tl site (y=0.05) compared
to significant alkaline deficiency (y=0.2-0.3) in analogous A1-yFe2-zSe2 (A =
K, Rb, Cs), and the resulting differences in iron valency, density of states
and doping, are suggested to explain this absence of superconductivity.
Compounds substituted with tellurium, at least up to x=0.25, show
superconducting transitions but probably due to tetragonal Fe(Se,Te) impurity
phase. Transmission electron microscopy confirmed the existence of ordered iron
vacancies network in the samples from the
Tl$_{0.8}$Fe$_{1.5}$(Se$_{1-x}$S$_{x}$)$_{2}$ series in the form of the
tetragonal $\sqrt{5}$ a $\times \sqrt{5}$ a $\times$ c superstructure
(\textit{I4/m}) (mixed with the orthorhombic $\sqrt{2}$ a $\times 2\sqrt{2}$ a
$\times$ c form (\textit{Ibam}) if the iron vacancies level is increased). The
N\'{e}el temperature (T$_{N}$) indicating the onset of antiferromagnetism order
in the $\sqrt{5}$ a $\times \sqrt{5}$ a $\times$ c supercell decreases from
450K in the selenide (x=0) to 330K in the sulfide (x=1). We finally demonstrate
a direct linear relationship between $T_{N\acute{e}el}$ and the Fe-(Se,S) bond
length (or Fe-(Se,S) height).
|
1203.6181v2
|
2012-05-21
|
Very High Resolution Solar X-ray Imaging Using Diffractive Optics
|
This paper describes the development of X-ray diffractive optics for imaging
solar flares with better than 0.1 arcsec angular resolution. X-ray images with
this resolution of the \geq10 MK plasma in solar active regions and solar
flares would allow the cross-sectional area of magnetic loops to be resolved
and the coronal flare energy release region itself to be probed. The objective
of this work is to obtain X-ray images in the iron-line complex at 6.7 keV
observed during solar flares with an angular resolution as fine as 0.1 arcsec -
over an order of magnitude finer than is now possible. This line emission is
from highly ionized iron atoms, primarily Fe xxv, in the hottest flare plasma
at temperatures in excess of \approx10 MK. It provides information on the flare
morphology, the iron abundance, and the distribution of the hot plasma.
Studying how this plasma is heated to such high temperatures in such short
times during solar flares is of critical importance in understanding these
powerful transient events, one of the major objectives of solar physics. We
describe the design, fabrication, and testing of phase zone plate X-ray lenses
with focal lengths of \approx100 m at these energies that would be capable of
achieving these objectives. We show how such lenses could be included on a
two-spacecraft formation-flying mission with the lenses on the spacecraft
closest to the Sun and an X-ray imaging array on the second spacecraft in the
focal plane \approx100 m away. High resolution X-ray images could be obtained
when the two spacecraft are aligned with the region of interest on the Sun.
Requirements and constraints for the control of the two spacecraft are
discussed together with the overall feasibility of such a formation-flying
mission.
|
1205.4762v1
|
2012-07-04
|
Nematic and meta-nematic transitions in the iron pnictides
|
Strongly interacting electrons can exhibit novel collective phases, among
which the electronic nematic phases are perhaps the most surprising as they
spontaneously break rotational symmetry of the underlying crystal lattice. The
electron nematicity has been recently observed in the iron-pnictide and cuprate
high-temperature superconductors. Whether such a tendency of electrons to
self-organise unidirectionally has a common feature in these superconductors
is, however, a highly controversial issue. In the cuprates, the nematicity has
been suggested as a possible source of the pseudogap phase, whilst in the
iron-pnictides, it has been commonly associated with the
tetragonal-to-orthorhombic structural phase transition at $T_s$. Here, we
provide the first thermodynamic evidence in BaFe2(As1-xPx)2 that the nematicity
develops well above the structural transition and persists to the nonmagnetic
superconducting regime, resulting in a new phase diagram strikingly similar to
the pseudogap phase diagram in the cuprates. Our highly sensitive magnetic
anisotropy measurements using microcantilever torque-magnetometry under
in-plane field rotation reveal pronounced two-fold oscillations, which break
the tetragonal symmetry. Combined with complementary high-resolution
synchrotron X-ray and resistivity measurements, our results consistently
identify two distinct temperatures - one at $T^{\ast}$, signifying a true
nematic transition, and the other at $T_s (< T^{\ast})$, which we show to be
not a true phase transition, but rather what we refer to as a "meta-nematic
transition", in analogy to the well-known metamagnetic transition in the theory
of magnetism. Our observation of the extended nematic phase above the
superconducting dome establishes that the nematicity has primarily an
electronic origin, inherent in the normal state of high-temperature
superconductors.
|
1207.1045v1
|
2012-08-24
|
Failed-Detonation Supernovae: Sub-Luminous Low-Velocity Ia Supernovae and Their Kicked Remnant White Dwarfs with Iron-Rich Cores
|
Type Ia supernovae (SNe Ia) originate from the thermonuclear explosions of
carbon-oxygen (C-O) white dwarfs (WDs). The single-degenerate scenario is a
well-explored model of SNe Ia where unstable thermonuclear burning initiates in
an accreting, Chandrasekhar-mass WD and forms an advancing flame. By several
proposed physical processes the rising, burning material triggers a detonation,
which subsequently consumes and unbinds the WD. However, if a detonation is not
triggered and the deflagration is too weak to unbind the star, a completely
different scenario unfolds. We explore the failure of the
Gravitationally-Confined Detonation (GCD) mechanism of SNe Ia, and demonstrate
through 2D and 3D simulations the properties of failed-detonation SNe. We show
that failed-detonation SNe expel a few 0.1 solar masses of burned and
partially-burned material and that a fraction of the material falls back onto
the WD, polluting the remnant WD with intermediate-mass and iron-group
elements, that likely segregate to the core forming an WD whose core is iron
rich. The remaining material is asymmetrically ejected at velocities comparable
to the escape velocity from the WD, and in response, the WD is kicked to
velocities of a few hundred km/s. These kicks may unbind the binary and eject a
runaway/hyper-velocity WD. Although the energy and ejected mass of the
failed-detonation SN are a fraction of typical thermonuclear SNe, they are
likely to appear as sub-luminous low-velocity SNe Ia. Such failed detonations
might therefore explain or are related to the observed branch of peculiar SNe
Ia, such as the family of low-velocity sub-luminous SNe (SN 2002cx/SN
2008ha-like SNe).
|
1208.5069v2
|
2012-11-26
|
Local Structure and Hyperfine Interactions of 57Fe in NaFeAs Studied by Mossbauer Spectroscopy
|
Detailed 57Fe Mossbauer spectroscopy measurements on superconducting NaFeAs
powder samples have been performed in the temperature range 13 K < T < 300 K.
The 57Fe spectra recorded in the paramagnetic range (T > TN ~ 46 K) were
discussed supposing that most of the Fe2+ ions are located in distorted (FeAs4)
tetrahedral of NaFeAs phase, while additional minor (< 10%) component of the
spectra corresponds to impurity or intergrowth NaFe2As2 phase with a nominal
composition near "NaFe2As2". Our results reveal that the structural transition
(TS ~ 55K) has a weak effect on the electronic structure of iron ions, while at
T < TN the spectra show a continuous distribution of hyperfine fields HFe .
Shape of these spectra was analyzed in term of two models: (i) an
incommensurate spin density wave modulation of iron magnetic structure, (ii) a
formation of a microdomain structure or phase separation. It was shown that the
hyperfine parameters obtained using these two methods have very similar values
over the whole temperature range. The analysis of the temperature dependence
HFe(T) with the Bean-Rodbell model leads to ksi = 1.16 +/- 0.05, suggesting
that the magnetic phase transition is first-order in nature. A sharp evolution
of the VZZ(T) and etha(T) parameters of the full Hamiltonian of hyperfine
interactions near T ~ (TN, TS), were interpreted as a manifestation of the
anisotropic electron redistribution between the dxz, dyz- and dxy-orbitals of
the iron ions.
|
1211.5967v3
|
2012-12-02
|
Tunable interplay between 3d and 4f electrons in Co-doped iron pnictides
|
We study the interplay of 3d and 4f electrons in the iron pnictides
CeFe$_{1-x}$Co$_x$AsO and GdFe$_{1-y}$Co$_y$AsO, which correspond to two very
different cases of $4f$-magnetic moment. Both CeFeAsO and GdFeAsO undergo a
spin-density-wave (SDW) transition associated with Fe 3d electrons at high
temperatures, which is rapidly suppressed by Fe/Co substitution.
Superconductivity appears in a narrow doping range: $0.05 < x < 0.2$ for
CeFe$_{1-x}$Co$_x$AsO and $0.05 < y < 0.25$ for GdFe$_{1-y}$Co$_y$AsO, showing
a maximum transition temperature $T_\textup{sc}$ of about 13.5 K for Ce and 19
K for Gd. In both compounds, the $4f$-electrons form an antiferromagnetic (AFM)
order at low temperatures over the entire doping range and Co 3d electrons are
ferromagnetically ordered on the Co-rich side; the Curie temperature reaches
$T_\textup{C}^\textup{Co} \approx$ 75 K at $x = 1$ and $y = 1$. In the
Ce-compounds, the N\'{e}el temperature $T_\textup{N}^\textup{Ce}$ increases
upon suppressing the SDW transition of Fe and then remains nearly unchanged
with further increasing Co concentration up to $x \simeq 0.8$
($T_\textup{N}^\textup{Ce}\approx$ 4 K). Furthermore, evidence of Co-induced
polarization on Ce-moments is observed on the Co-rich side. In the
Gd-compounds, the two magnetic species of Gd and Co are coupled
antiferromagnetically to give rise to ferrimagnetic behavior in the magnetic
susceptibility on the Co-rich side. For $0.7 \leq y < 1.0$, the system
undergoes a possible magnetic reorientation below the N\'{e}el temperature of
Gd ($T_\textup{N}^\textup{Gd}$). Our results suggest that the effects of both
electron hybridizations and magnetic exchange coupling between the 3d-4f
electrons give rise to a rich phase diagram in the rare-earth iron pnictides.
|
1212.0221v2
|
2013-01-15
|
Pulse phase and precession phase resolved spectroscopy of Her X-1: studying a representative Main-On with RXTE
|
We performed a detailed pulse phase resolved spectroscopy of the accreting
binary X-ray pulsar Her X-1 in the energy range 3.5-75 keV and have established
pulse phase profiles for all spectral parameters. For the centroid of the
cyclotron line, the photon index and the flux of the 6.4 keV iron line, we have
studied the variation as a function of 35 d phase. We analyzed RXTE
observations of the Main-On of November 2002. Four different time intervals of
about 1 d duration were selected to provide a good coverage of a complete
Main-On. The intervals are centered at 35 d phase 0.03, 0.10, 0.15, and 0.20,
respectively. All spectral parameters show a strong modulation with pulse
phase. While the centroid energy of the cyclotron line follows roughly the
shape of the pulse profile, both the photon index and the iron line intensity
exhibit distinct minima around the peak of the X-ray pulse. With respect to
variations of the observed profiles with 35 d phase, we find that there is a
clear evolution of the shape of the pulse profiles (flux versus pulse phase), a
moderate increase of the maximum cyclotron line energy (found around pulse
phase 0.7), but no significant evolution of the shape of the pulse phase
profiles of the cyclotron line energy, the spectral power law index or the iron
line intensity. The variation of spectral parameters as a function of the pulse
phase provides important information about the system: 1. the disappearance of
the Fe line flux near the highest continuum flux may be an indication of a
hollow cone geometry of the accretion structure; ii. the apparent
non-dependence of the cyclotron line energy profiles on 35 d phase provides a
new possibility to test the model of free precession of the neutron star,
proposed to be responsible for the systematic variations in the pulse profiles.
|
1301.3378v1
|
2013-01-24
|
The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI
|
We study the circumstellar environment of the M-type AGB star RT Vir using
mid-infrared high spatial resolution observations from the ESO-VLTI focal
instrument MIDI. The aim of this study is to provide observational constraints
on theoretical prediction that the winds of M-type AGB objects can be driven by
photon scattering on iron-free silicate grains located in the close environment
(about 2 to 3 stellar radii) of the star. We interpreted
spectro-interferometric data, first using wavelength-dependent geometric
models. We then used a self-consistent dynamic model atmosphere containing a
time-dependent description of grain growth for pure forsterite dust particles
to reproduce the photometric, spectrometric, and interferometric measurements
of RT Vir. Since the hydrodynamic computation needs stellar parameters as
input, a considerable effort was first made to determine these parameters. MIDI
differential phases reveal the presence of an asymmetry in the stellar
vicinity. Results from the geometrical modeling give us clues to the presence
of aluminum and silicate dust in the close circumstellar environment (< ~5
stellar radii). Comparison between spectro-interferometric data and a
self-consistent dust-driven wind model reveals that silicate dust has to be
present in the region between 2 to 3 stellar radii to reproduce the 59 and 63 m
baseline visibility measurements around 9.8 micron. This gives additional
observational evidence in favor of winds driven by photon scattering on
iron-free silicate grains located in the close vicinity of an M-type star.
However, other sources of opacity are clearly missing to reproduce the 10-13
micron visibility measurements for all baselines. This study is a first attempt
to understand the wind mechanism of M-type AGB stars by comparing photometric,
spectrometric, and interferometric measurements with state-of-the-art,
self-consistent dust-driven wind models. The agreement of the dynamic model
atmosphere with interferometric measurements in the 8-10 micron spectral region
gives additional observational evidence that the winds of M-type stars can be
driven by photon scattering on iron-free silicate grains. Finally, a larger
statistical study and progress in advanced self-consistent 3D modeling are
still required to solve the remaining problems.
|
1301.5872v1
|
2013-03-15
|
The 69 micron forsterite band in spectra of protoplanetary disks - Results from the Herschel DIGIT programme
|
Context: We have analysed Herschel-PACS spectra of 32 circumstellar disks
around Herbig Ae/Be and T-Tauri stars obtained within the Herschel key
programme DIGIT. In this paper we focus on the 69mu emission band of the
crystalline silicate forsterite.
Aims: This work provides an overview of the 69mu forsterite bands in the
DIGIT sample. We aim to derive the temperature and composition of the
forsterite grains. With this information, constraints can be placed on the
spatial distribution of the forsterite in the disk and its formation history.
Methods: Position and shape of the 69mu band are used to derive the
temperature and composition of the dust by comparison to laboratory spectra of
that band. We combine our data with existing Spitzer IRS spectra to compare the
presence and strength of the 69mu band to the forsterite bands at shorter
wavelengths.
Results: A total of 32 sources have been observed, 8 of them show a 69mu
emission band that can be attributed to forsterite. With the exception of the
T-Tauri star AS205, all of the detections are for disks associated with Herbig
Ae/Be stars. Most of the forsterite grains that give rise to the 69mu bands are
warm (~100-200 K) and iron-poor (less than ~2% iron). Only AB-Aur requires
approximately 3-4% of iron.
Conclusions: Our findings support the hypothesis that the forsterite grains
form through an equilibrium condensation process at high temperatures. The
connection between the strength of the 69 and 33mu bands shows that at least
part of the emission in these bands originates from the same dust grains.
Further, any model that explains the PACS and the Spitzer IRS observations must
take the effects of a wavelength dependent optical depth into account. We find
indications of a correlation of the detection rate of the 69mu band with the
spectral type of the host stars. However, our sample is too small to obtain a
definitive result.
|
1303.3744v1
|
2013-04-30
|
Superconductivity induced by hydrogen anion substitution in 1111-type iron arsenides
|
Hydrogen is the simplest bipolar element and its valence state can be
controlled from +1 to -1. We synthesized the 1111-type CaFeAsH and LnFeAsO1-xHx
(Ln = lanthanide) with the ZrCuSiAs type structure by a high-pressure synthesis
method. The position and valence state of the substituted H were determined by
neutron diffraction and density functional theory calculations. The close
similarity in the structural and electrical properties of CaFeAsH and CaFeAsF
indicated the formation of the hydride ion (H-), which is isovalent with the
fluoride ion (F-), in the 1111-type iron arsenides. When some of the O2- ions
in LnFeAsO are replaced by H-, superconductivity is induced by electron doping
to the FeAs-layer to maintain charge neutrality. Since the substitution limit
of hydrogen in LnFeAsO (x~0.5) is much higher than that of fluorine (x~0.2),
the hydrogen substitution technique provides an effective pathway for
high-density electron-doping, making it possible to draw the complete
electronic phase diagram of LnFeAsO. The x-T diagrams of LnFeAsO1-xHx (Ln = La,
Ce, Sm, Gd) have a wide superconducting (SC) region spanning the range x = 0.04
to 0.4, which is far from the parent antiferromagnetic region near x = 0.0. For
Ln=La, another SC dome region was found in the range x = ~ 0.2 to ~0.5 with a
maximum Tc = 36 K, in addition to a conventional SC dome located at x ~0.08
with maximum Tc = 29 K. Density functional theory calculations performed for
LaFeAsO1-xHx indicated that the newly observed Tc is correlated with the
appearance of degeneration of the Fe 3d bands (dxy, dyz and dzx), which is
caused not only by regularization of the tetrahedral shape of FeAs4 due to
chemical pressure effects but also by selective band occupation with doped
electrons. In this article, we review the recent progress of superconductivity
in iron (oxy)arsenides and related compounds induced by hydrogen substitution.
|
1304.7900v1
|
2013-05-23
|
On magnetic inhibition of photospheric macro-turbulence generated in the iron-bump opacity zone of O-stars
|
Massive, hot OB-stars show clear evidence of strong macroscopic broadening
(in addition to rotation) in their photospheric spectral lines. This paper
examines the occurrence of such "macro-turbulence" in slowly rotating O-stars
with strong, organised surface magnetic fields. Focusing on the CIV 5811A line,
we find evidence for significant macro-turbulent broadening in all stars except
NGC1624-2, which also has (by far) the strongest magnetic field. Instead, the
very sharp CIV lines in NGC1624-2 are dominated by magnetic Zeeman broadening,
from which we estimate a dipolar field of approximately 20 kG. By contrast,
magnetic broadening is negligible in the other stars (due to their weaker field
strengths, on order 1 kG), and their CIV profiles are typically very broad and
similar to corresponding lines observed in non-magnetic O-stars. Quantifying
this by an isotropic, Gaussian macro-turbulence, we derive vmac = 2.2 (+-
0.9/2.2) km/s for NGC1624-2, and vmac = 20-65 km/s for the rest of the magnetic
sample. We use these observational results to test the hypothesis that the
field can stabilise the atmosphere and suppress the generation of
macro-turbulence down to stellar layers where the magnetic pressure PB and the
gas pressure Pg are comparable. Using a simple grey atmosphere to estimate the
temperature T0 at which PB = Pg, we find that T0 > Teff for all investigated
magnetic stars, but that T0 reaches the ~160000 K layers associated with the
iron opacity-bump in hot stars only for NGC1624-2. This is consistent with the
view that the responsible physical mechanism for photospheric O-star
macro-turbulence may be stellar gravity-mode oscillations excited by
sub-surface convection zones, and suggests that a sufficiently strong magnetic
field can suppress such iron-bump generated convection and associated
pulsational excitation.
|
1305.5549v1
|
2013-06-14
|
Dynamic Transcript Profiling of Candida Albicans Infection in Zebrafish: a Pathogen-Host Interaction Study
|
Candida albicans is responsible for a number of life-threatening infections
and causes considerable morbidity and mortality in immunocompromised patients.
