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2020-04-23
|
Chandra High Energy Transmission Gratings Spectra of V3890 Sgr
|
The recurrent nova (RN) V3890 Sgr was observed during the 7th day after the
onset of its most recent outburst, with the Chandra ACIS-S camera and High
Energy Transmission Gratings (HETG). A rich emission line spectrum was
detected, due to transitions of Fe-L and K-shell ions ranging from neon to
iron. The measured absorbed flux is $\approx 10^{-10}$ erg cm$^{-2}$ s$^{-1}$
in the 1.4-15 Angstrom range (0.77-8.86 keV). The line profiles are asymmetric,
blue-shifted and skewed towards the blue side, as if the ejecta moving towards
us are less absorbed than the receding ones. The full width at half maximum of
most emission lines is 1000-1200 km s$^{-1}$, with some extended blue wings.
The spectrum is thermal and consistent with a plasma in collisional ionization
equilibrium with column density 1.3 $\times 10^{22}$ cm$^{-2}$ and at least two
components at temperatures of about 1 keV and 4 keV, possibly a forward and a
reverse shock, or regions with differently mixed ejecta and red giant wind. The
spectrum is remarkably similar to the symbiotic RNe V745 Sco and RS Oph, but we
cannot distinguish whether the shocks occurred at a distance of few AU from the
red giant, or near the giant's photosphere, in a high density medium containing
only a small mass. The ratios of the flux in lines of aluminum, magnesium and
neon relative to the flux in lines of silicon and iron probably indicate a
carbon-oxygen white dwarf (CO WD).
|
2004.11263v2
|
2020-05-19
|
ARES II: Characterising the Hot Jupiters WASP-127 b, WASP-79 b and WASP-62 b with HST
|
This paper presents the atmospheric characterisation of three large, gaseous
planets: WASP-127b, WASP-79b and WASP-62b. We analysed spectroscopic data
obtained with the G141 grism (1.088 - 1.68 $\mu$m) of the Wide Field Camera 3
(WFC3) onboard the Hubble Space Telescope (HST) using the Iraclis pipeline and
the TauREx3 retrieval code, both of which are publicly available. For WASP-127
b, which is the least dense planet discovered so far and is located in the
short-period Neptune desert, our retrieval results found strong water
absorption corresponding to an abundance of log(H$_2$O) =
-2.71$^{+0.78}_{-1.05}$, and absorption compatible with an iron hydride
abundance of log(FeH)=$-5.25^{+0.88}_{-1.10}$, with an extended cloudy
atmosphere. We also detected water vapour in the atmospheres of WASP-79 b and
WASP-62 b, with best-fit models indicating the presence of iron hydride, too.
We used the Atmospheric Detectability Index (ADI) as well as Bayesian log
evidence to quantify the strength of the detection and compared our results to
the hot Jupiter population study by Tsiaras et al. 2018. While all the planets
studied here are suitable targets for characterisation with upcoming facilities
such as the James Webb Space Telescope (JWST) and Ariel, WASP-127 b is of
particular interest due to its low density, and a thorough atmospheric study
would develop our understanding of planet formation and migration.
|
2005.09615v2
|
2020-05-29
|
Cold, dry, windy, and UV irradiated -- surveying Mars relevant conditions in Ojos del Salado Volcano (Andes Mountains, Chile)
|
The Special Collection of papers in this issue of Astrobiology provide an
overview of the characteristics and potential for future exploration of the
Ojos del Salado volcano, located in the Andes Mountains in front of the Atacama
Desert in northern Chile. The main benefits of this site compared with others
are the combination of strong UV radiation, the presence of permafrost, and
geothermal activity within a dry terrain. The interaction between limited snow
events and wind results in snow patches buried under a dry soil surface. This
leads to ephemeral water streams that only flow duringdaytime hours. On this
volcano, which has the highest located subsurface temperature monitoring
systems reported to date, seasonal melting of the permafrost is followed by
fast percolation events. This is due to the high porosity of these soils. The
results are landforms that shaped by the strong winds. At this site, both
thermal springs and lakes (the latter arising from melting ice) provide
habitats for life; a 6480m high lake heated by volcanic activity shows both
warm and cold sediments that contain a number of different microbial species,
including psychrophiles. Where the permafrost melts, thawing ponds have formed
at 5900m that is dominated by populations of Bacteroidetes and Proteobacteria,
while in the pond sediments and the permafrost itself Acidobacteria,
Actinobacteria, Bacteroidetes, Patescibacteria, Proteobacteria, and
Verrucomicrobia are abundant. In turn, fumaroles show the presence of
acidophilic iron-oxidizers and iron-reducing species. In spite of the extreme
conditions reported at Ojos del Salado, this site is easily accessible.
|
2005.14450v1
|
2020-06-05
|
Chemically peculiar A and F stars with enhanced s-process and iron-peak elements: stellar radiative acceleration at work
|
We present $\gtrsim 15,000$ metal-rich (${\rm [Fe/H]}>-0.2$dex) A and F stars
whose surface abundances deviate strongly from Solar abundance ratios and
cannot plausibly reflect their birth material composition. These stars are
identified by their high [Ba/Fe] abundance ratios (${\rm [Ba/Fe]}>1.0$dex) in
the LAMOST DR5 spectra analyzed by Xiang et al. (2019). They are almost
exclusively main sequence and subgiant stars with $T_{\rm eff}\gtrsim6300$K.
Their distribution in the Kiel diagram ($T_{\rm eff}$--$\log g$) traces a sharp
border at low temperatures along a roughly fixed-mass trajectory (around
$1.4M_\odot)$ that corresponds to an upper limit in convective envelope mass
fraction of around $10^{-4}$. Most of these stars exhibit distinctly enhanced
abundances of iron-peak elements (Cr, Mn, Fe, Ni) but depleted abundances of Mg
and Ca. Rotational velocity measurements from GALAH DR2 show that the majority
of these stars rotate slower than typical stars in an equivalent temperature
range. These characteristics suggest that they are related to the so-called
Am/Fm stars. Their abundance patterns are qualitatively consistent with the
predictions of stellar evolution models that incorporate radiative
acceleration, suggesting they are a consequence of stellar internal evolution
particularly involving the competition between gravitational settling and
radiative acceleration. These peculiar stars constitute 40% of the whole
population of stars with mass above 1.5$M_\odot$, affirming that "peculiar"
photospheric abundances due to stellar evolution effects are a ubiquitous
phenomenon for these intermediate-mass stars. This large sample of Ba-enhanced
chemically peculiar A/F stars with individual element abundances provides the
statistics to test more stringently the mechanisms that alter the surface
abundances in stars with radiative envelopes.
|
2006.03329v1
|
2020-06-29
|
Impact of the returning radiation on the analysis of the reflection spectra of black holes
|
A fraction of the electromagnetic radiation emitted from the surface of a
geometrically thin and optically thick accretion disk of a black hole returns
to the disk because of the strong light bending in the vicinity of the compact
object (returning radiation). While such radiation clearly affects the observed
spectrum of the source, it is often neglected in theoretical models. In the
present paper, we study the impact of the returning radiation on relativistic
reflection spectra. Assuming neutral material in the disk, we estimate the
systematic uncertainties on the measurement of the properties of the system
when we fit the data with a theoretical model that neglects the returning
radiation. Our NICER simulations show that the inclination angle of the disk
and the black hole spin parameter tend to be overestimated for low viewing
angles, while no clear bias is observed for high viewing angles. The iron
abundance of the disk is never overestimated. In the most extreme cases (in
particular, for maximally rotating black holes) the returning radiation
flattens the radial emissivity beyond a few gravitational radii. In such cases,
it also produces residuals that cannot be compensated by adjusting the
parameters of models neglecting the returning radiation. This may be an
important issue for interpretation of data from future X-ray missions (e.g.
Athena). When we simulate some observations with NuSTAR and we fit data above
10 keV, we find that some conclusions valid for the NICER simulations are not
true any longer (e.g., we can get a high iron abundance).
|
2006.15838v2
|
2020-07-02
|
Iron in X-COP: tracing enrichment in cluster outskirts with high accuracy abundance profiles
|
We present the first metal abundance profiles for a representative sample of
massive clusters. Our measures extend to $R_{500}$ and are corrected for a
systematic error plaguing previous outskirt estimates. Our profiles flatten out
at large radii, admittedly not a new result, however the radial range and
representative nature of our sample extends its import well beyond previous
findings. We find no evidence of segregation between cool-core and
non-cool-core systems beyond $\sim 0.3 R_{500}$, implying that, as was found
for thermodynamic properties (Ghirardini et al, 2019), the physical state of
the core does not affect global cluster properties. Our mean abundance within
$R_{500}$ shows a very modest scatter, $< $15%, suggesting the enrichment
process must be quite similar in all these massive systems. This is a new
finding and has significant implications on feedback processes. Together with
results from thermodynamic properties presented in a previous X-COP paper, it
affords a coherent picture where feedback effects do not vary significantly
from one system to another. By combing ICM with stellar measurements we have
found the amount of Fe diffused in the ICM to be about ten times higher than
that locked in stars. Although our estimates suggest, with some strength, that
the measured iron mass in clusters is well in excess of the predicted one,
systematic errors prevent us from making a definitive statement. Further
advancements will only be possible when systematic uncertainties, principally
those associated to stellar masses, both within and beyond $R_{500}$, can be
reduced.
|
2007.01084v1
|
2020-07-21
|
Unveiling the Planet Population at Birth
|
The radius distribution of small, close-in exoplanets has recently been shown
to be bimodal. The photoevaporation model predicted this bimodality. In the
photoevaporation scenario, some planets are completely stripped of their
primordial H/He atmospheres, whereas others retain them. Comparisons between
the photoevaporation model and observed planetary populations have the power to
unveil details of the planet population inaccessible by standard observations,
such as the core mass distribution and core composition. In this work, we
present a hierarchical inference analysis on the distribution of close-in
exoplanets using forward-models of photoevaporation evolution. We use this
model to constrain the planetary distributions for core composition, core mass
and initial atmospheric mass fraction. We find that the core-mass distribution
is peaked, with a peak-mass of $\sim 4$M$_\oplus$. The bulk core-composition is
consistent with a rock/iron mixture that is ice-poor and ``Earth-like''; the
spread in core-composition is found to be narrow ($\lesssim 16\%$ variation in
iron-mass fraction at the 2$\sigma$ level) and consistent with zero. This
result favours core formation in a water/ice poor environment. We find the
majority of planets accreted a H/He envelope with a typical mass fraction of
$\sim 4\%$; only a small fraction did not accrete large amounts of H/He and
were ``born-rocky''. We find four-times as many super-Earths were formed
through photoevaporation, as formed without a large H/He atmosphere. Finally,
we find core-accretion theory over-predicts the amount of H/He cores would have
accreted by a factor of $\sim 5$, pointing to additional mass-loss mechanisms
(e.g. ``boil-off'') or modifications to core-accretion theory.