Previous studies of C. albicans pathogenesis have suggested several steps must
occur before virulent infection, including early adhesion, invasion, and late
tissue damage. However, the mechanism that triggers C. albicans transformation
from yeast to hyphae form during infection has yet to be fully elucidated. This
study used a systems biology approach to investigate C. albicans infection in
zebrafish. The surviving fish were sampled at different post-infection time
points to obtain time-lapsed, genome-wide transcriptomic data from both
organisms, which were accompanied with in sync histological analyses. Principal
component analysis (PCA) was used to analyze the dynamic gene expression
profiles of significant variations in both C. albicans and zebrafish. The
results categorized C. albicans infection into three progressing phases:
adhesion, invasion, and damage. Such findings were highly supported by the
corresponding histological analysis. Furthermore, the dynamic interspecies
transcript profiling revealed that C. albicans activated its filamentous
formation during invasion and the iron scavenging functions during the damage
phases, whereas zebrafish ceased its iron homeostasis function following
massive hemorrhage during the later stages of infection. This was followed by
massive hemorrhaging toward the end stage of infection. Most of the immune
related genes were expressed as the infection progressed from invasion to the
damage phase. Such global, inter-species evidence of virulence-immune and iron
competition dynamics during C. albicans infection could be crucial in
understanding control fungal pathogenesis.
|
1306.3312v1
|
2013-10-09
|
Electron doping evolution of the magnetic excitations in BaFe2-xNixAs2
|
We use inelastic neutron scattering (INS) spectroscopy to study the magnetic
excitations spectra throughout the Brioullion zone in electron-doped iron
pnictide superconductors BaFe$_{2-x}$Ni$_{x}$As$_{2}$ with $x=0.096,0.15,0.18$.
While the $x=0.096$ sample is near optimal superconductivity with $T_c=20$ K
and has coexisting static incommensurate magnetic order, the $x=0.15,0.18$
samples are electron-overdoped with reduced $T_c$ of 14 K and 8 K,
respectively, and have no static antiferromagnetic (AF) order. In previous INS
work on undoped ($x=0$) and electron optimally doped ($x=0.1$) samples, the
effect of electron-doping was found to modify spin waves in the parent compound
BaFe$_2$As$_2$ below $\sim$100 meV and induce a neutron spin resonance at the
commensurate AF ordering wave vector that couples with superconductivity. While
the new data collected on the $x=0.096$ sample confirms the overall features of
the earlier work, our careful temperature dependent study of the resonance
reveals that the resonance suddenly changes its $Q$-width below $T_c$ similar
to that of the optimally hole-doped iron pnictides
Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_2$. In addition, we establish the dispersion of
the resonance and find it to change from commensurate to transversely
incommensurate with increasing energy. Upon further electron-doping to
overdoped iron pnictides with $x=0.15$ and 0.18, the resonance becomes weaker
and transversely incommensurate at all energies, while spin excitations above
$\sim$100 meV are still not much affected. Our absolute spin excitation
intensity measurements throughout the Brillouin zone for $x=0.096,0.15,0.18$
confirm the notion that the low-energy spin excitation coupling with itinerant
electron is important for superconductivity in these materials, even though the
high-energy spin excitations are weakly doping dependent.
|
1310.2333v1
|
2013-10-11
|
The return to the hard state of GX 339-4 as seen by Suzaku
|
The microquasar GX 339-4 was observed by Suzaku five times, spaced by a few
days, during its transition back to the hard state at the end of its 2010-2011
outburst. The 2-10 keV source flux decreases by a factor ~10 between the
beginning and the end of the monitoring. Simultaneous radio and OIR
observations highlighted the re-ignition of the radio emission just before the
beginning of the campaign, the maximum radio emission being reached between the
two first Suzaku pointings, while the IR peaked a few weeks latter. A
fluorescent iron line is always significantly detected. Fits with a gaussian or
Laor profiles give statistically equivalent results. In the case of a Laor
profile, fits of the five data sets simultaneously agree with a disk
inclination angle of ~20 degrees. The disk inner radius is <10-30 R_g in the
first two observations but almost unconstrained in the last three. A soft X-ray
excess is also present in these two first observations. Fits with a multicolor
disk component give disk inner radii in agreement with those obtained with the
iron line fits. The use of a physically more realistic model, including a
blurred reflection component and a comptonization continuum, give some hints of
the increase of the disk inner radius but the significances are always weak.
Interestingly, the addition of warm absorption significantly improves the fit
of OBS1 while it is not needed in the other observations. The radio-jet
re-ignition occurring between OBS1 and OBS2, these absorption features may
indicate the natural evolution from a disk wind and a jet. The comparison with
a long 2008 Suzaku observation of GX 339-4 in a persistent faint hard state
where a narrow iron line clearly indicates a disk recession, is discussed.
|
1310.3039v2
|
2013-10-30
|
On the Origin of the 6.4 keV Line in the Galactic Center Region
|
We analyse the 6.4 keV iron line component produced in the Galactic Center
(GC) region by cosmic rays in dense molecular clouds (MCs) and in the diffuse
molecular gas. We showed that this component, in principle, can be seen in
several years in the direction of the cloud Srg B2. If this emission is
produced by low energy CRs which ionize the interstellar molecular gas the
intensity of the line is quite small, < 1%. However, we cannot exclude that
local sources of CRs or X-ray photons nearby the cloud may provide much higher
intensity of the line from there. Production of the line emission from
molecular clouds depends strongly on processes of CR penetration into them. We
show that turbulent motions of neutral gas may generate strong magnetic
fluctuations in the clouds which prevent free penetration of CRs into the
clouds from outside. We provide a special analysis of the line production by
high energy electrons. We concluded that these electrons hardly provide the
diffuse 6.4 keV line emission from the GC because their density is depleted by
ionization losses. We do not exclude that local sources of electrons may
provide an excesses of the 6.4 keV line emission in some molecular clouds and
even reproduce a relatively short time variations of the iron line emission.
However, we doubt whether a single electron source provides the simultaneous
short time variability of the iron line emission from clouds which are distant
from each other on hundred pc as observed for the GC clouds. An alternative
speculation is that local electron sources could also provide the necessary
effect of the line variations in different clouds that are seen simultaneously
by chance that seems, however, very unlikely.
|
1310.8177v1
|
2013-11-04
|
On the role of AGN feedback on the thermal and chemodynamical properties of the hot intra-cluster medium
|
We present an analysis of the properties of the ICM in an extended set of
cosmological hydrodynamical simulations of galaxy clusters and groups performed
with the TreePM+SPH GADGET-3 code. Besides a set of non-radiative simulations,
we carried out two sets of simulations including radiative cooling, star
formation, metal enrichment and feedback from supernovae, one of which also
accounts for the effect of feedback from AGN resulting from gas accretion onto
super-massive black holes. These simulations are analysed with the aim of
studying the relative role played by SN and AGN feedback on the general
properties of the diffuse hot baryons in galaxy clusters and groups: scaling
relations, temperature, entropy and pressure radial profiles, and ICM chemical
enrichment. We find that simulations including AGN feedback produce scaling
relations that are in good agreement with X-ray observations at all mass
scales. However, our simulations are not able to account for the observed
diversity between CC and NCC clusters: unlike for observations, we find that
temperature and entropy profiles of relaxed and unrelaxed clusters are quite
similar and resemble more the observed behaviour of NCC clusters. As for the
pattern of metal enrichment, we find that an enhanced level of iron abundance
is produced by AGN feedback with respect to the case of purely SN feedback. As
a result, while simulations including AGN produce values of iron abundance in
groups in agreement with observations, they over-enrich the ICM in massive
clusters. The efficiency of AGN feedback in displacing enriched gas from halos
into the inter-galactic medium at high redshift also creates a widespread
enrichment in the outskirts of clusters and produces profiles of iron abundance
whose slope is in better agreement with observations.
|
1311.0818v1
|
2013-11-07
|
GIANO-TNG spectroscopy of red supergiants in the young star cluster RSGC2
|
The inner disk of the Galaxy has a number of young star clusters dominated by
red supergiants that are heavily obscured by dust extinction and observable
only at infrared wavelengths. These clusters are important tracers of the
recent star formation and chemical enrichment history in the inner Galaxy.
During the technical commissioning and as a first science verification of the
GIANO spectrograph at the Telescopio Nazionale Galileo, we secured
high-resolution (R~50,000) near-infrared spectra of three red supergiants in
the young Scutum cluster RSGC2. Taking advantage of the full YJHK spectral
coverage of GIANO in a single exposure, we were able to identify several tens
of atomic and molecular lines suitable for chemical abundance determinations.
By means of spectral synthesis and line equivalent width measurements, we
obtained abundances of Fe and other iron-peak elements such as V, Cr, Ni, of
alpha (O, Mg, Si, Ca and Ti) and other light elements (C, N, Na, Al, K, Sc),
and of some s-process elements (Y, Sr). We found iron abundances between half
and one third solar and solar-scaled [X/Fe] abundance patterns of iron-peak,
alpha and most of the light elements, consistent with a thin-disk chemistry. We
found a depletion of [C/Fe] and enhancement of [N/Fe], consistent with CN
burning, and low 12C/13C abundance ratios (between 9 and 11), requiring
extra-mixing processes in the stellar interiors during the post-main sequence
evolution. Finally, we found a slight [Sr/Fe] enhancement and a slight [Y/Fe]
depletion (by a factor of <=2), with respect to solar.
|
1311.1639v1
|
2013-12-20
|
Nematic order in iron superconductors - who is in the driver's seat?
|
Although the existence of nematic order in iron-based superconductors is now
a well-established experimental fact, its origin remains controversial. Nematic
order breaks the discrete lattice rotational symmetry by making the $x$ and $y$
directions in the Fe plane non-equivalent. This can happen because of (i) a
tetragonal to orthorhombic structural transition, (ii) a spontaneous breaking
of an orbital symmetry, or (iii) a spontaneous development of an Ising-type
spin-nematic order - a magnetic state that breaks rotational symmetry but
preserves time-reversal symmetry. The Landau theory of phase transitions
dictates that the development of one of these orders should immediately induce
the other two, making the origin of nematicity a physics realization of a
"chicken and egg problem". The three scenarios are, however, quite different
from a microscopic perspective. While in the structural scenario lattice
vibrations (phonons) play the dominant role, in the other two scenarios
electronic correlations are responsible for the nematic order. In this review,
we argue that experimental and theoretical evidence strongly points to the
electronic rather than phononic mechanism, placing the nematic order in the
class of correlation-driven electronic instabilities, like superconductivity
and density-wave transitions. We discuss different microscopic models for
nematicity in the iron pnictides, and link nematicity to other ordered states
of the global phase diagram of these materials -- magnetism and
superconductivity. In the magnetic model nematic order pre-empts stripe-type
magnetic order, and the same interaction which favors nematicity also gives
rise to an unconventional $s^{+-}$ superconductivity. In the charge/orbital
model magnetism appears as a secondary effect of ferro-orbital order, and the
interaction which favors nematicity gives rise to a conventional $s^{++}$
superconductivity.
|
1312.6085v1
|
2014-03-24
|
On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk
|
We present homogeneous and accurate iron abundances for 42 Galactic Cepheids
based on high-spectral resolution (R~38,000) high signal-to-noise ratio
(SNR>100) optical spectra collected with UVES at VLT (128 spectra). The above
abundances were complemented with high-quality iron abundances provided either
by our group (86) or available in the literature. We paid attention in deriving
a common metallicity scale and ended up with a sample of 450 Cepheids. We also
estimated for the entire sample accurate individual distances by using
homogeneous near-infrared photometry and the reddening free Period-Wesenheit
relations. The new metallicity gradient is linear over a broad range of
Galactocentric distances (Rg~5-19 kpc) and agrees quite well with similar
estimates available in the literature (-0.060+/-0.002 dex/kpc). We also uncover
evidence which suggests that the residuals of the metallicity gradient are
tightly correlated with candidate Cepheid Groups (CGs). The candidate CGs have
been identified as spatial overdensities of Cepheids located across the thin
disk. They account for a significant fraction of the residual fluctuations, and
in turn for the large intrinsic dispersion of the metallicity gradient. We
performed a detailed comparison with metallicity gradients based on different
tracers: OB stars and open clusters. We found very similar metallicity
gradients for ages younger than 3 Gyrs, while for older ages we found a
shallower slope and an increase in the intrinsic spread. The above findings
rely on homogeneous age, metallicity and distance scales. Finally we found, by
using a large sample of Galactic and Magellanic Cepheids for which are
available accurate iron abundances, that the dependence of the luminosity
amplitude on metallicity is vanishing.
|
1403.6128v1
|
2014-07-03
|
The Detection of Earth-mass Planets around Active Stars: The Mass of Kepler-78b
|
Kepler-78b is a transiting Earth-mass planet in an 8.5 hr orbit discovered by
the Kepler Space Mission. We performed an analysis of the published radial
velocity measurements for Kepler-78 in order to derive a refined measurement
for the planet mass. Kepler-78 is an active star and radial velocity variations
due to activity were removed using a Floating Chunk Offset (FCO) method where
an orbital solution was made to the data by allowing the velocity offsets of
individual nights to vary. We show that if we had no a priori knowledge of the
transit period the FCO method used as a periodogram would still have detected
Kepler-78b in the radial velocity data. It can thus be effective at finding
unknown short-period signals in the presence of significant activity noise.
Using the FCO method while keeping the ephemeris and orbital phase fixed to the
photometric values and using only data from nights where 6-10 measurements were
taken results in a K-amplitude of 1.34 +/- 0.25 m/s. a planet mass of 1.31 +/-
0.24 M_Earth, and a planet density of rho = 4.5 (-2.0/+2.2) g/cm^3. Allowing
the orbital phase to be a free parameter reproduces the transit phase to within
the uncertainty. The corresponding density implies that Kepler-78b may have a
structure that is deficient in iron and is thus more like the Moon. Although
the various approaches that were used to filter out the activity of Kepler 78
produce consistent radial velocity amplitudes to within the errors, these are
still too large to constrain the structure of this planet. The uncertainty in
the mass for Kepler-78b is large enough to encompass models with structures
ranging from Mercury-like (iron enriched) to Moon-like (iron deficient). A more
accurate K-amplitude as well as a better determination of the planet radius are
needed to distinguish between these models.
|
1407.0853v1
|
2014-08-06
|
Theory of the evolution of magnetic order in Fe$_{1+y}$Te compounds with increasing interstitial iron
|
We examine the influence of the excess of interstitial Fe on the magnetic
properties of Fe$_{1+y}$Te compounds. Because in iron chalcogenides the
correlations are stronger than in the iron arsenides, we assume in our model
that some of the Fe orbitals give rise to localized magnetic moments. These
moments interact with each other via exchange interactions as well as
phonon-mediated biquadratic interactions that favor a collinear double-stripe
state, corresponding to the ordering vectors $\left(\pm\pi/2,\pm\pi/2\right)$.