|
2007.11006v2
|
2020-07-24
|
The Most Metal-poor Stars in the Inner Bulge
|
The bulge is the oldest component of the Milky Way. Since numerous
simulations of Milky Way formation have predicted that the oldest stars at a
given metallicity are found on tightly bound orbits, the Galaxy's oldest stars
are likely metal-poor stars in the inner bulge with small apocenters (i.e.,
$R_{\mathrm{apo}}\lesssim4$ kpc). In the past, stars with these properties have
been impossible to find due to extreme reddening and extinction along the line
of sight to the inner bulge. We have used the mid-infrared metal-poor star
selection of Schlaufman & Casey (2014) on Spitzer/GLIMPSE data to overcome
these problems and target candidate inner bulge metal-poor giants for
moderate-resolution spectroscopy with AAT/AAOmega. We used those data to select
three confirmed metal-poor giants ($[\mathrm{Fe/H}]=-3.15,-2.56,-2.03$) for
follow-up high-resolution Magellan/MIKE spectroscopy. A comprehensive orbit
analysis using Gaia DR2 astrometry and our measured radial velocities confirms
that these stars are tightly bound inner bulge stars. We determine the
elemental abundances of each star and find high titanium and iron-peak
abundances relative to iron in our most metal-poor star. We propose that the
distinct abundance signature we detect is a product of nucleosynthesis in the
Chandrasekhar-mass thermonuclear supernova of a CO white dwarf accreting from a
helium star with a delay time of about 10 Myr. Even though chemical evolution
is expected to occur quickly in the bulge, the intense star formation in the
core of the nascent Milky Way was apparently able to produce at least one
Chandrasekhar-mass thermonuclear supernova progenitor before chemical evolution
advanced beyond $[\mathrm{Fe/H}]\sim-3$.
|
2007.12728v1
|
2021-02-01
|
Connecting the Light Curves of Type IIP Supernovae to the Properties of their Progenitors
|
Observations of core-collapse supernovae (CCSNe) reveal a wealth of
information about the dynamics of the supernova ejecta and its composition but
very little direct information about the progenitor. Constraining properties of
the progenitor and the explosion requires coupling the observations with a
theoretical model of the explosion. Here, we begin with the CCSN simulations of
Couch et al 2020 ApJ 890 127, which use a non-parametric treatment of the
neutrino transport while also accounting for turbulence and convection. In this
work we use the SuperNova Explosion Code to evolve the CCSN hydrodynamics to
later times and compute bolometric light curves. Focusing on SNe IIP, we then
(1) directly compare the theoretical STIR explosions to observations and (2)
assess how properties of the progenitor's core can be estimated from optical
photometry in the plateau phase alone. First, the distribution of plateau
luminosities (L$_{50}$) and ejecta velocities achieved by our simulations is
similar to the observed distributions. Second, we fit our models to the light
curves and velocity evolution of some well-observed SNe. Third, we recover
well-known correlations, as well as the difficulty of connecting any one SN
property to zero-age main sequence mass. Finally, we show that there is a
usable, linear correlation between iron core mass and L$_{50}$ such that
optical photometry alone of SNe IIP can give us insights into the cores of
massive stars. Illustrating this by application to a few SNe, we find iron core
masses of 1.3-1.5 solar masses with typical errors of ~0.05 solar masses. Data
are publicly available online (\url{https://doi.org/10.5281/zenodo.6631964}).
|
2102.01118v2
|
2021-06-09
|
Turbulent transport of impurities in 3D devices
|
A large diffusive turbulent contribution to the radial impurity transport in
Wendelstein 7-X (W7-X) plasmas has been experimentally inferred during the
first campaigns and numerically confirmed by means of gyrokinetic simulations
with the code stella. In general, the absence of strong impurity accumulation
during the initial W7-X campaigns is attributed to this diffusive term. In the
present work the diffusive contribution is also calculated in other stellarator
plasmas. In particular, the diffusion (D) and convection (V) coefficients of
carbon and iron impurities produced by ion-temperature-gradient (ITG)
turbulence are obtained for W7-X, LHD, TJ-II and NCSX. The results show that,
although the size of D and V can differ across the four devices, inward
convection is found for all of them. For W7-X, TJ-II and NCSX the two
coefficients are comparable and the turbulent peaking factor is surprisingly
similar. In LHD, appreciably weaker diffusive and convective impurity transport
and significantly larger turbulent peaking factor are predicted. All this
suggests that ITG turbulence, although not strongly, would lead to negative
impurity density gradients in stellarators. Then, considering mixed ITG/Trapped
Electron Mode (TEM) turbulence for the specific case of W7-X, it has been
quantitatively assessed to what degree pellet fueled reduced turbulence
scenarios feature reduced turbulent transport of impurities as well. The
results for trace iron impurities show that, although their turbulent transport
is not entirely suppressed, a significant reduction of V and a stronger
decrease of D are found. Although the diffusion is still above neoclassical
levels, the neoclassical convection would gain under such conditions a greater
specific weight on the dynamics of impurities in comparison with standard ECRH
scenarios.
|
2106.05017v2
|
2021-06-21
|
Correlations Among STM Observables in Disordered Unconventional Superconductors
|
New developments in scanning tunneling spectroscopy now allow for the
spatially resolved measurement of the Josephson critical current $I_c$ between
a tip and a superconducting sample, a nearly direct measurement of the true
superconducting order parameter. However, it is unclear how these $I_c$
measurements are correlated with previous estimates of the spectral gap taken
from differential conductance measurements. In particular, recent such
experiments on an iron-based superconductor found almost no correlation between
$I_c$ and the spectral gap obtained from differential conductance $g=dI/dV$
spectra, reporting instead a more significant correlation between $I_c$ and the
the coherence-peak height. Here we point out that the correlation--or the lack
thereof--between these various quantities can be naturally explained by the
effect of disorder on unconventional superconductivity. Using large scale
numerical simulations of a BCS $d$-wave pair Hamiltonian with many-impurity
potentials, we observe that "substitutional" disorder models with weak
pointlike impurities lead to a situation in which the true superconducting
order parameter and $I_c$ are both uncorrelated with the spectral gap from
$dI/dV$ measurements and highly correlated with the coherence-peak heights. The
underlying mechanism appears to be the disorder-induced transfer of spectral
weight away from the coherence peaks. On the other hand, smooth impurity
potentials with a length scale larger than the lattice constant lead to a large
positive correlation between the true superconducting order parameter and the
spectral gap, in addition to a large correlation between the order parameter
and the coherence-peak height. We discuss the applicability of our results to
recent Josephson scanning tunneling spectroscopy experiments on iron-based and
cuprate high-temperature superconductors.
|
2106.11091v2
|
2021-06-24
|
X-ray Emission of the $γ$-ray Loud Young Radio Galaxy NGC 3894
|
The radio source 1146+596 is hosted by an elliptical/S0 galaxy NGC\,3894,
with a low-luminosity active nucleus. The radio structure is compact,
suggesting a very young age of the jets in the system. Recently, the source has
been confirmed as a high-energy (HE, $>0.1$\,GeV) $\gamma$-ray emitter, in the
most recent accumulation of the {\it Fermi} Large Area Telescope (LAT) data.
Here we report on the analysis of the archival {\it Chandra} X-ray Observatory
data for the central part of the galaxy, consisting of a single 40\,ksec-long
exposure. We have found that the core spectrum is best fitted by a combination
of an ionized thermal plasma with the temperature of $\simeq 0.8$\,keV, and a
moderately absorbed power-law component (photon index $\Gamma = 1.4\pm 0.4$,
hydrogen column density $N_{\rm H}/10^{22}$\,cm$^{-2}$\,$= 2.4\pm 0.7$). We
have also detected the iron K$\alpha$ line at $6.5\pm 0.1$\,keV, with a large
equivalent width of EW\,$= 1.0_{-0.5}^{+0.9}$\,keV. Based on the simulations of
the {\it Chandra}'s Point Spread Function (PSF), we have concluded that, while
the soft thermal component is extended on the scale of the galaxy host, the
hard X-ray emission within the narrow photon energy range 6.0--7.0\,keV
originates within the unresolved core (effectively the central kpc radius). The
line is therefore indicative of the X-ray reflection from a cold neutral gas in
the central regions of NGC\,3894. We discuss the implications of our findings
in the context of the X-ray Baldwin effect. NGC\,3894 is the first young radio
galaxy detected in HE $\gamma$-rays with the iron K$\alpha$ line.
|
2106.13193v2
|
2021-06-30
|
The Inhomogeneity of Composition along the Magnetic Cloud Axis
|
It is generally accepted that CMEs result from eruptions of magnetic flux
ropes, which are dubbed as magnetic clouds in interplanetary space. The
composition (including the ionic charge states and elemental abundances) is
determined prior to and/or during CME eruptions in the solar atmosphere, and
does not alter during magnetic cloud propagation to 1 AU and beyond. It has
been known that the composition is not uniform within a cross section
perpendicular to magnetic cloud axis, and the distribution of ionic charge
states within a cross section provides us an important clue to investigate the
formation and eruption processes of flux ropes due to the freeze-in effect. The
flux rope is a three dimensional magnetic structure intrinsically, and it
remains unclear whether the composition is uniform along the flux rope axis as
most magnetic clouds are only detected by one spacecraft. In this paper we
report a magnetic cloud that was observed by ACE near 1 AU on 1998 March 4--6
and Ulysses near 5.4 AU on March 24--28 sequentially. At these times, both
spacecraft were located around the ecliptic plane, and the latitudinal and
longitudinal separations between them were $\sim$2.2$^{\circ}$ and
$\sim$5.5$^{\circ}$, respectively. It provides us an excellent opportunity to
explore the axial inhomogeneity of flux rope composition, as both spacecraft
almost intersected the cloud center at different sites along its axis. Our
study shows that the average values of ionic charge states exhibit significant
difference along the axis for carbon, and the differences are relatively slight
but still obvious for charge states of oxygen and iron, as well as the
elemental abundances of iron and helium. Besides the means, the composition
profiles within the cloud measured by both spacecraft also exhibit some
discrepancies. We conclude that the inhomogeneity of composition exists along
the cloud axis.
|
2106.15834v1
|
2021-06-30
|
Spectroscopy and photometry of the least-massive Type-II globular clusters: NGC1261 AND NGC6934
|
Recent work has revealed two classes of Globular Clusters (GCs), dubbed
Type-I and Type-II. Type-II GCs are characterized by a blue- and a red- red
giant branch composed of stars with different metallicities, often coupled with
distinct abundances in the slow-neutron capture elements (s-elements). Here we
continue the chemical tagging of Type-II GCs by adding the two least-massive
clusters of this class, NGC1261 and NGC6934. Based on both spectroscopy and
photometry, we find that red stars in NGC1261 are slightly enhanced in [Fe/H]
by ~0.1 dex and confirm that red stars of NGC 6934 are enhanced in iron by ~0.2
dex. Neither NGC1261 nor NGC6934 show internal variations in the s-elements,
which suggests a GC mass threshold for the occurrence of s-process enrichment.