The remaining Fe orbitals are assumed to be itinerant, giving rise to the
first-principle derived Fermi surface displaying nesting features at momenta
$\left(\pi,0\right)/\left(0,\pi\right)$. Increasing the amount of itinerant
electrons due to excess Fe, $y$, leads to changes in the Fermi surface and to
the suppression of its nesting properties. As a result, due to the Hund's
coupling between the itinerant and localized moments, increasing $y$ leads to
modifications in the local moments' exchange interactions via the multi-orbital
generalization of the long-range Ruderman-Kittel-Kasuya-Yosida (RKKY)
interaction. By numerically computing the RKKY corrections and minimizing the
resulting effective exchange Hamiltonian, we find, in general, that the excess
electrons introduced in the system change the classical magnetic ground state
from a double-stripe state to an incommensurate spiral, consistent with the
experimental observations. We show that these results can be understood as a
result of the suppression of magnetic spectral weight of the itinerant
electrons at momenta $\left(\pi,0\right)/\left(0,\pi\right)$, combined with the
transfer of broad magnetic spectral weight from large to small momenta,
promoted by the introduction of excess Fe.
|
1408.1418v2
|
2014-11-28
|
Superconducting properties and pseudogap from preformed Cooper pairs in the triclinic (CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$
|
Using a combination of muon-spin relaxation ($\mu$SR), inelastic neutron
scattering (INS) and nuclear magnetic resonance (NMR), we investigated the
novel iron-based superconductor with a triclinic crystal structure
(CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$ (T$_{\rm c}$ = 13 K), containing
platinum-arsenide intermediary layers. The temperature dependence of the
superfluid density obtained from the $\mu$SR relaxation-rate measurements
indicates the presence of two superconducting gaps,
$\Delta_\text{1}\gg\Delta_\text{2}$. According to our INS measurements,
commensurate spin fluctuations are centered at the ($\pi$, 0) wave vector, like
in most other iron arsenides. Their intensity remains unchanged across
T$_\text{c}$, indicating the absence of a spin resonance typical for many
Fe-based superconductors. Instead, we observed a peak in the spin-excitation
spectrum around $\hslash\omega_0=\,$7 meV at the same wave vector, which
persists above T$_{\rm c}$ and is characterized by the ratio
$\hslash\omega_0/k_\text{B}T_\text{c}\approx\,$6.2, which is significantly
higher than typical values for the magnetic resonant modes in iron pnictides
(~4.3). The temperature dependence of magnetic intensity at 7 meV revealed an
anomaly around T* = 45 K related to the disappearance of this new mode. A
suppression of the spin-lattice relaxation rate, $1/T_1T$, observed by NMR
immediately below T* without any notable subsequent anomaly at T$_{\rm c}$,
indicates that T* could mark the onset of a pseudogap in
(CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$, which is likely associated with the
emergence of preformed Cooper pairs.
|
1411.7858v1
|
2014-12-30
|
An XSPEC model to explore spectral features from black-hole sources - II. The relativistic iron line in the lamp-post geometry
|
In X-ray spectra of several active galactic nuclei and Galactic black hole
binaries a broad relativistically smeared iron line is observed. This feature
arises by fluorescence when the accretion disc is illuminated by hot corona
above it. Due to central location of the corona the illumination and thus also
the line emission decrease with radius. It was reported in the literature that
this decrease is very steep in some of the sources, suggesting a highly compact
corona.
We revisit the lamp-post setup in which the corona is positioned on the axis
above the rotating black hole and investigate to what extent the steep
emissivity can be explained by this scenario. We show the contributions of the
relativistic effects to the disc illumination by the primary source - energy
shift, light bending and aberration. The lamp-post radial illumination pattern
is compared to the widely used radial broken power-law emissivity profile. We
find that very steep emissivities require the primary illuminating source to be
positioned very near the black hole horizon and/or the spectral power-law index
of the primary emission to be very high. The broken power-law approximation of
the illumination can be safely used when the primary source is located at
larger heights. However, for low heights the lamp-post illumination
considerably differs from this approximation.
We also show the variations of the iron line local flux over the disc due to
the flux dependence on incident and emission angles. The former depends mainly
on the height of the primary source while the latter depends on the inclination
angle of the observer. Thus the strength of the line varies substantially
across the disc. This effect may contribute to the observed steeper emissivity.
|
1412.8627v1
|
2015-03-14
|
Deep SDSS optical spectroscopy of distant halo stars II. Iron, calcium, and magnesium abundances
|
We analyze a sample of 3,944 low-resolution (R ~ 2000) optical spectra from
the Sloan Digital Sky Survey (SDSS), focusing on stars with effective
temperatures 5800 < Teff < 6300 K, and distances from the Milky Way plane in
excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. We followed
the same methodology as in the previous paper in this series, deriving
atmospheric parameters by chi2 minimization, but this time we obtained the
abundances of individual elements by fitting their associated spectral lines.
Distances were calculated from absolute magnitudes obtained by a statistical
comparison of our stellar parameters with stellar-evolution models. The
observations reveal a decrease in the abundances of iron, calcium, and
magnesium at large distances from the Galactic center. The median abundances
for the halo stars analyzed are fairly constant up to a Galactocentric distance
r ~ 20 kpc, rapidly decrease between r ~ 20 and r ~ 40 kpc, and flatten out to
significantly lower values at larger distances, consistent with previous
studies. In addition, we examine the [Ca/Fe] and [Mg/Fe] as a function of Fe/H
and Galactocentric distance. Our results show that the most distant parts of
the halo show a steeper variation of the [Ca/Fe] and [Mg/Fe] with iron. We
found that at the range -1.6 < [Fe/H] < -0.4 [Ca/Fe] decreases with distance,
in agreement with earlier results based on local stars. However, the opposite
trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the
halo are more metal-poor than the inner regions, based on in situ observations
of distant stars, agrees with recent results based on inferences from the
kinematics of more local stars, and with predictions of recent galaxy formation
simulations for galaxies similar to the Milky Way.
|
1503.04362v1
|
2015-05-29
|
NuSTAR and Suzaku observations of the hard state in Cygnus X-1: locating the inner accretion disk
|
We present simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR ) and
Suzaku observations of the X-ray binary Cygnus X-1 in the hard state. This is
the first time this state has been observed in Cyg X-1 with NuSTAR, which
enables us to study the reflection and broad-band spectra in unprecedented
detail. We confirm that the iron line cannot be fit with a combination of
narrow lines and absorption features, and instead requires a relativistically
blurred profile in combination with a narrow line and absorption from the
companion wind. We use the reflection models of Garcia et al. (2014) to
simultaneously measure the black hole spin, disk inner radius, and coronal
height in a self-consistent manner. Detailed fits to the iron line profile
indicate a high level of relativistic blurring, indicative of reflection from
the inner accretion disk. We find a high spin, a small inner disk radius, and a
low source height, and rule out truncation to greater than three gravitational
radii at the 3{\sigma} confidence level. In addition, we find that the line
profile has not changed greatly in the switch from soft to hard states, and
that the differences are consistent with changes in the underlying reflection
spectrum rather than the relativistic blurring. We find that the blurring
parameters are consistent when fitting either just the iron line or the entire
broad-band spectrum, which is well modelled with a Comptonized continuum plus
reflection model.
|
1506.00007v1
|
2015-06-03
|
The complex accretion geometry of GX 339-4 as seen by NuSTAR and Swift
|
We present spectral analysis of five NuSTAR and Swift observations of GX
339-4 taken during a failed outburst in summer 2013. These observations cover
Eddington luminosity fractions in the range ~0.9-6%. Throughout this outburst,
GX 339-4 stayed in the hard state, and all five observations show similar X-ray
spectra with a hard power-law with a photon index near 1.6 and significant
contribution from reflection. Using simple reflection models we find
unrealistically high iron abundances. Allowing for different photon indices for
the continuum incident on the reflector relative to the underlying observed
continuum results in a statistically better fit and reduced iron abundances.
With a photon index around 1.3, the input power-law on the reflector is
significantly harder than that which is directly observed. We study the
influence of different emissivity profiles and geometries and consistently find
an improvement when using separate photon indices. The inferred inner accretion
disk radius is strongly model dependent, but we do not find evidence for a
truncation radius larger than 100 r_g in any model. The data do not allow
independent spin constraints but the results are consistent with the literature
(i.e., a>0). Our best-fit models indicate an inclination angle in the range
40-60 degrees, consistent with limits on the orbital inclination but higher
than reported in the literature using standard reflection models. The iron line
around 6.4 keV is clearly broadened, and we detect a superimposed narrow core
as well. This core originates from a fluorescence region outside the influence
of the strong gravity of the black hole and we discuss possible geometries.
|
1506.01381v1
|
2015-06-05
|
NuSTAR and Swift observations of the black hole candidate XTE J1908+094 during its 2013 outburst
|
The black hole candidate XTE J1908+094 went into outburst for the first time
since 2003 in October 2013. We report on an observation with the Nuclear
Spectroscopic Telescope Array (NuSTAR) and monitoring observations with Swift
during the outburst. NuSTAR caught the source in the soft state: the spectra
show a broad relativistic iron line, and the light curves reveal a ~40 ks flare
with the count rate peaking about 40% above the non-flare level and with
significant spectral variation. A model combining a multi-temperature thermal
component, a power-law, and a reflection component with an iron line provides a
good description of the NuSTAR spectrum. Although relativistic broadening of
the iron line is observed, it is not possible to constrain the black hole spin
with these data. The variability of the power-law component, which can also be
modeled as a Comptonization component, is responsible for the flux and spectral
change during the flare, suggesting that changes in the corona (or possibly
continued jet activity) are the likely cause of the flare.
|
1506.02001v2
|
2015-07-28
|
Probing M subdwarf metallicity with an esdK5+esdM5.5 binary
|
We present a spectral analysis of the binary G 224-58 AB that consists of the
coolest M extreme subdwarf (esdM5.5) and a brighter primary (esdK5). This
binary may serve as a benchmark for metallicity measurement calibrations and as
a test-bed for atmospheric and evolutionary models for esdM objects.
We determine abundances primarily using high resolution optical spectra of
the primary. Other parameters were determined from the fits of synthetic
spectra computed with these abundances to the observed spectra from 0.4 to 2.5
microns for both components.
We determine \Tef =4625 $\pm$ 100 K, \logg = 4.5 $\pm$ 0.5 for the A
component and \Tef = 3200 $\pm$ 100 K, \logg = 5.0 $\pm$ 0.5, for the B
component. We obtained abundances of [Mg/H]=$-$1.51$\pm$0.08,
[Ca/H]=$-$1.39$\pm$0.03, [Ti/H]=$-$1.37$\pm$0.03 for alpha group elements and
[CrH]=$-$1.88$\pm$0.07, [Mn/H]=$-$1.96$\pm$0.06, [Fe/H]=$-$1.92$\pm$0.02,
[Ni/H]=$-$1.81$\pm$0.05 and [Ba/H]W=$-$1.87$\pm$0.11 for iron group elements
from fits to the spectral lines observed in the optical and infrared spectral
regions of the primary. We find consistent abundances with fits to the
secondary albeit at lower signal-to-noise.
Abundances of elements in \ga and \gb atmospheres cannot be described by one
metallicity parameter. The offset of $\sim$ 0.4 dex between the abundances
derived from alpha element and iron group elements corresponds with our
expectation for metal-deficient stars. We thus clarify that some indices used
to date to measure metallicities for establishing esdM stars based on CaH, MgH
and TiO band system strength ratios in the optical and H$_2$O in the infrared
relate to abundances of alpha-element group rather than to iron peak elements.
For metal deficient M dwarfs with [Fe/H] < -1.0, this provides a ready
explanation for apparently inconsistent "metallicities" derived using different
methods.
|
1507.07740v1
|
2015-10-16
|
The Transient Accereting X-Ray Pulsar XTE J1946+274: Stability of the X-Ray Properties at Low Flux and Updated Orbital Solution
|
We present a timing and spectral analysis of the X-ray pulsar XTE J1946+274
observed with Suzaku during an outburst decline in 2010 October and compare
with previous results. XTE J1946+274 is a transient X-ray binary consisting of
a Be-type star and a neutron star with a 15.75 s pulse period in a 172 d orbit
with 2-3 outbursts per orbit during phases of activity. We improve the orbital
solution using data from multiple instruments. The X-ray spectrum can be
described by an absorbed Fermi-Dirac cutoff power law model along with a narrow
Fe K line at 6.4 keV and a weak Cyclotron Resonance Scattering Feature (CRSF)
at ~35 keV. The Suzaku data are consistent with the previously observed
continuum flux versus iron line flux correlation expected from fluorescence
emission along the line of sight. However, the observed iron line flux is
slightly higher, indicating the possibility of a higher iron abundance or the
presence of non-uniform material. We argue that the source most likely has only
been observed in the subcritical (non-radiation dominated) state since its
pulse profile is stable over all observed luminosities and the energy of the
CRSF is approximately the same at the highest (~$5 \times 10^{37} $erg
s$^{-1}$) and lowest (~$5 \times 10^{36} $erg s$^{-1}$) observed 3-60 keV
luminosities.
|
1510.05032v1
|
2015-11-02
|
Electron correlation effects and scattering rates in Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductor
|
Using angle-resolved photoemission spectroscopy we have studied the
low-energy electronic structure and the Fermi surface topology of
Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductors. Similar to the known iron pnictides
we observe hole pockets at the center and electron pockets at the corner of the
Brillouin zone (BZ). However, on a finer level, the electronic structure around
the $\Gamma$- and $Z$-points in $k$-space is substantially different from other
iron pnictides, in that we observe two hole pockets at the $\Gamma$-point, and
more interestingly only one hole pocket is seen at the $Z$-point, whereas in
$1111$-, $111$-, and $122$-type compounds, three hole pockets could be readily
found at the zone center. Another major difference noted in the
Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductors is that the top of innermost
hole-like band moves away from the Fermi level to higher binding energy on
going from $\Gamma$ to $Z$, quite opposite to the iron pnictides. The
polarization dependence of the observed features was used to aid the
attribution of the orbital character of the observed bands. Photon energy
dependent measurements suggest a weak $k_z$ dispersion for the outer hole
pocket and a moderate $k_z$ dispersion for the inner hole pocket. By evaluating
the momentum and energy dependent spectral widths, the single-particle
self-energy was extracted and interestingly this shows a pronounced non-Fermi
liquid behaviour for these compounds. The experimental observations are
discussed in context of electronic band structure calculations and models for
the self-energy such as the spin-fermion model and the marginal-Fermi liquid.
|
1511.00424v1
|
2015-12-03
|
Strong anisotropy in nearly ideal-tetrahedral superconducting FeS single crystals
|
We report the novel preparation of single crystals of tetragonal iron
sulfide, FeS, which exhibits a nearly ideal tetrahedral geometry with S--Fe--S
bond angles of 110.2(2) $^\circ$ and 108.1(2) $^\circ$. Grown via hydrothermal
de-intercalation of K${_x}$Fe${_{2-y}}$S${_2}$ crystals under basic and
reducing conditions, the silver, plate-like crystals of FeS remain stable up to
200 $^\circ$C under air and 250 $^\circ$C under inert conditions, even though
the mineral "mackinawite" (FeS) is known to be metastable. FeS single crystals
exhibit a superconducting state below $T_c=4$ K as determined by electrical
resistivity, magnetic susceptibility, and heat capacity measurements,
confirming the presence of a bulk superconducting state. Normal state
measurements yield an electronic specific heat of 5~mJ/mol-K$^2$, and
paramagnetic, metallic behavior with a low residual resistivity of
250~$\mu\Omega\cdot$cm. Magnetoresistance measurements performed as a function
of magnetic field angle tilted toward both transverse and longitudinal
orientations with respect to the applied current reveal remarkable
two-dimensional behavior. This is paralleled in the superconducting state,
which exhibits the largest known upper critical field $H_{c2}$ anisotropy of
all iron-based superconductors, with $H_{c2}^{||ab}(0) /
H_{c2}^{||c}(0)=$(2.75~T)/(0.275~T)=10. Comparisons to theoretical models for
2D and anisotropic-3D superconductors, however, suggest that FeS is the latter
case with a large effective mass anisotropy. We place FeS in context to other
closely related iron-based superconductors and discuss the role of structural
parameters such as anion height on superconductivity.
|
1512.01245v3
|
2016-01-26
|
Low-energy Population III supernovae and the origin of extremely metal-poor stars
|
Some ancient, dim, metal-poor stars may have formed in the ashes of the first
supernovae (SNe). If their chemical abundances can be reconciled with the
elemental yields of specific Population III (Pop III) explosions, they could
reveal the properties of primordial stars. But multidimensional simulations of
such explosions are required to predict their yields because dynamical
instabilities can dredge material up from deep in the ejecta that would
otherwise be predicted to fall back on to the central remnant and be lost in
one-dimensional (1D) models. We have performed two-dimensional (2D) numerical
simulations of two low-energy Pop III SNe, a 12.4 Msun explosion and a 60 Msun
explosion, and find that they produce elemental yields that are a good fit to
those measured in the most iron-poor star discovered to date, SMSS
J031300.36-670839.3 (J031300). Fallback on to the compact remnant in these weak
explosions accounts for the lack of measurable iron in J031300 and its low
iron-group abundances in general. Our 2D explosions produce higher abundances
of heavy elements (atomic number Z > 20) than their 1D counterparts due to
dredge-up by fluid instabilities. Since almost no Ni is ejected by these weak
SNe, their low luminosities will prevent their detection in the near-infrared
with the James Webb Space Telescope and future 30-m telescopes on the ground.
The only evidence that they ever occurred will be in the fossil abundance
record.
|
1601.06896v3
|
2016-02-29
|
Andreev spectroscopy of iron-based superconductors: temperature dependence of the order parameters and scaling of the $Δ_{L,S}$ with $T_C$
|
We studied iron-based superconductors of various families with critical
temperatures covering almost all range $T_C = 9 - 53$ K. In natural arrays of
contacts formed in these materials we observed intrinsic multiple Andreev
reflections effect (IMARE). By using IMARE spectroscopy, we detected the
two-gap superconductivity, determined the value of the large and the small
superconducting gaps, and the corresponding BCS-ratios. The temperature
dependencies of the large and the small gaps $\Delta_{L,S}(T)$ are similar for
various families of the Fe-based superconductors and could be well-fitted in
the framework of the two-band model by Moskalenko and Suhl. We concluded on the
extended s-wave symmetry of the $\Delta_L$ order parameter (20-30 % anisotropy
in k-space) and on the absence of nodes for $\Delta_S$. The BCS-ratio
$2\Delta_L/k_BT_C \approx 5.2$ is nearly constant within the whole range of
$T_C$ (this means that coupling rate is unchanged), reflecting the 20 %
reduction of the $T_C^{local}$ in relation to the eigen $T_C^L$, and the large
gap roughly corresponds to the energy of magnetic resonance $2\Delta_L \approx
E_{res}$. This result requires a special theoretical consideration. Our
estimation of the relative coupling constants and eigen parameters of each
condensate (in a hypothetical case of a zero interband interaction)
$2\Delta_L/k_BT_C^L = 4.2 - 4.8$ and $2\Delta_S/k_BT_C^S = 3.5 - 4.5$ leads to
indirect conclusion that namely a strong electron-phonon interaction in each
condensate described in the framework of the Eliashberg theory plays the key
role in the superconductivity of iron-based oxypnictides. With it, the two
condensates interact weakly with each other. The observed scaling of
$\Delta_{L,S}$ with $T_C$, as was discussed above, is caused mainly by changing
of the density of states $N_{L,S}$ in the bands, whereas Ln-O spacers act as
charge reservoirs.
|
1602.09114v2
|
2016-03-01
|
X-ray-binary spectra in the lamp post model
|
[Abridged] Context. The high-energy radiation from black-hole binaries may be
due to the reprocessing of a lamp located on the black hole axis, emitting
X-rays. The observed spectrum is made of 3 components: the direct spectrum; the
thermal bump; and the reflected spectrum made of the Compton hump and the
iron-line complex.
Aims. We aim at computing accurately the complete reprocessed spectrum
(thermal bump + reflected) of black-hole binaries over the entire X-ray band.
We also determine the strength of the direct component. Our choice of
parameters is adapted to a source showing an important thermal component.
Methods. We compute in full GR the illumination of a thin disk by a lamp
along the rotation axis. We use the ATM21 radiative transfer code to compute
the spectrum emitted along the disk. We ray trace this local spectrum to
determine the reprocessed spectrum as observed at infinity. We discuss the
dependence of the local and ray-traced spectra on the emission angle and spin.
Results. We show the importance of the angle dependence of the total disk
specific intensity spectrum emitted by the illuminated atmosphere when the
thermal disk emission if fully taken into account. High spin implies high
temperature in the inner regions, so the emitted thermal disk spectrum covers
the iron-line complex. As a result we locally observe absorption lines produced
in the hot disk atmosphere. Absorption lines are narrow and disappear after ray
tracing the local spectrum.
Conclusions. Our results mainly highlight the importance of considering the
angle dependence of the local spectrum when computing reprocessed spectra, as
was already found in a recent study. The main new result of our work is to show
the importance of computing the thermal bump of the spectrum, as this feature
can change considerably the observed iron-line complex.
|
1603.00185v2
|
2016-09-15
|
Self-consistent mapping: Effect of local environment on formation of magnetic moment in alpha-FeSi2
|
We suggest here the method of the mapping of DFT calculations on the
multiorbital model in a following way: the parameters of the formulated
multiorbital model should be determined from the requirement that the
self-consistent charge and spin densities found from the ab initio and model
calculations have to be as close to each other as possible. The analysis of the
model allows for detailed understanding of the role played by different
parameters of the model in the physics of interest. As an example of the
approach we present the study of the effect of silicon atoms substitution by
the iron atoms and vice versa on the magnetic properties in the iron silicide
alpha-FeSi2. The DFT+GGA calculations are mapped to the model with intraatomic
Coulomb and exchange interactions, hoppings to nearest and next nearest atoms
and exchange of the delocalized electrons between iron atoms; the magnetic
moments on atoms and charge densities of the material are found
self-consistently within the Hartree-Fock approximation. We find that while the
stoichiometric alpha-FeSi2 is nonmagnetic, the substitutions generate different
magnetic structures. For example, the substitution of three Si atoms by the Fe
atoms results in the ferrimagnetic structure whereas the substitution of four
Si atoms by Fe atoms gives rise to either the nonmagnetic or the ferromagnetic
state depending on the type of local enviroment of the substitutional Fe atoms.
Besides, contrary to the commonly accepted statement that the destruction of
the magnetic moment is controlled only by the number of Fe-Si nearest
neighbors, we find that actually it is controlled by the Fe-Fe
next-nearest-neighbors. This finding led us to the counterintuitive conclusion:
an increase of Si concentration in Fe1-xSi2+x ordered alloys may lead to a
ferromagnetism. This conclusion is confirmed by the calculation within
GGA-to-DFT.
|
1609.04624v1
|
2016-09-26
|
Phase Transition of Iron-based Single Crystals at Extreme Strain Rates under Dynamic Loadings
|
Phase transition of iron, as a prototype of martensite phase transition under
dynamic loadings, exhibits huge diverges in its TP among experiments with
different pressure medium and loading rates, even in the same initial samples.
Great achievements are made in understanding strain or stress dependence of the
TP under dynamic loadings. However, present understandings on the strain rate
dependence of the TP are far from clear, even a virgin for extreme high strain
rates. In this work, large scale NEMD simulations are conducted to study the
effects of strain rates on the phase transition of iron-based single crystals.
Our results show that the phase transition is preceded by lattice instabilities
under ramp compressions, but present theory, represented by modified Born
criteria, cannot correctly predict observed onsets of the instability. Through
considering both strain and strain gradient disturbances, new instability
criteria are proposed, which could be generally applied for studying
instabilities under either static or dynamic loadings. For the ramp with a
strain rate smaller than about 1010s-1, the observed onset of instabilities is
indeed equal to the one predicted by the new instability criteria under small
gradient disturbances. The observed onsets deviates from the predicted one at
lager strain rates because of finite strain gradient effect. Interestingly, the
strain rate dependence of the TP also exhibits an obvious change at the same
strain rate, i.e., 1010 s-1. When 1010 s-1, a certain power law is obeyed, but
it is not applicable at larger strain rates. This strain rate effect on the TP
is well interpreted with nucleation time and the finite strain gradient effect.
According to these basic understandings, the roles of strain rates on
nucleation and growth of the phase transition are studied.
|
1609.07829v2
|
2017-01-24
|
Nucleosynthesis in the Innermost Ejecta of Neutrino-Drive Supernova Explosions in Two Dimensions
|
We examine the nucleosynthesis in the innermost, neutrino-processed ejecta (a
few $10^{-3}\,M_\odot$) of self-consistent, two-dimensional explosion models of
core-collapse supernovae for six progenitor stars with different initial
masses. Three models have initial masses near the low-mass end of the supernova
range, $8.8\,M_\odot$ (e8.8; electron-capture supernova), $9.6\,M_\odot$
(z9.6), and $8.1\,M_\odot$ (u8.1), with initial metallicities of 1, 0, and
$10^{-4}$ times the solar metallicity, respectively. The other three are
solar-metallicity models with initial masses of $11.2\,M_\odot$ (s11),
$15\,M_\odot$ (s15), and $27\,M_\odot$ (s27). The low-mass models e8.8, z9.6,
and u8.1 exhibit high production factors (nucleosynthetic abundances relative
to the solar ones) of 100--200 for light trans-iron elements from Zn to Zr.
This is associated with appreciable ejection of neutron-rich matter in these
models. Remarkably, the nucleosynthetic outcomes for progenitors e8.8 and z9.6
are almost identical, including interesting productions of $^{48}$Ca and
$^{60}$Fe, irrespective of their quite different (O-Ne-Mg and Fe) cores prior
to collapse. In the more massive models s11, s15, and s27, several proton-rich
isotopes of light trans-iron elements, including the $p$-isotope $^{92}$Mo (for
s27) are made, up to production factors of $\sim$30. Both electron-capture and
core-collapse supernovae near the low-mass end can therefore be dominant
contributors to the Galactic inventory of light trans-iron elements from Zn to
Zr and probably $^{48}$Ca and live $^{60}$Fe. The innermost ejecta of more
massive supernovae may have only sub-dominant contributions to the chemical
enrichment of the Galaxy except for $^{92}$Mo.
|
1701.06786v2
|
2017-04-04
|
Competing spin density wave, collinear, and helical magnetism in Fe1+xTe
|
The Fe1+xTe phase diagram consists of two distinct magnetic structures with
collinear order present at low interstitial iron concentrations and a helical
phase at large values of x with these phases separated by a Lifshitz point. We
use unpolarized single crystal diffraction to confirm the helical phase for
large interstitial iron concentrations and polarized single crystal diffraction
to demonstrate the collinear order for the iron deficient side of the Fe1+xTe
phase diagram. Polarized neutron inelastic scattering show that the
fluctuations associated with this collinear order are predominately transverse
at low energy transfers, consistent with a localized magnetic moment picture.
We then apply neutron inelastic scattering and polarization analysis to
investigate the dynamics and structure near the boundary between collinear and
helical order in the Fe1+xTe phase diagram. We first show that the phase
separating collinear and helical order is characterized by a spin-density wave
with a single propagation wave vector of (~ 0.45, 0, 0.5). We do not observe
harmonics or the presence of a charge density wave. The magnetic fluctuations
associated with this wavevector are different from the collinear phase being
strongly longitudinal in nature and correlated anisotropically in the (H,K)
plane. The excitations preserve the C4 symmetry of the lattice, but display
different widths in momentum along the two tetragonal directions at low energy
transfers. While the low energy excitations and minimal magnetic phase diagram
can be understood in terms of localized interactions, we suggest that the
presence of density wave phase implies the importance of electronic and orbital
properties.
|
1704.01111v1
|
2017-05-07
|
The peculiar Na-O anticorrelation of the bulge globular cluster NGC 6440
|
Context. Galactic Globular Clusters (GCs) are essential tools to understand
the earliest epoch of the Milky Way, since they are among the oldest objects in
the Universe and can be used to trace its formation and evolution. Current
studies using high resolution spectroscopy for many stars in each of a large
sample of GCs allow us to develop a detailed observational picture about their
formation and their relation with the Galaxy. However, it is necessary to
complete this picture by including GCs that belong to all major Galactic
components, including the Bulge. Aims. Our aim is to perform a detailed
chemical analyses of the bulge GC NGC 6440 in order to determine if this object
has Multiple Populations (MPs) and investigate its relation with the Bulge of
the Milky Way and with the other Galactic GCs, especially those associated with
the Bulge, which are largely poorly studied. Methods. We determined the stellar
parameters and the chemical abundances of light elements (Na, Al), iron-peak
elements (Fe, Sc, Mn, Co, Ni), $\alpha$-elements (O, Mg, Si, Ca, Ti) and heavy
elements (Ba, Eu) in seven red giant members of NGC 6440 using high resolution
spectroscopy from FLAMES@UVES. Results. We found a mean iron content of
[Fe/H]=-0.50$\pm$0.03 dex in agreement with other studies. We found no internal
iron spread. On the other hand, Na and Al show a significant intrinsic spread,
but the cluster has no significant O-Na anticorrelation nor exhibits a Mg-Al
anticorrelation. The $\alpha$-elements show good agreement with the Bulge field
star trend, although they are at the high alpha end and are also higher than
those of other GCs of comparable metallicity. The heavy elements are dominated
by the r-process, indicating a strong contribution by SNeII. The chemical
analysis suggests an origin similar to that of the Bulge field stars.
|
1705.02684v1
|
2017-05-10
|
Direct Measurement of Nuclear Dependence of Charged Current Quasielastic-like Neutrino Interactions using MINERvA
|
Charged-current $\nu_{\mu}$ interactions on carbon, iron, and lead with a
final state hadronic system of one or more protons with zero mesons are used to
investigate the influence of the nuclear environment on quasielastic-like
interactions. The transfered four-momentum squared to the target nucleus,
$Q^2$, is reconstructed based on the kinematics of the leading proton, and
differential cross sections versus $Q^2$ and the cross-section ratios of iron,
lead and carbon to scintillator are measured for the first time in a single
experiment. The measurements show a dependence on atomic number. While the
quasielastic-like scattering on carbon is compatible with predictions, the
trends exhibited by scattering on iron and lead favor a prediction with
intranuclear rescattering of hadrons accounted for by a conventional particle
cascade treatment. These measurements help discriminate between different
models of both initial state nucleons and final state interactions used in the
neutrino oscillation experiments.
|
1705.03791v1
|
2017-05-30
|
Chemical Complexity in the Eu-enhanced Monometallic Globular Cluster NGC 5986
|
NGC 5986 is a poorly studied but relatively massive Galactic globular cluster
that shares several physical and morphological characteristics with
"iron-complex" clusters known to exhibit significant metallicity and heavy
element dispersions. In order to determine if NGC 5986 joins the iron-complex
cluster class, we investigated the chemical composition of 25 red giant branch
and asymptotic giant branch cluster stars using high resolution spectra
obtained with the Magellan-M2FS instrument. Cluster membership was verified
using a combination of radial velocity and [Fe/H] measurements, and we found
the cluster to have a mean heliocentric radial velocity of +99.76 km s^-1
(sigma = 7.44 km s^-1). We derived a mean metallicity of [Fe/H] = -1.54 dex
(sigma = 0.08 dex), but the cluster's small dispersion in [Fe/H] and low
[La/Eu] abundance preclude it from being an iron-complex cluster. NGC 5986 has
<[Eu/Fe]> = +0.76 dex (sigma = 0.08 dex), which is among the highest ratios
detected in a Galactic cluster. NGC 5986 exhibits classical globular cluster
characteristics, such as uniformly enhanced [alpha/Fe] ratios, a small
dispersion in Fe-peak abundances, and (anti-)correlated light element
variations. Similar to NGC 2808, we find evidence that NGC 5986 may host at
least 4-5 populations with distinct light element compositions, and the
presence of a clear Mg-Al anti-correlation along with an Al-Si correlation
suggests that the cluster gas experienced processing at temperatures >65-70 MK.
However, the current data do not support burning temperatures exceeding ~100
MK. We find some evidence that the first and second generation stars in NGC
5986 may be fully spatially mixed, which could indicate that the cluster has
lost a significant fraction of its original mass. [abridged]
|
1705.10840v1
|
2017-07-11
|
First results from the IllustrisTNG simulations: A tale of two elements -- chemical evolution of magnesium and europium
|
The distribution of elements in galaxies provides a wealth of information
about their production sites and their subsequent mixing into the interstellar
medium. Here we investigate the distribution of elements within stars in the
IllustrisTNG simulations. In particular, we analyze the abundance ratios of
magnesium and europium in Milky Way-like galaxies from the TNG100 simulation
(stellar masses ${\log} (M_\star / {\rm M}_\odot) \sim 9.7 - 11.2$). As
abundances of magnesium and europium for individual stars in the Milky Way are
observed across a variety of spatial locations and metallicities, comparison
with the stellar abundances in our more than $850$ Milky Way-like galaxies
provides stringent constraints on our chemical evolutionary methods. To this
end we use the magnesium to iron ratio as a proxy for the effects of our SNII
and SNIa metal return prescription, and a means to compare our simulated
abundances to a wide variety of galactic observations. The europium to iron
ratio tracks the rare ejecta from neutron star -- neutron star mergers, the
assumed primary site of europium production in our models, which in turn is a
sensitive probe of the effects of metal diffusion within the gas in our
simulations. We find that europium abundances in Milky Way-like galaxies show
no correlation with assembly history, present day galactic properties, and
average galactic stellar population age. In general, we reproduce the europium
to iron spread at low metallicities observed in the Milky Way, with the level
of enhancement being sensitive to gas properties during redshifts $z \approx
2-4$. We show that while the overall normalization of [Eu/Fe] is susceptible to
resolution and post-processing assumptions, the relatively large spread of
[Eu/Fe] at low [Fe/H] when compared to that at high [Fe/H] is very robust.
|
1707.03401v2
|
2017-08-20
|
Large anomalous Nernst and spin Nernst effects in noncollinear antiferromagnets Mn$_3X$ ($X$ = Sn, Ge, Ga)
|
Noncollinear antiferromagnets have recently been attracting considerable
interest partly due to recent surprising discoveries of the anomalous Hall
effect (AHE) in them and partly because they have promising applications in
antiferromagnetic spintronics. Here we study the anomalous Nernst effect (ANE),
a phenomenon having the same origin as the AHE, and also the spin Nernst effect
(SNE) as well as AHE and the spin Hall effect (SHE) in noncollinear
antiferromagnetic Mn$_3X$ ($X$ = Sn, Ge, Ga) within the Berry phase formalism
based on {\it ab initio} relativistic band structure calculations. For
comparison, we also calculate the anomalous Nernst conductivity (ANC) and
anomalous Hall conductivity (AHC) of ferromagnetic iron as well as the spin
Nernst conductivity (SNC) of platinum metal. Remarkably, the calculated ANC at
room temperature (300 K) for all three alloys is huge, being up to 5 times
larger than that of iron. Moreover, the calculated SNC for Mn$_3$Sn and
Mn$_3$Ga is also large, being as large as that of platinum. This suggests that
these anitferromagnets would be useful materials for thermoelectronic devices
and spin caloritronic devices. The calculated ANC of Mn$_3$Sn and iron are in
reasonably good agreement with the very recent experiments. The calculated SNC
of platinum also agrees with the very recent experiments in both sign and
magnitude. The calculated thermoelectric and thermomagnetic properties are
analyzed in terms of the band structures as well as the energy-dependent AHC,
ANC, SNC and spin Hall conductivity via the Mott relations.
|
1708.05933v3
|
2017-09-15
|
Investigating the interstellar dust through the Fe K-edge
|
The chemical and physical properties of interstellar dust in the densest
regions of the Galaxy are still not well understood. X-rays provide a powerful
probe since they can penetrate gas and dust over a wide range of column
densities (up to $10^{24}\ \rm{cm}^{-2}$). The interaction (scattering and
absorption) with the medium imprints spectral signatures that reflect the
individual atoms which constitute the gas, molecule, or solid. In this work we
investigate the ability of high resolution X-ray spectroscopy to probe the
properties of cosmic grains containing iron. Although iron is heavily depleted
into interstellar dust, the nature of the Fe-bearing grains is still largely
uncertain. In our analysis we use iron K-edge synchrotron data of minerals
likely present in the ISM dust taken at the European Synchrotron Radiation
Facility. We explore the prospects of determining the chemical composition and
the size of astrophysical dust in the Galactic centre and in molecular clouds
with future X-ray missions. The energy resolution and the effective area of the
present X-ray telescopes are not sufficient to detect and study the Fe K-edge,
even for bright X-ray sources. From the analysis of the extinction cross
sections of our dust models implemented in the spectral fitting program SPEX,
the Fe K-edge is promising for investigating both the chemistry and the size
distribution of the interstellar dust. We find that the chemical composition
regulates the X-ray absorption fine structures in the post edge region, whereas
the scattering feature in the pre-edge is sensitive to the mean grain size.