We found a significant correlation between the additional Fe locked in the red
stars of Type-II GCs and the present-day mass of the cluster. Nevertheless,
most Type II GCs retained a small fraction of Fe produced by SNe II, lower than
the 2%; NGC6273, M54 and omega Centauri are remarkable exceptions. In the
appendix, we infer for the first time chemical abundances of Lanthanum, assumed
as representative of the s-elements, in M54, the GC located in the nucleus of
the Sagittarius dwarf galaxy. Red-sequence stars are marginally enhanced in
[La/Fe] by 0.10\pm0.06 dex, in contrast with the large [La/Fe] spread of most
Type II GCs. We suggest that different processes are responsible for the
enrichment in iron and s-elements in Type-II GCs.
|
2106.15978v1
|
2021-07-02
|
Ti I lines at 2.2 $μ$m as probes of the cool parts of sunspots
|
The sunspot umbra harbors the coolest plasma on the solar surface due to the
presence of strong magnetic fields. The routinely used atomic lines to observe
the photosphere have weak signals in the umbra and are often swamped by
molecular lines. This makes it harder to infer the properties of the umbra,
especially in the darkest regions. The lines of the Ti I multiplet at 2.2
$\mu$m are formed mainly at temperatures $\le\!4500$ K and are not known to be
affected by molecular blends in sunspots. Since the first systematic
observations in the 1990's, these lines have been seldom observed due to the
instrumental challenges involved at these longer wavelengths. We revisit these
lines and investigate their formation in different solar features. We
synthesize the Ti I multiplet using a snapshot from 3D MHD simulation of a
sunspot and explore the properties of two of its lines in comparison with two
commonly used iron lines at 630.25 nm and $1.5648\,\mu$m. We find that the Ti I
lines have stronger signals than the Fe I lines in both intensity and
polarization in the sunspot umbra and in penumbral spines. They have little to
no signal in the penumbral filaments and the quiet Sun, at $\mu=1$. Their
strong and well-split profiles in the dark umbra are less affected by stray
light. Consequently, inside the sunspot it is easier to invert these lines and
to infer the atmospheric properties, compared to the iron lines. The Cryo-NIRSP
instrument at the DKIST will provide the first ever high resolution
observations in this wavelength range. In this preparatory study, we
demonstrate the unique temperature and magnetic sensitivities of the Ti
multiplet, by probing the Sun's coolest regions which are not favourable for
the formation of other commonly used spectral lines. We thus expect such
observations to advance our understanding of sunspot properties.
|
2107.01247v1
|
2022-07-23
|
Data Adaptive Regularization for Abdominal Quantitative Susceptibility Mapping
|
Purpose: To improve repeatability and reproducibility across acquisition
parameters and reduce bias in quantitative susceptibility mapping (QSM) of the
liver, through development of an optimized regularized reconstruction algorithm
for abdominal QSM.
Theory and Methods: An optimized approach to estimation of magnetic
susceptibility distribution is formulated as a constrained reconstruction
problem that incorporates estimates of the input data reliability and
anatomical priors available from chemical shift-encoded imaging. The proposed
data-adaptive method was evaluated with respect to bias, repeatability, and
reproducibility in a patient population with a wide range of liver iron
concentration (LIC). The proposed method was compared to the state-of-the-art
approach in liver QSM for two multi-echo SGRE protocols with different
acquisition parameters at 3T.
Results: The data-adaptive method produced susceptibility maps with higher
subjective quality due to reduced shading artifacts. For both acquisition
protocols, higher linear correlation with both $R_2$ and $R_2^*$-based
measurements were observed for the data-adaptive method ($r^2=0.74/0.72$ for
$R_2$, $0.98/0.99$ for $R_2^*$) than the standard method ($r^2=0.62/0.67$ and
$0.84/0.91$). For both protocols, the data-adaptive method enabled better
test-retest repeatability (repeatability coefficients 0.14/0.14ppm for the
data-adaptive method, 0.26/0.31ppm for the standard method) and reproducibility
across protocols (reproducibility coefficient 0.25ppm vs 0.36ppm) than the
standard method.
Conclusions: The proposed data-adaptive QSM algorithm may enable
quantification of liver iron concentration with improved
repeatability/reproducibility across different acquisition parameters as 3T.
|
2207.11416v1
|
2022-07-27
|
The breakdown of both strange metal and superconducting states at a pressure-induced quantum critical point in iron-pnictide superconductors
|
The strange metal (SM) state, characterized by a linear-in-temperature
resistivity, is often seen in the normal state of high temperature
superconductors. It is believed that the SM state is one of the keys to
understand the underlying mechanism of high-Tc superconductivity. Here we
report the first observation of the concurrent breakdown of the SM normal state
and superconductivity at a pressure-induced quantum critical point in an
iron-pnictide superconductor, Ca10(Pt4As8)((Fe0.97Pt0.03)2As2)5. We find that,
upon suppressing the superconducting state by applying pressure, the power
exponent changes from 1 to 2, and the corresponding coefficient A, the slope of
the temperature-linear resistivity per FeAs layer, gradually diminishes. At a
critical pressure (12.5 GPa), A and Tc go to zero concurrently,where a quantum
phase transition (QPT) from a superconducting state with a SM normal state to a
non-superconducting Fermi liquid state takes place. Scaling analysis on the
results obtained from the pressurized 1048 superconductor reveals that A and Tc
have a positive relation, which exhibits a similarity with that is seen in
other chemically-doped unconventional superconductors, regardless of the type
of the tuning method (doping or pressurizing), the crystal structure, the bulk
or film superconductors and the nature of dopant. These results suggest that
there is a simple but powerful organizational principle of connecting the SM
normal state with the high-Tc superconductivity.