Finally, we note that the Fe K-edge is insensitive to other dust properties,
such as the porosity and the geometry of the dust.
|
1709.05359v1
|
2017-10-11
|
Fermi surface with Dirac fermions in CaFeAsF determined via quantum oscillation measurements
|
Despite the fact that 1111-type iron arsenides hold the record transition
temperature of iron-based superconductors, their electronic structures have not
been studied much because of the lack of high-quality single crystals. In this
study, we completely determine the Fermi surface in the antiferromagnetic state
of CaFeAsF, a 1111 iron-arsenide parent compound, by performing quantum
oscillation measurements and band-structure calculations. The determined Fermi
surface consists of a symmetry-related pair of Dirac electron cylinders and a
normal hole cylinder. From analyses of quantum-oscillation phases, we
demonstrate that the electron cylinders carry a nontrivial Berry phase $\pi$.
The carrier density is of the order of 10$^{-3}$ per Fe. This unusual metallic
state with the extremely small carrier density is a consequence of the
previously discussed topological feature of the band structure which prevents
the antiferromagnetic gap from being a full gap. We also report a nearly
linear-in-$B$ magnetoresistance and an anomalous resistivity increase above
about 30 T for $B \parallel c$, the latter of which is likely related to the
quantum limit of the electron orbit. Intriguingly, the electrical resistivity
exhibits a nonmetallic temperature dependence in the paramagnetic tetragonal
phase ($T >$ 118 K), which may suggest an incoherent state. Our study provides
a detailed knowledge of the Fermi surface in the antiferromagnetic state of
1111 parent compounds and moreover opens up a new possibility to explore
Dirac-fermion physics in those compounds.
|
1710.03938v3
|
2017-10-26
|
X-ray reflection from cold white dwarfs in magnetic cataclysmic variables
|
We model X-ray reflection from white dwarfs (WD) in magnetic cataclysmic
variables (mCVs) with a Monte Carlo simulation. A point source with a power-law
spectrum or a realistic post-shock accretion column (PSAC) source irradiates a
cool and spherical WD. The PSAC source emits thermal spectra of various
temperatures stratified along the column according to the PSAC model. In the
point source simulation, we confirm (1) a source harder and nearer to the WD
enhances the reflection, (2) higher iron abundance enhances the equivalent
widths (EWs) of fluorescent iron K${\alpha}_{1,2}$ lines and their Compton
shoulder, and increases cut-off energy of a Compton hump, and (3) significant
reflection appears from an area that is more than 90 deg apart from the
position right under the point X-ray source because of the WD curvature. The
PSAC simulation reveals that (1) a more massive WD basically enhances the
intensities of the fluorescent iron K${\alpha}_{1,2}$ lines and the Compton
hump, except for some specific accretion rate, because the more massive WD
makes the hotter PSAC from which higher energy X-rays are preferentially
emitted, (2) a larger specific accretion rate monotonically enhances the
reflection because it makes the hotter and shorter PSAC, and (3) the intrinsic
thermal component hardens by occultation of the cool base of the PSAC by the
WD. We quantitatively evaluate influences of the parameters on the EWs and the
Compton hump with both types of sources. We also calculate X-ray modulation
profiles brought about by the WD spin. They depend on angles of the spin axis
from the line of sight and from the PSAC, and whether the two PSACs can be
seen. The reflection spectral model and the modulation model involve the
fluorescent lines and the Compton hump and can directly be compared to the data
which allow us to evaluate these geometrical parameters with unprecedented
accuracy.
|
1710.09931v1
|
2017-12-19
|
Magnetic phase diagram of the iron pnictides in the presence of spin-orbit coupling: Frustration between $C_2$ and $C_4$ magnetic phases
|
We investigate the impact of spin anisotropic interactions, promoted by
spin-orbit coupling, on the magnetic phase diagram of the iron-based
superconductors. Three distinct magnetic phases with Bragg peaks at $(\pi,0)$
and $(0,\pi)$ are possible in these systems: one $C_2$ (i.e. orthorhombic)
symmetric stripe magnetic phase and two $C_4$ (i.e. tetragonal) symmetric
magnetic phases. While the spin anisotropic interactions allow the magnetic
moments to point in any direction in the $C_2$ phase, they restrict the
possible moment orientations in the $C_4$ phases. As a result, an interesting
scenario arises in which the spin anisotropic interactions favor a $C_2$ phase,
but the other spin isotropic interactions favor a $C_4$ phase. We study this
frustration via both mean-field and renormalization-group approaches. We find
that, to lift this frustration, a rich magnetic landscape emerges well below
the magnetic transition temperature, with novel $C_2$, $C_4$, and mixed
$C_2$-$C_4$ phases. Near the putative magnetic quantum critical point, spin
anisotropies promote a stable Gaussian fixed point in the renormalization-group
flow, which is absent in the spin isotropic case, and is associated with a
near-degeneracy between $C_2$ and $C_4$ phases. We argue that this frustration
is the reason why most $C_4$ phases in the iron pnictides only appear inside
the $C_2$ phase, and discuss additional manifestations of this frustration in
the phase diagrams of these materials.
|
1712.07192v4
|
2018-01-03
|
Sequential structural and antiferromagnetic transitions in BaFe$_2$Se$_3$ under pressure
|
The discovery of superconductivity in the two-leg ladder compound
BaFe$_2$S$_3$ has established the 123-type iron chalcogenides as a novel and
interesting subgroup of the iron-based superconductors family. However, in this
123 series, BaFe$_2$Se$_3$ is an exceptional member, with a magnetic order and
crystalline structure different from all others. Recently, an exciting
experiment reported the emergence of superconductivity in BaFe$_2$Se$_3$ at
high pressure [J.-J. Ying, et al., Phys. Rev. B 95, 241109 (R) (2017)]. In this
publication, we report a first principles study of BaFe$_2$Se$_3$. Our analysis
unveils a variety of qualitative differences between BaFe$_2$S$_3$ and
BaFe$_2$Se$_3$, including in the latter an unexpected chain of transitions with
increasing pressure. First, by gradually reducing the tilting angle of iron
ladders, the crystalline structure smoothly transforms from Pnma to Cmcm at ~6
GPa. Second, the system becomes metallic at 10.4 GPa. Third, its unique ambient
pressure Block antiferromagnetic ground state is replaced by the more common CX
antiferromagnetic order at ~12 GPa, the same magnetic state of the 123-S
ladder. This transition is found at a pressure very similar to the experimental
superconducting transition. Finally, all magnetic moments vanish at 30 GPa.
This reported theoretical diagram of the complete phase evolution is important
because of the technical challenges to capture many physical properties in
high-pressure experiments. The information obtained in our calculations suggest
different characteristics for superconductivity in BaFe$_2$Se$_3$ and
BaFe$_2$S$_3$: in 123-S pairing occurs when magnetic moments vanish, while in
123-Se the transition region from Block- to CX-type magnetism appears to
catalyze superconductivity. Finally, an additional superconducting dome above
~30 GPa is expected to occur.
|
1801.00877v1
|
2018-02-12
|
Superconducting transition temperatures in the electronic and magnetic phase diagrams of Sr2VFeAsO3-delta, a superconductor
|
We elucidate the magnetic phases and superconducting transition temperatures
(Tc) in Sr2VFeAsO3-delta (21113V), an iron-based superconductor with a
thick-blocking layer fabricated from a perovskite-related transition metal
oxide. At low temperatures (T < 37.1 K), 21113V exhibited a superconducting
phase in the range 0.031 =< delta =< 0.145 and an antiferromagnetic (AFM) iron
sublattice in the range 0.267 =< delta =< 0.664. Mixed-valent vanadium
exhibited a dominant AFM phase in 0.031 =< delta =< 0.088, and a partial
ferrimagnetic (Ferri.) phase in the range 0.124 =< delta =< 0.664. The Ferri.
phase was the most dominant at a delta value of 0.267, showing an AFM phase of
Fe at T < 20 K. Increasing the spontaneous magnetic moments reduced the
magnetic shielding volume fraction due to the superconducting phase. This
result was attributed to the magnetic phase of vanadium, which dominates the
superconductivity of Fe in 21113V. The Tc-delta curve showed two maxima. The
smaller and larger of Tc maxima occurred at delta = 0.073 and delta = 0.145,
respectively; the latter resides on the phase boundary between AFM and the
partial Ferri. phases of vanadium. 21113V is a useful platform for verifing new
mechanisms of Tc enhancement in iron-based superconductors.
|
1802.03907v2
|
2018-07-20
|
Measurement of the Iron Spectrum in Cosmic Rays by VERITAS
|
We present a new measurement of the energy spectrum of iron nuclei in cosmic
rays from 20 to 500 TeV. The measurement makes use of a template-based analysis
method, which, for the first time, is applied to the energy reconstruction of
iron-induced air showers recorded by the VERITAS array of imaging atmospheric
Cherenkov telescopes. The event selection makes use of the direct Cherenkov
light which is emitted by charged particles before the first interaction, as
well as other parameters related to the shape of the recorded air shower
images. The measured spectrum is well described by a power law
$\frac{\mathrm{d} F}{\mathrm{d} E}=f_0\cdot
\left(\frac{E}{E_0}\right)^{-\gamma}$ over the full energy range, with $\gamma
= 2.82 \pm 0.30 \mathrm{(stat.)} ^{+0.24}_{-0.27} \mathrm{(syst.)}$ and $f_0 =
\left( 4.82 \pm 0.98 \mathrm{(stat.)}^{+2.12}_{-2.70} \mathrm{(syst.)}
\right)\cdot 10^{-7}$m$^{-2}$s$^{-1}$sr$^{-1}$TeV$^{-1}$ at $E_0=50$TeV, with
no indication of a cutoff or spectral break. The measured differential flux is
compatible with previous results, with improved statistical uncertainty at the
highest energies.
|
1807.08010v1
|
2018-07-23
|
The Quantum Field Of A Magnet Shown By A Nanomagnetic Ferrolens
|
It has been more than two hundred years since the first iron filings
experiment, showing us the 2D macroscopic magnetic imprint of the field of a
permanent magnet. However, latest developments in modern nanomagnetic passive
direct observation devices reveal in real-time and color a more intriguing 3D
dynamic and detailed image of the field of a magnet, with surprising new
findings, that can change our perspective for dipole magnetism forever and lead
to new research. This research is a continuation of our previous work (DOI:
10.1016/j.jmmm.2017.12.023). The magnetostatic fields were under our scope and
examined with the aid of the ferrolens. We are presenting experimental and
photographical evidence, demonstrating the true complex 3D Euclidian geometry
of the quantum field of permanent magnets that have never been seen before and
the classic iron filings experiment, apart of its 2D limitations, fails to
depict. An analysis of why and what these iron filings inherent limitations
are, giving us an incomplete and also in some degree misguiding image of the
magnetic field of a magnet is carried out, whereas, as we prove the ferrolens
is free of these limitations and its far more advanced visualization
capabilities is allowing it to show the quantum image with depth of field
information, of the dipole field of a permanent magnet. For the first time the
domain wall (i.e. Bloch or Neel wall) region of the field of a magnet is
clearly made visible by the ferrolens along with what phenomenon is actually
taking place there, leading to the inescapable conclusion, novel observation
and experimental evidence that the field of any dipole magnet actually consists
of two distinct and separate toroidal shaped 3D magnetic bubbles, each located
at either side of the dipole around the exact spatial regions where the two
poles of the magnet reside.
|
1807.08751v1
|
2018-10-05
|
(16) Psyche: A mesosiderite-like asteroid?
|
Asteroid (16) Psyche is the target of the NASA Psyche mission. It is
considered one of the few main-belt bodies that could be an exposed
proto-planetary metallic core and that would thus be related to iron
meteorites. Such an association is however challenged by both its near- and
mid-infrared spectral properties and the reported estimates of its density.
Here, we aim to refine the density of (16) Psyche to set further constraints on
its bulk composition and determine its potential meteoritic analog.
We observed (16) Psyche with ESO VLT/SPHERE/ZIMPOL as part of our large
program (ID 199.C-0074). We used the high angular resolution of these
observations to refine Psyche's three-dimensional (3D) shape model and
subsequently its density when combined with the most recent mass estimates. In
addition, we searched for potential companions around the asteroid. We derived
a bulk density of 3.99\,$\pm$\,0.26\,g$\cdot$cm$^{-3}$ for Psyche. While such
density is incompatible at the 3-sigma level with any iron meteorites
($\sim$7.8\,g$\cdot$cm$^{-3}$), it appears fully consistent with that of
stony-iron meteorites such as mesosiderites (density
$\sim$4.25\,$\cdot$cm$^{-3}$). In addition, we found no satellite in our images
and set an upper limit on the diameter of any non-detected satellite of
1460\,$\pm$\,200}\,m at 150\,km from Psyche (0.2\%\,$\times$\,R$_{Hill}$, the
Hill radius) and 800\,$\pm$\,200\,m at 2,000\,km (3\%\,$\times$\,$R_{Hill}$).
Considering that the visible and near-infrared spectral properties of
mesosiderites are similar to those of Psyche, there is merit to a
long-published initial hypothesis that Psyche could be a plausible candidate
parent body for mesosiderites.
|
1810.02771v1
|
2018-11-26
|
Confirming the presence of second population stars and the iron discrepancy along the AGB of the globular cluster NGC 6752
|
Asymptotic giant branch (AGB) stars in the globular cluster NGC6752 have been
found to exhibit some chemical peculiarities with respect to the red giant
branch (RGB) stars. A discrepancy between [FeI/H] and [FeII/H] (not observed in
RGB stars) has been detected adopting spectroscopic temperatures. Moreover, a
possible lack of second-population stars along the AGB was claimed. The use of
photometric temperatures based on (V-K) colors was proposed to erase this iron
discrepancy. Also, ad hoc scenarios have been proposed to explain the absence
of second-population AGB stars. Here we analyzed a sample of 19 AGB and 14 RGB
stars of NGC6752 observed with the spectrographs UVES. The two temperature
scales agree very well for the RGB stars while for the AGB stars there is a
systematic offset of ~100 K. We found that even if the photometric temperatures
alleviate the iron discrepancy with respect to the spectroscopic ones, a
systematic difference between [FeI/H] and [FeII/H] is still found among the AGB
stars. An unexpected result is that the photometric temperatures do not satisfy
the excitation equilibrium in the AGB stars. This suggests that standard 1D-LTE
model atmospheres are unable to properly describe the thermal structure of AGB
stars, at variance with the RGB stars. The use of photometric temperatures
confirms the previous detection of second-population AGB stars in this cluster,
with the presence of clear correlations/anticorrelations among the light
element abundances. This firmly demonstrates that both first and
second-population stars evolve along the AGB of NGC6752.
|
1811.10626v1
|
2019-03-06
|
Lattice Disorder Effect on Magnetic Ordering of Iron Arsenides
|
This study investigates the changes of magnetic ordering temperature via
nano- and mesoscale structural features in an iron arsenide. Although magnetic
ground states in quantum materials can be theoretically predicted from known
crystal structures and chemical compositions, the ordering temperature is
harder to pinpoint due to such local lattice variations. In this work we find
surprisingly that a locally disordered material can exhibit a significantly
larger Neel temperature (TN) than an ordered material of precisely the same
chemical stoichiometry. Here, a EuFe2As2 crystal, which is a 122 parent of iron
arsenide superconductors, is found through synthesis to have ordering below TN
= 195 K (for the disordered crystal) or TN = 175 K (for the ordered crystal).
In the higher TN crystals, there are shorter planar Fe-Fe bonds [2.7692(2) A
vs. 2.7745(3) A], a randomized in-plane defect structure, and diffuse
scattering along the [00L] crystallographic direction that manifests as a
rather broad specific heat peak. For the lower TN crystals, the a-lattice
parameter is larger and the in-plane microscopic structure shows defect
ordering along the antiphase boundaries, giving a larger TN and a higher
superconducting temperature (Tc) upon the application of pressure. First
principles calculations find a strong interaction between c-axis strain and
interlayer magnetic coupling, but little impact of planar strain on the
magnetic order. Neutron single-crystal diffraction shows that the
low-temperature magnetic phase transition due to localized Eu moments is not
lattice or disorder sensitive, unlike the higher-temperature Fe sublattice
ordering. This study demonstrates a higher magnetic ordering point arising from
local disorder in 122.
|
1903.02545v2
|
2019-05-24
|
How much of the Solar System should we leave as Wilderness?
|
"How much of the Solar System should we reserve as wilderness, off-limits to
human development?" We argue that, as a matter of policy, development should be
limited to one eighth, with the remainder set aside. We argue that adopting a
"1/8 principle" is far less restrictive, overall, than it might seem. One
eighth of the iron in the asteroid belt is more than a million times greater
than all of the Earth's estimated iron reserves and may suffice for centuries.
A limit of some sort is needed because of the problems associated with
exponential growth. Humans are poor at estimating the pace of such growth, so
the limitations of a resource are hard to recognize before the final three
doubling times which take utilization successively from 1/8 to 1/4 to 1/2, and
then to the point of exhaustion. Population growth and climate change are
instances of unchecked exponential growth. Each places strains upon ouru
available resources. Each is a problem we would like to control but attempts to
do so at this comparatively late stage have not been encouraging. Our limited
ability to see ahead suggests that we should set ourselves a 'tripwire' that
gives us at least 3 doubling times as leeway, i.e. when 1/8 of Solar System
resources are close to being exploited. At a 3.5 percent growth rate for the
space economy, comparable to that of the iron use from the beginning of the
Industrial Revolution until now, the 1/8 point would be reached after 400
years. At that point the 20 year doubling time of a 3.5 percent growth rate
means that only 60 years would remain to transition the economic system to new
"steady state" conditions. The rationale for adopting the 1/8 principle now is
that it may be far easier to implement in principle restrictions at an early
stage, rather than later, when vested and competing interests have come into
existence.
|
1905.13681v1
|
2019-07-26
|
Stabilization of $\varepsilon$-Fe$_2$O$_3$ epitaxial layer on MgO(111)/GaN via an intermediate $γ$-Fe$_2$O$_3$-phase
|
In the present study we have demonstrated epitaxial stabilization of the
metastable magnetically-hard $\varepsilon$-Fe$_2$O$_3$ phase on top of a thin
MgO(111) buffer layer grown onto the GaN (0001) surface. The primary purpose to
introduce a 4\,nm-thick buffer layer of MgO in between Fe$_2$O$_3$ and GaN was
to stop thermal migration of Ga into the iron oxide layer. Though such
migration and successive formation of the orthorhombic GaFeO$_3$ was supposed
earlier to be a potential trigger of the nucleation of the isostructural
$\varepsilon$-Fe$_2$O$_3$, the present work demonstrates that the growth of
single crystalline uniform films of epsilon ferrite by pulsed laser deposition
is possible even on the MgO capped GaN. The structural properties of the 60\,nm
thick Fe$_2$O$_3$ layer on MgO / GaN were probed by electron and x-ray
diffraction, both suggesting that the growth of $\varepsilon$-Fe$_2$O$_3$ is
preceded by formation of a thin layer of $\gamma$-Fe$_2$O$_3$. The presence of
the magnetically hard epsilon ferrite was independently confirmed by
temperature dependent magnetometry measurements. The depth-resolved x-ray and
polarized neutron reflectometry reveal that the 10\,nm iron oxide layer at the
interface has a lower density and a higher magnetization than the main volume
of the $\varepsilon$-Fe$_2$O$_3$ film. The density and magnetic moment depth
profiles derived from fitting the reflectometry data are in a good agreement
with the presence of the magnetically degraded $\gamma$-Fe$_2$O$_3$ transition
layer between MgO and $\varepsilon$-Fe$_2$O$_3$. The natural occurrence of the
interface between magnetoelectric $\varepsilon$- and spin caloritronic
$\gamma$- iron oxide phases can enable further opportunities to design novel
all-oxide-on-semiconductor devices.
|
1907.11611v2
|
2019-08-06
|
On the use of field RR Lyrae as Galactic probes: I. The Oosterhoff dichotomy based on fundamental variables
|
We collected a large data set of field RR Lyrae stars (RRLs) by using
catalogues already available in the literature and Gaia DR2. We estimated the
iron abundances for a sub-sample of 2,382 fundamental RRLs ($\Delta$S method:
CaIIK, H$\beta$, H$\gamma$ and H$\delta$ lines) for which are publicly
available medium-resolution SDSS-SEGUE spectra. We also included similar
estimates available in the literature ending up with the largest and most
homogeneous spectroscopic data set ever collected for RRLs (2,903). The
metallicity scale was validated by using iron abundances based on high
resolution spectra for a fundamental field RRL (V~Ind), for which we collected
X-shooter spectra covering the entire pulsation cycle. The peak
([Fe/H]=-1.59$\pm$0.01) and the standard deviation ($\sigma$=0.43 dex) of the
metallicity distribution agree quite well with similar estimates available in
the literature. The current measurements disclose a well defined metal-rich
tail approaching Solar iron abundance. The spectroscopic sample plotted in the
Bailey diagram (period vs luminosity amplitude) shows a steady variation when
moving from the metal-poor ([Fe/H]=-3.0/-2.5) to the metal-rich
([Fe/H]=-0.5/0.0) regime. The smooth transition in the peak of the period
distribution as a function of the metallicity strongly indicates that the
long-standing problem of the Oosterhoff dichotomy among Galactic globulars is
the consequence of the lack of metal-intermediate clusters hosting RRLs. We
also found that the luminosity amplitude, in contrast with period, does not
show a solid correlation with metallicity. This suggests that
period-amplitude-metallicity relations should be cautiously treated.
|
1908.02064v2
|
2019-08-27
|
Carbon, oxygen, and iron abundances in disk and halo stars. Implications of 3D non-LTE spectral line formation
|
The abundances of carbon, oxygen, and iron in late-type stars are important
parameters in exoplanetary and stellar physics, as well as key tracers of
stellar populations and Galactic chemical evolution. We carried out
three-dimensional (3D) non-LTE radiative transfer calculations for CI and OI,
and 3D LTE radiative transfer calculations for FeII, across the STAGGER-grid of
3D hydrodynamic model atmospheres. The absolute 3D non-LTE versus 1D LTE
abundance corrections can be as severe as $-0.3$ dex for CI lines in
low-metallicity F dwarfs, and $-0.6$ dex for OI lines in high-metallicity F
dwarfs. The 3D LTE versus 1D LTE abundance corrections for FeII lines are less
severe, typically less than $+0.15$ dex. We used the corrections in a
re-analysis of carbon, oxygen, and iron in $187$ F and G dwarfs in the Galactic
disk and halo. Applying the differential 3D non-LTE corrections to 1D LTE
abundances visibly reduces the scatter in the abundance plots. The thick disk
and high-$\alpha$ halo population rise in carbon and oxygen with decreasing
metallicity, and reach a maximum of [C/Fe]$\approx0.2$ and a plateau of
[O/Fe]$\approx0.6$ at [Fe/H]$\approx-1.0$. The low-$\alpha$ halo population is
qualitatively similar, albeit offset towards lower metallicities and with
larger scatter. Nevertheless, these populations overlap in the [C/O] versus
[O/H] plane, decreasing to a plateau of [C/O]$\approx-0.6$ below
[O/H]$\approx-1.0$. In the thin-disk, stars having confirmed planet detections
tend to have higher values of C/O at given [O/H]; this potential signature of
planet formation is only apparent after applying the abundance corrections to
the 1D LTE results. Our grids of line-by-line abundance corrections are
publicly available and can be readily used to improve the accuracy of
spectroscopic analyses of late-type stars.
|
1908.10319v2
|
2020-01-03
|
The chemical composition of the accretion disk and donor star in Ultra Compact X-ray Binaries: A comprehensive X-ray analysis
|
We have analyzed the X-ray spectra of all known Ultra Compact X-ray Binaries
(UCXBs), with the purpose of constraining the chemical composition of their
accretion disk and donor star. Our investigation was focused on the presence
(or absence) of the Fe K${\alpha}$ emission line, which was used as the probe
of chemical composition of the disk, based on previously established
theoretical predictions for the reflection of X-ray radiation off the surface
of C/O-rich or He-rich accretion disks in UCXBs. We have contrasted the results
of our spectral analysis to the history of type I X-ray bursts from these
systems, which can also indicate donor star composition. We found that UCXBs
with prominent and persistent iron K${\alpha}$ emission also featured repeat
bursting activity. On the other hand, the UCXBs for which no iron line was
detected, appear to have few or no type I X-ray bursts detected over more than
a decade of monitoring. Based on Monte Carlo simulations, demonstrating a
strong correlation between the Fe K${\alpha}$ line strength and the abundance
of C and O in the accretion disk material and given the expected correlation
between the H/He abundance and the recurrence rate of type I X-ray bursts, we
propose that there is a considerable likelihood that UCXBs with persistent iron
emission have He-rich donors, while those that do not, likely have C/O or
O/Ne/Mg-rich donors. Our results strongly advocate for the development of more
sophisticated simulations of X-ray reflection from hydrogen-poor accretion
disks.
|
2001.00716v2
|
2020-01-20
|
The extreme CNO-enhanced composition of the primitive iron-poor dwarf star J0815+4729
|
We present an analysis of high-resolution Keck/HIRES spectroscopic
observations of J0815+4729, an extremely carbon-enhanced, iron-poor dwarf star.
These high-quality data allow us to derive a metallicity of
[Fe/H]$=-5.49{\pm}0.14$ from the three strongest \ion{Fe}{1} lines and to
measure a high [Ca/Fe]~$=0.75{\pm}0.14$. The large carbon abundance of
A(C)~$=7.43{\pm}0.17$ (or [C/Fe]~$\sim 4.49{\pm}0.11$) places this star in the
upper boundary of the low-carbon band in the A(C)-[Fe/H] diagram, suggesting no
contamination from a binary AGB companion. We detect the oxygen triplet at
777nm for the first time in an ultra-metal poor star, indicating a large
oxygen-to-iron abundance ratio of [O/Fe]~$=4.03{\pm}0.12$
(A(O)~$=7.23{\pm}0.14$), significantly higher than the previously most
metal-poor dwarf J2209-0028 with an oxygen triplet detection with
[O/Fe]~$\sim2.2$~dex at [Fe/H]~$\sim -3.9$. Nitrogen is also dramatically
enhanced with (A(N)~$=6.75{\pm}0.08$) and an abundance ratio [N/Fe]~$\sim
4.41{\pm}0.08$. We also detect Ca, Na and Mg, while provide upper limits for
eight other elements. The abundance pattern of J0815+4729 resembles that of
HE~1327-2326, indicating that both are second-generation stars contaminated by
a $\sim 21-27$~\msun~single, zero-metallicity low-energy supernova with very
little mixing and substantial fallback. The absence of lithium implies an
upper-limit abundance A(Li)~$<1.3$~dex, about 0.7~dex below the detected Li
abundance in J0023+0307 which has a similar metallicity, exacerbating the
cosmological lithium problem.
|
2001.07257v2
|
2020-01-21
|
Observation of magnetic adatom-induced Majorana vortex and its hybridization with field-induced Majorana vortex in an iron-based superconductor
|
Braiding Majorana zero modes is essential for fault-tolerant topological
quantum computing. Iron-based superconductors with nontrivial band topology
have recently emerged as a surprisingly promising platform for creating
distinct Majorana zero modes in magnetic vortices in a single material and at
relatively high temperatures. The magnetic field-induced Abrikosov vortex
lattice makes it difficult to braid a set of Majorana zero modes or to study
the coupling of a Majorana doublet due to overlapping wave functions. Here we
report the observation of the proposed quantum anomalous vortex with integer
quantized vortex core states and the Majorana zero mode induced by magnetic Fe
adatoms deposited on the surface. We observe its hybridization with a nearby
field-induced Majorana vortex in iron-based superconductor FeTe0.55Se0.45. We
also observe vortex-free Yu-Shiba-Rusinov bound states at the Fe adatoms with a
weaker coupling to the substrate, and discover a reversible transition between
Yu-Shiba-Rusinov states and Majorana zero mode by manipulating the exchange
coupling strength. The dual origin of the Majorana zero modes, from magnetic
adatoms and external magnetic field, provides a new single-material platform
for studying their interactions and braiding in superconductors bearing
topological band structures.
|
2001.07376v2
|
2020-06-05
|
Chandra View of the LINER-type Nucleus in the Radio-Loud Galaxy CGCG 292-057: Ionized Iron Line and Jet-ISM Interactions
|
We present an analysis of the new, deep (94\,ksec) {\it Chandra} ACIS-S
observation of radio-loud active galaxy CGCG\,292$-$057, characterized by a
LINER-type nucleus and a complex radio structure that indicates intermittent
jet activity. On the scale of the host galaxy bulge, we detected excess X-ray
emission with a spectrum best fit by a thermal plasma model with a temperature
of $\sim 0.8$\,keV. We argue that this excess emission results from compression
and heating of the hot diffuse fraction of the interstellar medium displaced by
the expanding inner, $\sim 20$\, kpc-scale lobes observed in this restarted
radio galaxy. The nuclear X-ray spectrum of the target clearly displays an
ionized iron line at $\sim 6.7$\,keV, and is best fitted with a
phenomenological model consisting of a power-law (photon index $\simeq 1.8$)
continuum absorbed by a relatively large amount of cold matter (hydrogen column
density $\simeq 0.7 \times 10^{23}$\,cm$^{-2}$), and partly scattered (fraction
$\sim 3\%$) by ionized gas, giving rise to a soft excess component and
K$\alpha$ line from iron ions. We demonstrate that the observed X-ray spectrum,
particularly the equivalent width of Fe\,\texttt{XXV} K$\alpha$ (of order
$0.3$\,keV) can in principle, be explained in a scenario involving a
Compton-thin gas located at the scale of the broad-lined region in this source
and photoionized by nuclear illumination. We compare the general spectral
properties of the CGCG\,292$-$057 nucleus, with those of other nearby LINERs
studied in X-rays.
|
2006.03717v2
|
2020-06-29
|
No Redshift Evolution in the Broad Line Region Metallicity up to z=7.54: Deep NIR Spectroscopy of ULAS J1342+0928
|
We present deep (9 hours) Gemini-N/GNIRS near-infrared spectroscopic
observations of ULAS J1342+0928, a luminous quasar at z=7.54. Various broad
emission lines were detected, as well as the underlying continuum and iron
forests over the rest-frame wavelength 970-2930A. There is a clear trend that
higher-ionization emission lines show larger blueshifts with CIV1549 exhibiting
5510^{+240}_{-110} km s-1 blueshift with respect to the systematic redshift
from the far-infrared [CII] 158um emission line. Those high ionization lines
have wide profiles with FWHM more than 10000 km s-1. A modest blueshift of
340^{+110}_{-80} km s-1 is also seen in MgII, the lowest ionization line
identified in the spectrum. The updated MgII-based black hole mass of
M_BH=9.1_{-1.3}^{+1.4} x 10^8 M_sun and the Eddington ratio of
L_bol/L_Edd=1.1_{-0.2}^{+0.2} confirm that ULAS J1342+0928 is powered by a
massive and actively accreting black hole. There is no significant difference
in the emission line ratios such as SiIV/CIV and AlIII/CIV when compared to
lower-redshift quasars in a similar luminosity range, which suggests early
metal pollution of the broad-line-region clouds. This trend also holds for the
FeII/MgII line ratio, known as a cosmic clock that traces the iron enrichment
in the early universe. Different iron templates and continuum fitting ranges
were used to explore how the FeII/MgII measurement changes as a function of
spectral modeling. Quasars at even higher redshift or at fainter luminosity
range (L_bol<10^46 erg s-1) are needed to probe the sites of early metal
enrichment and a corresponding change in the FeII/MgII ratio.
|
2006.16268v1
|
2020-08-05
|
Orbital-selective spin waves in detwinned NaFeAs
|
The existence of orbital-dependent electronic correlations has been
recognized as an essential ingredient to describe the physics of iron-based
superconductors. NaFeAs, a parent compound of iron based superconductors,
exhibits a tetragonal-to-orthorhombic lattice distortion below $T_s\approx 60$
K, forming an electronic nematic phase with two 90$^\circ$ rotated (twinned)
domains, and orders antiferromagnetically below $T_N\approx 42$ K. We use
inelastic neutron scattering to study spin waves in uniaxial pressure-detwinned
NaFeAs. By comparing the data with combined density functional theory and
dynamical mean-field theory calculations, we conclude that spin waves up to an
energy scale of $E_\text{crossover} \approx 100$ meV are dominated by
$d_{yz}$-$d_{yz}$ intra-orbital scattering processes, which have the two-fold
($C_2$) rotational symmetry of the underlying lattice. On the other hand, the
spin wave excitations above $E_\text{crossover}$, which have approximately
fourfold ($C_4$) rotational symmetry, arise from the $d_{xy}$-$d_{xy}$
intra-orbital scattering that controls the overall magnetic bandwidth in this
material. In addition, we find that the low-energy ($E\approx 6$ meV) spin
excitations change from approximate $C_4$ to $C_2$ rotational symmetry below a
temperature $T^\ast$ ($>T_s$), while spin excitations at energies above
$E_\text{crossover}$ have approximate $C_4$ rotational symmetry and are weakly
temperature dependent. These results are consistent with angle resolved
photoemission spectroscopy measurements, where the presence of an uniaxial
strain necessary to detwin NaFeAs also raises the onset temperature $T^\ast$ of
observable orbital-dependent band splitting to above $T_s$, thus supporting the
notion of orbital selective spin waves in the nematic phase of iron-based
superconductors.
|
2008.02314v1
|
2020-10-26
|
Simulation of hydrogen permeation through pure iron for trapping and surface phenomena characterisation
|
There is a need for numerical models capable of predicting local accumulation
of hydrogen near stress concentrators and crack tips to prevent and mitigate
hydrogen assisted fracture in steels. The experimental characterisation of
trapping parameters in metals, which is required for an accurate simulation of
hydrogen transport, is usually performed through the electropermeation test. In
order to study grain size influence and grain boundary trapping during
permeation, two modelling approaches are explored; a 1D Finite Element model
including trap density and binding energy as input parameters and a
polycrystalline model based on the assignment of a lower diffusivity and
solubility to the grain boundaries. Samples of pure iron after two different
heat treatments - 950C for 40 minutes and 1100C for 5 minutes - are tested
applying three consecutive rising permeation steps and three decaying steps.