|
2207.13272v2
|
2017-05-29
|
Baby MIND: A magnetized segmented neutrino detector for the WAGASCI experiment
|
T2K (Tokai-to-Kamioka) is a long-baseline neutrino experiment in Japan
designed to study various parameters of neutrino oscillations. A near detector
complex (ND280) is located 280~m downstream of the production target and
measures neutrino beam parameters before any oscillations occur. ND280's
measurements are used to predict the number and spectra of neutrinos in the
Super-Kamiokande detector at the distance of 295~km. The difference in the
target material between the far (water) and near (scintillator, hydrocarbon)
detectors leads to the main non-cancelling systematic uncertainty for the
oscillation analysis. In order to reduce this uncertainty a new
WAter-Grid-And-SCintillator detector (WAGASCI) has been developed. A magnetized
iron neutrino detector (Baby MIND) will be used to measure momentum and charge
identification of the outgoing muons from charged current interactions. The
Baby MIND modules are composed of magnetized iron plates and long plastic
scintillator bars read out at the both ends with wavelength shifting fibers and
silicon photomultipliers. The front-end electronics board has been developed to
perform the readout and digitization of the signals from the scintillator bars.
Detector elements were tested with cosmic rays and in the PS beam at CERN. The
obtained results are presented in this paper.
|
1705.10406v3
|
2017-06-15
|
$η$ Carinae's Dusty Homunculus Nebula from Near-Infrared to Submillimeter Wavelengths: Mass, Composition, and Evidence for Fading Opacity
|
Infrared observations of the dusty, massive Homunculus Nebula around the
luminous blue variable $\eta$ Carinae are crucial to characterize the mass-loss
history and help constrain the mechanisms leading to the Great Eruption. We
present the 2.4 - 670 $\mu$m spectral energy distribution, constructed from
legacy ISO observations and new spectroscopy obtained with the {\em{Herschel
Space Observatory}}. Using radiative transfer modeling, we find that the two
best-fit dust models yield compositions which are consistent with CNO-processed
material, with iron, pyroxene and other metal-rich silicates, corundum, and
magnesium-iron sulfide in common. Spherical corundum grains are supported by
the good match to a narrow 20.2 $\mu$m feature. Our preferred model contains
nitrides AlN and Si$_3$N$_4$ in low abundances. Dust masses range from 0.25 to
0.44 $M_\odot$ but $M_{\rm{tot}} \ge$ 45 $M_\odot$ in both cases due to an
expected high Fe gas-to-dust ratio. The bulk of dust is within a 5$"$ $\times$
7$"$ central region. An additional compact feature is detected at 390 $\mu$m.
We obtain $L_{\rm{IR}}$ = 2.96 $\times$ 10$^6$ $L_\odot$, a 25\% decline from
an average of mid-IR photometric levels observed in 1971-1977. This indicates a
reduction in circumstellar extinction in conjunction with an increase in visual
brightness, allowing 25-40\% of optical and UV radiation to escape from the
central source. We also present an analysis of $^{12}$CO and $^{13}$CO $J =
5-4$ through $9-8$ lines, showing that the abundances are consistent with
expectations for CNO-processed material. The [$^{12}$C~{\sc{ii}}] line is
detected in absorption, which we suspect originates in foreground material at
very low excitation temperatures.
|
1706.05112v1
|
2017-06-29
|
X-shooter spectroscopy of young stellar objects in Lupus: Lithium, iron, and barium elemental abundances
|
With the purpose of performing a homogeneous determination of elemental
abundances for members of the Lupus T association, we analyzed three chemical
elements: lithium, iron, and barium. The aims were: to derive the Li abundance
for ~90% of known class II stars in the Lupus I, II, III, IV clouds; to perform
chemical tagging of a region where few Fe abundance measurements have been
obtained in the past, and no determination of the Ba content has been done up
to now. We also investigated possible Ba enhancement, as this element has
become increasingly interesting in the last years following the evidence of Ba
over-abundance in young clusters, the origin of which is still unknown. Using
X-shooter@VLT, we analyzed the spectra of 89 cluster members, both class II and
III stars. We measured the strength of the Li line and derived the abundance of
this element through equivalent width measurements and curves of growth. For
six class II stars we also measured the Fe and Ba abundances using the spectral
synthesis and the code MOOG. The veiling contribution was taken into account
for all three elements. We find a dispersion in the strength of the Li line at
low Teff and identify three targets with severe Li depletion. The nuclear age
inferred for these highly Li-depleted stars is around 15 Myr, which exceeds the
isochronal one. As in other star-forming regions, no metal-rich members are
found in Lupus, giving support to a recent hypothesis that the Fe abundance
distribution of most of the nearby young regions could be the result of a
common and widespread star formation episode involving the Galactic thin disk.