Experimental results show that the finer grain microstructure promotes a
diffusion delay due to grain boundary trapping. The usual methodology for the
determination of trap densities and binding energies is revisited in which the
limiting diluted and saturated cases are considered. To this purpose, apparent
diffusivities are fitted including also the influence of boundary conditions
and comparing results provided by the constant concentration with the constant
flux assumption. Grain boundaries are characterised for pure iron with a
binding energy between 37.8 and 39.9 kJ/mol and a low trap density but it is
numerically demonstrated that saturated or diluted assumptions are not always
verified, and a univocal determination of trapping parameters requires a
broader range of charging conditions for permeation. The relationship between
surface parameters, i.e. charging current, recombination current and surface
concentrations, is also studied.
|
2010.13567v1
|
2020-11-11
|
Non-stoichiometric amorphous magnesium-iron silicates in circumstellar dust shells. Dust growth in outflows from supergiants
|
We investigate the dust growth in oxygen-rich stellar outflows for a set of
nine well-observed massive supergiants with optically thin dust shells. Models
of the infrared emission from their circumstellar dust shells are compared to
their observed infrared spectra so as to derive the essential parameters that
govern dust formation in the extended envelope of these stars. The results
obtained from the comparative study are also compared with the predictions of a
model for silicate dust condensation solely based on laboratory data and basic
stellar properties. The infrared emission in the wavelength range between 6 and
25 mu can be reproduced rather well by a mixture of non-stoichiometric
iron-bearing silicates, alumina, and metallic iron dust particles for all nine
objects. The observed spectra obtained from three objects, mu Cep, RW Cyg, and
RS Per, can be reproduced by a stationary and (essentially) spherically
symmetric outflow which enables a direct comparison with predictions from a
theoretical dust growth model. The temperature at the onset of massive silicate
dust growth is of the order of 920 K and the corresponding outflow velocity of
the order of the sound velocity for these objects. The condensation temperature
suggests that the silicate dust grows on the corundum dust grains that are
formed well in the interior of the silicate dust shell at a much higher
temperature. Our results propose that regarding the two major problems of dust
formation in stellar outflows: (i) formation of seed nuclei; (ii) their growth
to macroscopic dust grains, we are gradually coming close to a quantitative
understanding of the second item.
|
2011.05903v1
|
2020-11-17
|
On the Probability that a Rocky Planet's Composition Reflects its Host Star
|
The bulk density of a planet, as measured by mass and radius, is a result of
planet structure and composition. Relative proportions of iron core, rocky
mantle, and gaseous envelopes are degenerate for a given density. This
degeneracy is reduced for rocky planets without significant gaseous envelopes
when the structure is assumed to be a differentiated iron core and rocky
mantle, in which the core mass fraction (CMF) is a first-order description of a
planet's bulk composition. A rocky planet's CMF may be derived both from bulk
density and by assuming the planet reflects the host star's major rock-building
elemental abundances (Fe, Mg, and Si). Contrasting CMF measures, therefore,
shed light on the outcome diversity of planet formation from processes
including mantle stripping, out-gassing, and/or late-stage volatile delivery.
We present a statistically rigorous analysis of the consistency of these two
CMF measures accounting for observational uncertainties of planet mass and
radius and host-star chemical abundances. We find that these two measures are
unlikely to be resolvable as statistically different unless the bulk density
CMF is at least 40% greater than or 50% less than the CMF as inferred from the
host star. Applied to 11 probable rocky exoplanets, Kepler-107c has a CMF as
inferred from bulk density that is significantly greater than the inferred CMF
from its host star (2$\sigma$) and is therefore likely an iron-enriched
super-Mercury. K2-229b, previously described as a super-Mercury, however, does
not meet the threshold for a super-Mercury at a 1- or 2- $\sigma$ level.
|
2011.08893v4
|
2021-01-22
|
Spin dynamics in NaFeAs and NaFe$_{0.53}$Cu$_{0.47}$As probed by resonant inelastic X-ray scattering
|
The parent compounds of iron-based superconductors are magnetically-ordered
bad metals, with superconductivity appearing near a putative magnetic quantum
critical point. The presence of both Hubbard repulsion and Hund's coupling
leads to rich physics in these multiorbital systems, and motivated descriptions
of magnetism in terms of itinerant electrons or localized spins. The
NaFe$_{1-x}$Cu$_x$As series consists of magnetically-ordered bad metal ($x=0$),
superconducting ($x\approx0.02$) and magnetically-ordered
semiconducing/insulating ($x\approx0.5$) phases, providing a platform to
investigate the connection between superconductivity, magnetism and electronic
correlations. Here we use X-ray absorption spectroscopy and resonant inelastic
X-ray scattering to study the valence state of Fe and spin dynamics in two
NaFe$_{1-x}$Cu$_x$As compounds ($x=0$ and 0.47). We find that magnetism in both
compounds arises from Fe$^{2+}$ atoms, and exhibits underdamped dispersive spin
waves in their respective ordered states. The dispersion of spin excitations in
NaFe$_{0.53}$Cu$_{0.47}$As is consistent with being quasi-one-dimensional.
Compared to NaFeAs, the band top of spin waves in NaFe$_{0.53}$Cu$_{0.47}$As is
slightly softened with significantly more spectral weight of the spin
excitations. Our results indicate the spin dynamics in
NaFe$_{0.53}$Cu$_{0.47}$As arise from localized magnetic moments and suggest
the iron-based superconductors are proximate to a correlated insulating state
with localized iron moments.
|
2101.08948v1
|
2021-02-17
|
Single particle tunneling spectroscopy and superconducting gaps in layered iron based superconductor KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$
|
We perform scanning tunneling microscopy/spectroscopy study on the layered
iron based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ with a critical temperature
of about 33.5 K. Two types of terminated surfaces are generally observed after
cleaving the samples in vacuum. On one commonly obtained surface, we observe a
full gap feature with energy gap values close to 4.6 meV. This type of spectrum
shows a clean and uniform full gap in space, which indicates the absence of gap
nodes in this superconductor. Quasiparticle interference patterns have also
been measured, which show no scattering patterns between the hole and tiny
electron pockets, but rather an intra-band scattering pattern is observed
possibly due to the hole-like $\alpha$ pocket. The Fermi energy of this band is
only about $24\pm6$ meV as derived from the energy dispersion result.
Meanwhile, impurity induced bound-state peaks can be observed at about $\pm2.2$
meV on some spectra, and the peak value seems to be independent to magnetic
field. On the second type of surface which is rarely obtained, the fully gapped
feature can still be observed on the tunneling spectra, although multiple gaps
are obtained either from a single spectrum or separate ones, and the gap values
determined from coherence peaks locate mainly in the range from 4 to 8 meV. Our
results clearly indicate multiple and nodeless superconducting gap nature in
this layered superconductor KCa$_2$Fe$_4$As$_4$F$_2$, and the superfluid is
mainly contributed by the hole-like Fermi surfaces near $\Gamma$ point. This
would inspire further consideration on the effect of the shallow and incipient
bands near M point, and help to understand the pairing mechanism in this highly
layered iron-based superconductor.
|
2102.08785v1
|
2021-03-11
|
Intertwined charge, spin, and pairing orders in doped iron ladders
|
Motivated by recent experimental progress on iron-based ladder compounds, we
study the doped two-orbital Hubbard model for the two-leg ladder BaFe$_2$S$_3$.
The model is constructed by using {\it ab initio} hopping parameters and the
ground state properties are investigated using the density matrix
renormalization group method. We show that the $(\pi,0)$ magnetic ordering at
half-filling, with ferromagnetic rungs and antiferromagnetic legs, becomes
incommensurate upon hole doping. Moreover, depending on the strength of the
Hubbard $U$ coupling, other magnetic patterns, such as $(0,\pi)$, are also
stabilized. We found that the binding energy for two holes becomes negative for
intermediate Hubbard interaction strength, indicating hole pairing. Due to the
crystal-field split among orbitals, the holes primarily reside in one orbital,
with the other one remaining half-filled. This resembles orbital selective Mott
states. The formation of tight hole pairs continues with increasing hole
density, as long as the magnetic order remains antiferromagnetic in one
direction. The study of pair-pair correlations indicates the dominance of the
intra-orbital spin-singlet channel, as opposed to other pairing channels.
Although in a range of hole doping pairing correlations decay slowly, our
results can also be interpreted as corresponding to a charge-density-wave made
of pairs, a precursor of eventual superconductivity after interladder couplings
are included. Such scenario of intertwined orders has been extensively
discussed before in the cuprates, and our results suggest a similar physics
could exist in ladder iron-based superconductors. Finally, we also show that a
robust Hund's coupling is needed for pairing to occur.
|
2103.06407v2
|
2021-03-11
|
Enhancement of Superconductivity Linked with Linear-in-Temperature/Field Resistivity in Ion-Gated FeSe Films
|
Iron selenide (FeSe) - the structurally simplest iron-based superconductor,
has attracted tremendous interest in the past years. While the transition
temperature (Tc) of bulk FeSe is $\sim$ 8 K, it can be significantly enhanced
to 40 - 50 K by various ways of electron doping. However, the underlying
physics for such great enhancement of Tc and so the Cooper pairing mechanism
still remain puzzles. Here, we report a systematic study of the
superconducting- and normal-state properties of FeSe films via ionic liquid
gating. With fine tuning, Tc evolves continuously from below 10 K to above 40
K; in situ two-coil mutual inductance measurements unambiguously confirm the
gating is a uniform bulk effect. Close to Tc, the normal-state resistivity
shows a linear dependence on temperature and the linearity extends to lower
temperatures with the superconductivity suppressed by high magnetic fields. At
high fields, the normal-state magnetoresistance exhibits a linear-in-field
dependence and obeys a simple scaling relation between applied field and
temperature. Consistent behaviors are observed for different-Tc states
throughout the gating process, suggesting the pairing mechanism very likely
remains the same from low- to high-Tc state. Importantly, the coefficient of
the linear-in-temperature resistivity is positively correlated with Tc,
similarly to the observations in cuprates, Bechgaard salts and iron pnictide
superconductors. Our study points to a short-range antiferromagnetic exchange
interaction mediated pairing mechanism in FeSe.
|
2103.06512v2
|
2021-06-08
|
Atomic iron and nickel in the coma of C/1996 B2 (Hyakutake): production rates, emission mechanisms, and possible parents
|
Two papers recently reported the detection of gaseous nickel and iron in the
comae of over 20 comets from observations collected over two decades, including
interstellar comet 2I/Borisov. To evaluate the state of the laboratory data in
support of these identifications, we re-analyzed archived spectra of comet
C/1996 B2 (Hyakutake), one of the nearest and brightest comets of the last
century, using a combined experimental and computational approach. We developed
a new, many-level fluorescence model that indicates that the fluorescence
emission of Fe I and Ni I vary greatly with heliocentric velocity. Combining
this model with laboratory spectra of an Fe-Ni plasma, we identified 22 lines
of Fe I and 14 lines of Ni I in the spectrum of Hyakutake. Using Haser models,
we estimate the nickel and iron production rates as Q(Ni) = 2.6 - 4.1 x 10^22
s^-1 and Q(Fe) = 0.4 - 2.8 x 10^23 s^-1. From derived column densities, the
Ni/Fe abundance ratio log10[Ni/Fe] = -0.15 +/- 0.07 deviates significantly from
solar abundance ratios, and it is consistent with the ratios observed in solar
system comets. Possible production and emission mechanisms are analyzed in
context of existing laboratory measurements. Based on the observed spatial
distributions, excellent fluorescence model agreement, and Ni/Fe ratio, our
findings support an origin consisting of a short-lived unknown parent followed
by fluorescence emission. Our models suggest that the strong heliocentric
velocity dependence of the fluorescence efficiencies can provide a meaningful
test of the physical process responsible for the Fe I and Ni I emission.
|
2106.04701v2
|
2021-09-01
|
No umbrella needed: Confronting the hypothesis of iron rain on WASP-76b with post-processed general circulation models
|
High-resolution spectra are unique indicators of three-dimensional processes
in exoplanetary atmospheres. For instance, in 2020, Ehrenreich et al. reported
transmission spectra from the ESPRESSO spectrograph yielding an anomalously
large Doppler blueshift from the ultra-hot Jupiter WASP-76b. Interpretations of
these observations invoke toy model depictions of gas-phase iron condensation
in lower-temperature regions of the planet's atmosphere. In this work, we
forward model the atmosphere of WASP-76b with double-gray general circulation
models (GCMs) and ray-striking radiative transfer to diagnose the planet's
high-resolution transmission spectrum. We confirm that a physical mechanism
driving strong east-west asymmetries across the terminator must exist to
reproduce large Doppler blueshifts in WASP-76b's transmission spectrum. We
identify low atmospheric drag and a deep radiative-convective boundary as
necessary components of our GCM to produce this asymmetry (the latter is
consistent with existing Spitzer phase curves). However, we cannot reproduce
either the magnitude or the time-dependence of the WASP-76b Doppler signature
with gas-phase iron condensation alone. Instead, we find that high-altitude,
optically thick clouds composed of $\rm Al_2O_3$, Fe, or $\rm Mg_2SiO_4$
provide reasonable fits to the Ehrenreich et al. observations -- with marginal
contributions from condensation. This fit is further improved by allowing a
small orbital eccentricity ($e \approx 0.017$), consistent with prior WASP-76b
orbital constraints. We additionally validate our forward-modeled spectra by
reproducing lines of nearly all species detected in WASP-76b by Tabernero et
al. 2021. Our procedure's success in diagnosing phase-resolved Doppler shifts
demonstrates the benefits of physical, self-consistent, three-dimensional
simulations in modeling high-resolution spectra of exoplanet atmospheres.
|
2109.00163v2
|
2021-10-21
|
Heat Capacity of oxide scale in the range from 0 C to 1300 C: Generalized estimates with account for movability of phase transitions
|
The known data on the heat capacity of magnetite (Fe3O4), hematite (Fe2O3)
and iron (Fe) at different temperatures are approximated by formulas containing
phase transition temperatures as varying parameters. This allows to take into
account the effect of phase transition shifts, for example, due to impurities,
lattice defects, grain sizes or high cooling rates. For this purpose, the
entire target temperature range from 0 C to 1300 C is divided by phase
transition temperatures into separate intervals. The conjugation of the
approximating functions between the intervals at the magnetic transition point
is performed without a gap, and at the point of polymorphic transformation
(alpha Fe - gamma Fe) with a finite gap of heat capacity values. For wustite
Fe1-xO which does not experience phase transformations, the temperature
dependence of the heat capacity is approximated by a single smooth function. In
combination with previously obtained formulas for the density of iron oxides
and iron the proposed approximations allow us to estimate the specific mass
heat capacity of oxide scale depending of its structural composition and
temperature. By model calculations it is shown that at temperatures of 200 C
and 900 C specific mass heat capacity of oxide scale practically does not
depend on the percentage of its individual components and is approximately 750
and 850 J/(kg K) respectively. At a temperature of about 575 C, on contrary,
actually possible variations in the composition of oxide scale can lead to a
change in its specific heat capacity from 850 to 1150 J/(kg K). The obtained
dependencies are recommended for use in mathematical modeling of production and
processing of steel products in the presence of oxide scale on their surface
|
2110.11101v1
|
2021-11-20
|
Gaia-ESO Survey: Detailed elemental abundances in red giants of the peculiar globular cluster NGC 1851
|
Context. NGC 1851 is one of several globular clusters for which multiple
stellar populations of the subgiant branch have been clearly identified and a
difference in metallicity detected. A crucial piece of information on the
formation history of this cluster can be provided by the sum of A(C+N+O)
abundances. However, these values have lacked a general consensus thus far. The
separation of the subgiant branch can be based on age and/or A(C+N+O) abundance
differences. Aims. Our main aim was to determine carbon, nitrogen, and oxygen
abundances for evolved giants in the globular cluster NGC1851 in order to check
whether or not the double populations of stars are coeval. Methods.