We find that Ba is over-abundant by ~0.7 dex with respect to the Sun. Since
current theoretical models cannot reproduce this Ba abundance pattern, we
investigated whether this unusually large Ba content might be related to
effects due to stellar parameters, stellar activity, and accretion.
|
1706.09684v1
|
2018-02-02
|
Spectral Evidence for Amorphous Silicates in Least-processed CO Meteorites and Their Parent Bodies
|
Least-processed carbonaceous chondrites (carbonaceous chondrites that have
experienced minimal aqueous alteration and thermal metamorphism) are
characterized by their predominately amorphous iron-rich silicate
interchondrule matrices and chondrule rims. The presence of abundant amorphous
material in a meteorite indicates that the parent body, or at least a region of
the parent body, experienced minimal processing since the time of accretion.
The CO chemical group of carbonaceous chondrites has a significant number of
these least-processed samples. We present visible/near-infrared and
mid-infrared spectra of eight least-processed CO meteorites (petrologic type
3.0-3.1). In the visible/near-infrared, these COs are characterized by a broad
weak feature that was first observed by Cloutis et al. (2012) to be at 1.3-um
and attributed to iron-rich amorphous silicate matrix materials. This feature
is observed to be centered at 1.4-um for terrestrially unweathered,
least-processed CO meteorites. At mid-infrared wavelengths, a 21-um feature,
consistent with Si-O vibrations of amorphous materials and glasses, is also
present. This spectral signature is absent in both the near- and mid-infrared
spectra of higher metamorphic grade COs because this material has
recrystallized as crystalline olivine. Furthermore, spectra of least-processed
primitive meteorites from other chemical groups (CRs, MET 00426 and QUE 99177,
and C2-ungrouped Acfer 094), also exhibit a 21-um feature. Thus, we conclude
that the 1.4- and 21-umm features are characteristic of primitive
least-processed meteorites from all chemical groups of carbonaceous chondrites.
Finally, we present an IRTF+SPeX observation of asteroid (93) Minerva that has
spectral similarities in the visible/near-infrared to the least-processed CO
carbonaceous chondrites. Minerva is likely the least-processed CO-like asteroid
observed to date.
|
1802.00830v1
|
2018-02-08
|
Orbital Origin of Extremely Anisotropic Superconducting Gap in Nematic Phase of FeSe Superconductor
|
The iron-based superconductors are characterized by multiple-orbital physics
where all the five Fe 3$d$ orbitals get involved. The multiple-orbital nature
gives rise to various novel phenomena like orbital-selective Mott transition,
nematicity and orbital fluctuation that provide a new route for realizing
superconductivity. The complexity of multiple-orbital also asks to disentangle
the relationship between orbital, spin and nematicity, and to identify dominant
orbital ingredients that dictate superconductivity. The bulk FeSe
superconductor provides an ideal platform to address these issues because of
its simple crystal structure and unique coexistence of superconductivity and
nematicity. However, the orbital nature of the low energy electronic
excitations and its relation to the superconducting gap remain controversial.
Here we report direct observation of highly anisotropic Fermi surface and
extremely anisotropic superconducting gap in the nematic state of FeSe
superconductor by high resolution laser-based angle-resolved photoemission
measurements. We find that the low energy excitations of the entire hole pocket
at the Brillouin zone center are dominated by the single $d_{xz}$ orbital. The
superconducting gap exhibits an anti-correlation relation with the $d_{xz}$
spectral weight near the Fermi level, i.e., the gap size minimum (maximum)
corresponds to the maximum (minimum) of the $d_{xz}$ spectral weight along the
Fermi surface. These observations provide new insights in understanding the
orbital origin of the extremely anisotropic superconducting gap in FeSe
superconductor and the relation between nematicity and superconductivity in the
iron-based superconductors.
|
1802.02940v1
|
2018-02-13
|
Probing the use of spectroscopy to determine the meteoritic analogues of meteors
|
Determining the source regions of meteorites is one of the major goals of
current research in planetary science. Whereas asteroid observations are
currently unable to pinpoint the source regions of most meteorite classes,
observations of meteors with camera networks and the subsequent recovery of the
meteorite may help make progress on this question. The main caveat of such an
approach, however, is that the recovery rate of meteorite falls is low,
implying that the meteoritic analogues of at least 80% of the observed falls
remain unknown.
Aims: Spectroscopic observations of bolides may have the potential to
mitigate this problem by classifying the incoming material.
Methods: To probe the use of spectroscopy to determine the meteoritic
analogues of bolides, we collected emission spectra in the visible range
(320-880nm) of five meteorite types (H,L,LL,CM,eucrite) acquired in atmospheric
entry-like conditions in a plasma wind tunnel at the University of Stuttgart
(Germany). A detailed spectral analysis including line identification and mass
ratio determinations (Mg/Fe,Na/Fe) was subsequently performed on all spectra.
Results: Spectroscopy, via a simple line identification, allows us to
distinguish the main meteorite classes (chondrites, achondrites and irons) but
does not have the potential to distinguish for example an H from a CM
chondrite.
Conclusions: The source location within the main belt of the different
meteorite classes (H, L, LL, CM, etc.) should continue to be investigated via
fireball observation networks. Spectroscopy of incoming bolides only marginally
helps precisely classify the incoming material (iron meteorites only). To reach
a statistically significant sample of recovered meteorites along with accurate
orbits (>100) within a reasonable time frame (10-20 years), the optimal
solution may be the spatial extension of existing fireball observation
networks.
|
1802.04699v2
|
2018-02-14
|
Hydrodynamical Neutron-star Kicks in Electron-capture Supernovae and Implications for the CRAB Supernova
|
Neutron stars (NSs) obtain kicks of typically several 100 km/s at birth. The
gravitational tug-boat mechanism can explain these kicks as consequences of
asymmetric mass ejection during the supernova (SN) explosion. Support for this
hydrodynamic explanation is provided by observations of SN remnants with
associated NSs, which confirm the prediction that the bulk of the explosion
ejecta, in particular chemical elements between silicon and the iron group, are
dominantly expelled in the hemisphere opposite to the direction of the NS kick.