High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO
VLT telescope, were analysed using a differential model atmosphere method.
Results. We provide abundances of up to 29 chemical elements for a sample of 45
giants in NGC 1851. The investigated stars can be separated into two
populations with a difference of 0.07 dex in the mean metallicity, 0.3 dex in
the mean C/N, and 0.35 dex in the mean s-process dominated element-to-iron
abundance ratios [s/Fe]. No significant difference was determined in the mean
values of A(C+N+O) as well as in abundance to iron ratios of carbon, alpha- and
iron-peak-elements, and of europium. Conclusions. As the averaged A(C+N+O)
values between the two populations do not differ, additional evidence is given
that NGC 1851 is composed of two clusters, the metal-rich cluster being by
about 0.6 Gyr older than the metal-poor one. A global overview of NGC 1851
properties and the detailed abundances of chemical elements favour its
formation in a dwarf spheroidal galaxy that was accreted by the Milky Way.
|
2111.10684v1
|
2021-12-12
|
Top-heavy stellar mass distribution in galactic nuclei inferred from the universally high abundance ratio of [Fe/Mg]
|
Recent observations of active galactic nuclei (AGNs) have shown a high
Fe~II/Mg~II line-flux ratio in their broad-line regions, nearly independent of
redshift up to $z \gtrsim 6$. The high flux ratio requires rapid production of
iron in galactic nuclei to reach an abundance ratio of ${\rm [Fe/Mg]} \gtrsim
0.2$ as high as those observed in matured galaxies in the local universe. We
propose a possible explanation of rapid iron enrichment in AGNs by massive star
formation that follows a top-heavy initial mass function (IMF) with a power-law
index of $\Gamma$ larger than the canonical value of $\Gamma=-2.35$ for a
Salpeter IMF. Taking into account metal production channels from different
types of SNe, we find that the high value of ${\rm [Fe/Mg]} \gtrsim 0.2$
requires the IMF to be characterized with $\Gamma \gtrsim -1$ ($\Gamma \gtrsim
0$) and a high-mass cutoff at $M_{\rm max} \simeq 100$--$150~{\rm M_\odot}$
$(M_{\rm max} \gtrsim 250~{\rm M_\odot})$. Given the conditions, core-collapse
SNe with $M_\ast \gtrsim 70~{\rm M_\odot}$ and pair-instability SNe give a
major contribution for iron enrichment. Such top-heavy stellar IMFs would be a
natural consequence from mass growth of stars formed in dense AGN disks under
Bondi-like gas accretion that is regulated by feedback at $M_\ast \gtrsim
10~{\rm M_\odot}$. The massive stellar population formed in AGN disks also
leave stellar-mass black hole remnants, whose mergers associated with
gravitational-wave emission account for at most 10 \% of the merger rate
inferred from LIGO/Virgo observations to simultaneously explain the high ${\rm
[Fe/Mg]}$ ratio with metal ejection.
|
2112.06151v1
|
2022-03-16
|
Measurements of protons and charged pions emitted from $ν_μ$ charged-current interactions on iron at a mean neutrino energy of 1.49$\,$GeV using a nuclear emulsion detector
|
This study conducted an analysis of muons, protons, and charged pions emitted
from $\nu_{\mu}$ charged-current interactions on iron using a nuclear emulsion
detector. The emulsion detector with a 65$\,$kg iron target was exposed to a
neutrino beam corresponding to 4.0$\times$10$^{19}$ protons on target with a
mean neutrino energy of 1.49$\,$GeV. The measurements were performed at a
momentum threshold of 200 (50)$\,$MeV/$c$ for protons (pions), which are the
lowest momentum thresholds attempted up to now. The measured quantities are the
multiplicities, emission angles, and momenta of the muons, protons, and charged
pions. In addition to these inclusive measurements, exclusive measurements such
as the muon-proton emission-angle correlations of specific channels and the
opening angle between the protons of CC0$\pi$2$p$ events were performed. The
data were compared to Monte Carlo (MC) predictions and some significant
differences were observed. The results of the study demonstrate the capability
of detailed measurements of neutrino-nucleus interactions using a nuclear
emulsion detector to improve neutrino interaction models.
|
2203.08367v3
|
2022-07-06
|
Atomic diffusion and turbulent mixing in solar-like stars: Impact on the fundamental properties of FG-type stars
|
Chemical composition is an important factor that affects stellar evolution.
The element abundance on the stellar surface evolves along the lifetime of the
star because of transport processes, including atomic diffusion. However,
models of stars with masses higher than about 1.2Msun predict unrealistic
variations at the stellar surface. This indicates the need for competing
transport processes that are mostly computationally expensive for large grids
of stellar models. The purpose of this study is to implement turbulent mixing
in stellar models and assess the possibility of reproducing the effect of
radiative accelerations with turbulent mixing for elements like iron in order
to make the computation of large grids possible. We computed stellar models
with MESA and assessed the effects of atomic diffusion (with radiative
acceleration) in the presence of turbulent mixing. We parametrised the effect
of radiative accelerations on iron with a turbulent diffusion coefficient.
Finally, we tested this parametrisation by modelling two F-type stars of the
Kepler Legacy sample. We found that, for iron, a parametrisation of turbulent
mixing that simulates the effect of radiative acceleration is possible. This
leads to an increase in the efficiency of the turbulent mixing to counteract
the effect of gravitational settling. This approximation does not affect
significantly the surface abundances of the other elements we studied, except
for oxygen and calcium. We demonstrate that this parametrisation has a
negligible impact on the accuracy of the seismic properties inferred with these
models. Moreover, turbulent mixing makes the computation of realistic F-type
star models including the effect atomic diffusion possible. This leads to
differences of about 10% in the inferred ages compared to results obtained with
models that neglect these processes.
|
2207.02779v1
|
2022-07-28
|
Symmetry protected 1D chains in mixed-valence iron oxides
|
During the last decade of high-pressure research a whole new series of iron
oxides was discovered, like Fe$_4$O$_5$, Fe$_5$O$_6$, Fe$_7$O$_9$ etc.,
featuring closely related structures with arrays of one-dimensional (1D) chains
of trigonal prisms embedded between slabs of octahedra. Here, we develop a
unified approach to the series based on a specific crystallographic generation
mechanism which predicts the structures of these oxides and naturally
classifies them in terms of the slab cycle. When including magnetic
interactions, we show that the 1D chains have a symmetry protection against
magnetic perturbations from the iron ions in the slabs, and that the slab size
determines the type of magnetic order, which is either ferromagnetic or
antiferromagnetic. Dynamical mean-field theory calculations reveal the
orbitally selective Mott state of the Fe ions and tendency of conductivity to
low-dimensional behavior with particular enhancement along the 1D chains.
Across the series, the decoupling of the chains increases, and so with the
inherent charge ordering of the slabs, these structures have the potential to
allow experimental realization of the model system of coupled 1D wires. We
point out the possibility to stabilize these compounds in the thin-film form
that, together with a wide range of possible ionic substitutions and fact that
these compounds are recoverable at ambient pressure, makes them a very
promising platform to engineer physical systems with interesting
magnetotransport phenomena, as corroborated by the recent discovery of quantum
Hall effect in ZrTe$_5$.
|
2207.14111v1
|
2022-08-01
|
Sustainable steel through hydrogen plasma reduction of iron ore: process, kinetics, microstructure, chemistry
|
Fe- and steelmaking is the largest single industrial CO2 emitter, accounting
for 6.5% of all CO2 emissions on the planet. This fact challenges the current
technologies to achieve carbon-lean steel production and to align with the
requirement of a drastic reduction of 80% in all CO2 emissions by around 2050.
Thus, alternative reduction technologies have to be implemented for extracting
iron from its ores. The H-based direct reduction has been explored as a
sustainable route to mitigate CO2 emissions, where the reduction kinetics of
the intermediate oxide product FexO wustite into Fe is the rate-limiting step
of the process. The total reaction has an endothermic net energy balance.
Reduction based on a H plasma may offer an attractive alternative. Here, we
present a study about the reduction of hematite using H plasma. The evolution
of both, chemical composition and phase transformations was investigated in
several intermediate states. We found that hematite reduction kinetics depends
on the balance between the initial input mass and the arc power. For an
optimized input mass-arc power ratio, complete reduction was obtained within 15
min of exposure to the H plasma. The wustite reduction is also the
rate-limiting step towards complete reduction. Nonetheless, the reduction
reaction is exothermic, and its rates are comparable with those found in
H-based direct reduction. Chemical and microstructure analysis revealed that
the gangue elements partition to the remaining oxide regions, probed by energy
dispersive spectroscopy and atom probe tomography. Si-enrichment was observed
in the interdendritic fayalite domains, at the wustite/Fe hetero-interfaces and
in the primarily solidified oxide particles inside the Fe. With proceeding
reduction, however, such elements are gradually removed from the samples so
that the final iron product is nearly free of gangue-related impurities.
|
2208.00661v1
|
2022-09-27
|
3D non-LTE iron abundances in FG-type dwarfs
|
Spectroscopic measurements of iron abundances are prone to systematic
modelling errors. We present 3D non-LTE calculations across 32 STAGGER-grid
models with effective temperatures from 5000 K to 6500 K, surface gravities of
4.0 dex and 4.5 dex, and metallicities from $-$3 dex to 0 dex, and study the
effects on 171 Fe I and 12 Fe II optical lines. In warm metal-poor stars, the
3D non-LTE abundances are up to 0.5 dex larger than 1D LTE abundances inferred
from Fe I lines of intermediate excitation potential. In contrast, the 3D
non-LTE abundances can be 0.2 dex smaller in cool metal-poor stars when using
Fe I lines of low excitation potential. The corresponding abundance differences
between 3D non-LTE and 1D non-LTE are generally less severe but can still reach
$\pm$0.2 dex. For Fe II lines the 3D abundances range from up to 0.15 dex
larger, to 0.10 dex smaller, than 1D abundances, with negligible departures
from 3D LTE except for the warmest stars at the lowest metallicities. The
results were used to correct 1D LTE abundances of the Sun and Procyon (HD
61421), and of the metal-poor stars HD 84937 and HD 140283, using an
interpolation routine based on neural networks. The 3D non-LTE models achieve
an improved ionisation balance in all four stars. In the two metal-poor stars,
they remove excitation imbalances that amount to 250 K to 300 K errors in
effective temperature. For Procyon, the 3D non-LTE models suggest [Fe/H] = 0.11
$\pm$ 0.03, which is significantly larger than literature values based on
simpler models. We make the 3D non-LTE interpolation routine for FG-type dwarfs
publicly available, in addition to 1D non-LTE departure coefficients for
standard MARCS models of FGKM-type dwarfs and giants. These tools, together
with an extended 3D LTE grid for Fe II from 2019, can help improve the accuracy
of stellar parameter and iron abundance determinations for late-type stars.
|
2209.13449v3
|
2022-10-24
|
High-precision abundances of first population stars in NGC 2808: confirmation of a metallicity spread
|
Photometric investigations have revealed that Galactic globular clusters
exhibit internal metallicity variations amongst the so-called first-population
stars, until now considered to have a homogeneous initial chemical composition.
This is not fully supported by the sparse spectroscopic evidence, which so far
gives conflicting results. Here, we present a high-resolution re-analysis of
five stars in the Galactic globular cluster NGC 2808 taken from the literature.
Target stars are bright red giants with nearly identical atmospheric parameters
belonging to the first population according to their identification in the
chromosome map of the cluster, and we have measured precise differential
abundances for Fe, Si, Ca, Ti, and Ni to the ~0.03 dex level.
Thanks to the very small uncertainties associated to the differential
atmospheric parameters and abundance measurements, we find that target stars
span a range of iron abundance equal to 0.25 +/- 0.06 dex. The individual
elemental abundances are highly correlated with the position of the star along
the extended sequence described by first population objects in the cluster
chromosome map: bluer stars have a lower iron content. This agrees with
inferences from the photometric analysis.
The differential abundances for all other elements also show statistically
significant ranges that point to intrinsic abundance spreads. The Si, Ca, Ti,
and Ni variations are highly correlated with iron variations and the total
abundance spreads for all elements are consistent within the error bars. This
suggests a scenario in which short-lived massive stars exploding as supernovae
contributed to the self-enrichment of the gas in the natal cloud while star
formation was still ongoing.
|
2210.13369v1
|
2023-01-07
|
Changes in the distribution of circum-binary material around the HMXB GX 301-2 during a rapid spin-up episode of the neutron star
|
Some accretion powered X-ray pulsars with supergiant companion stars undergo
occasional rapid spin-up episodes that last for weeks to a few months. We
explore the changes in the accretion environment of the pulsar GX 301-2 during
its latest 80 days long spin-up episode in 2019 when the spin frequency of the
pulsar increased by ~2% over two orbits of the binary. By performing
time-resolved spectroscopy with the MAXI/GSC spectra of the source, we
estimated the equivalent hydrogen column density and equivalent width of the
iron fluorescence line during the spin-up episode, and compared them with the
long-term average values estimated by orbital-phase resolved spectroscopy. The
measured absorption column density during the spin-up episode is about twice
that of an average orbit, while the equivalent width of the iron line is less
than half of an average orbit. Though the spin-up episode started immediately
after a pre-periastron flare and lasted for the two consecutive orbits of the
binary, the associated enhancement in luminosity started a few days after the
pre-periastron flare and lasted only during the first orbit, and some
enhancement was seen again during the pre-periastron passage of the second
orbit. The absorption column density and iron line equivalent width vary
throughout the spin-up episode and are distinct from an average orbit. These
observations indicate a significant change in the accretion and reprocessing
environment in GX 301-2 during the spin-up episode and may hold important clues
for the phenomenon in this source and several other sources with supergiant
companions.
|
2301.02815v1
|
2023-03-09
|
Saline bolus for negative contrast perfusion imaging in magnetic particle imaging
|
Objective. Magnetic Particle Imaging (MPI) is capable of high temporal
resolution measurements of the spatial distribution of magnetic nanoparticles
and therefore well suited for perfusion imaging, which is an important tool in
medical diagnosis. Perfusion imaging in MPI usually requires a fresh bolus of
tracer material to capture the key signal dynamics. Here, we propose a method
to decouple the imaging sequence from the injection of additional tracer
material, without further increasing the administered iron dose in the body
with each image. Approach. A bolus of physiological saline solution without any
particles (negative contrast) diminishes the steady-state concentration of a
long-circulating tracer during passage. This depression in the measured
concentration contributes to the required contrast dynamics. The presence of a
long-circulating tracer is therefore a prerequisite to obtain the negative
contrast. As a quantitative tracer based imaging method, the signal is linear
in the tracer concentration for any location that contains nanoparticles and
zero in the surrounding tissue which does not provide any intrinsic signal.
After tracer injection, the concentration over time (positive contrast) can be
utilized to calculate dynamic diagnostic parameters like perfusion parameters
in vessels and organs. Every acquired perfusion image thus requires a new bolus
of tracer with a sufficiently large iron dose to be visible above the
background. Main results. Perfusion parameters are calculated based on the time
response of the proposed negative bolus and compared to a positive bolus.
Results from phantom experiments show that normalized signals from positive and
negative boli are concurrent and deviations of calculated perfusion maps are
low. Significance. Our method opens up the possibility to increase the total
monitoring time, while minimizing the iron dose per acquired image.
|
2303.05439v2
|
2023-03-19
|
In-situ visualization of local distortions in the high-$T_c$ molecule-intercalated $Li_x(C_5H_5N)_yFe_{2-z}Se_2$ superconductor
|
A time-resolved synchrotron X-ray total scattering study sheds light on the
evolution of the different structural length scales involved during the
intercalation of the layered iron-selenide host by organic molecular donors,
aiming at the formation of the expanded lattice $Li_x(C_5H_5N)_yFe_{2-z}Se_2$
hybrid superconductor. The intercalates are found to crystallize in the
tetragonal $ThCr_2Si_2$-type structure at the average level, however, with an
enhanced interlayer iron-selenide spacing (d= 16.2 \r{A}) that accommodates the
heterocyclic molecular spacers. Quantitative atomic pair distribution function
(PDF) analysis at variable times, suggests distorted $FeSe_4$ tetrahedral local
environments that appear swollen with respect to those in the parent
$\beta$-FeSe. Simultaneously acquired, in-situ synchrotron X-ray powder
diffraction data disclose that secondary phases ($\alpha$-Fe and $Li_2Se$),
grow significantly when higher Li-concentration is used in the solvothermal
reaction or when the solution is aged. These observations are in line with the
strongly reducing character of the intercalation medium's solvated electrons
that mediate the defect chemistry of the expanded lattice superconductor. In
the latter, intralayer correlated local distortions indicate electron donating
aspects that reflect in somewhat enlarged Fe-Se bonds. They also reveal a
degree of relief of chemical pressure associated with a large distance between
Fe and Se sheets ('taller' anion height) and a stretched Fe-Fe square planar
topology. The elongation of the latter, derived from the in-situ PDF study,
speaks for a plausible increase in the Fe-site vacancy concentration. The
evolution of the local structural parameters suggests an optimum reaction
window where kinetically stabilized phases resemble the distortions of the
edge-sharing Fe-Se tetrahedra, required for high-$T_c$ in expanded lattice
iron-chalcogenides.
|
2303.10698v1
|
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