Here, we present a large set of two- and three-dimensional explosion
simulations of electron-capture SNe, considering explosion energies between
~3x10^49 erg and ~1.6x10^50 erg. We find that the fast acceleration of the SN
shock in the steep density gradient delimiting the O-Ne-Mg core of the
progenitor enables such a rapid expansion of neutrino-heated matter that the
growth of neutrino-driven convection freezes out quickly in a high-mode
spherical harmonics pattern. Since the corresponding momentum asymmetry of the
ejecta is very small and the gravitational acceleration by the fast-expanding
ejecta abates rapidly, the NS kick velocities are at most a few km/s. The
extremely low core compactness of O-Ne-Mg-core progenitors therefore favors
hydrodynamic NS kicks much below the ~160 km/s measured for the Crab pulsar.
This suggests either that the Crab Nebula is not the remnant of an
electron-capture SN, but of a low-mass iron-core progenitor, or that the Crab
pulsar was not accelerated by the gravitational tug-boat mechanism but received
its kick by a non-hydrodynamic mechanism such as, e.g., anisotropic neutrino
emission.
|
1802.05274v2
|
2018-02-21
|
Can the relativistic light bending model explain X-ray spectral variations of Seyfert galaxies?
|
Many Seyfert galaxies are known to exhibit Fe-K broad emission line features
in their X-ray energy spectra. The observed lines have three distinct features;
(1) the line profiles are skewed and show significant low-energy tails, (2) the
Fe-K band have low variability, which produces a broad and deep dip in the
root-mean-square (rms) spectra, and (3) photons in this band have time lags
behind those in the adjacent energy bands with amplitudes of several $R_g/c$,
where $R_g$ is the gravitational radius. The "relativistic light bending model"
is proposed to explain these observed features, where a compact X-ray source
(lamp post) above an extreme Kerr black hole illuminates the innermost area of
the accretion disc. In this paper, we critically examine the relativistic light
bending model by computing the rms spectra and the lag features using a
ray-tracing technique, when a lamp post moves vertically on the black hole spin
axis. As a result, we found that the observed deep rms dip requires that the
iron is extremely overabundant ($\gtrsim10$ solar), whereas the observed lag
amplitude is consistent with the normal iron abundance. Furthermore,
disappearance of the lag in the high-flux state requires a source height as
high as $\sim40\,R_g$, which contradicts the relativistically broad emission
line feature. Our simulations agree with the data that the reverberation
feature moves to lower frequencies with larger source height, however, if this
scenario is correct, the simulations predict detection of a clear Fe-K lag at
low frequencies, which is not constrained in the data. Therefore, we conclude
that the relativistic light bending model may not explain the characteristic
Fe-K spectral variations in Seyfert galaxies.
|
1802.07554v2
|
2018-02-21
|
Broad absorption line symbiotic stars: highly ionized species in the fast outflow from MWC 560
|
In symbiotic binaries, jets and disk winds may be integral to the physics of
accretion onto white dwarfs from cool giants. The persistent outflow from
symbiotic star MWC 560 (=V694 Mon) is known to manifest as broad absorption
lines (BALs), most prominently at the Balmer transitions. We report the
detection of high-ionization BALs from C IV, Si IV, N V, and He II in
International Ultraviolet Explorer spectra obtained on 1990 April 29-30, when
an optical outburst temporarily erased the obscuring 'iron curtain' of
absorption troughs from Fe II and similar ions. The C IV and Si IV BALs reached
maximum radial velocities at least 1000 km/s higher than contemporaneous Mg II
and He II BALs; the same behaviors occur in the winds of quasars and
cataclysmic variables. An iron curtain lifts to unveil high-ionization BALs
during the P Cygni phase observed in some novae, suggesting by analogy a
temporary switch in MWC 560 from persistent outflow to discrete mass ejection.
At least three more symbiotic stars exhibit broad absorption with blue edges
faster than 1500 km/s; high-ionization BALs have been reported in AS 304
(=V4018 Sgr), while transient Balmer BALs have been reported in Z And and CH
Cyg. These BAL-producing fast outflows can have wider opening angles than has
been previously supposed. BAL symbiotics are short-timescale laboratories for
their giga-scale analogs, broad absorption line quasars (BALQSOs), which
display a similarly wide range of ionization states in their winds.
|
1802.07743v2
|
2018-02-28
|
Late metal-silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling
|
The short-lived $^{182}$Hf-$^{182}$W decay system is a powerful chronometer
for constraining the timing of metal-silicate separation and core formation in
planetesimals and planets. Neutron capture effects on W isotopes, however,
significantly hamper the application of this tool. In order to correct for
neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron
dosimeter. This study applies this method to IAB iron meteorites, in order to
constrain the timing of metal segregation on the IAB parent body. The
$\epsilon^{182}$W values obtained for the IAB iron meteorites range from -3.61
$\pm$ 0.10 to -2.73 $\pm$ 0.09. Correlating $\epsilon^{\mathrm{i}}$Pt with
$^{182}$W data yields a pre-neutron capture $^{182}$W of -2.90 $\pm$ 0.06. This
corresponds to a metal-silicate separation age of 6.0 $\pm$ 0.8 Ma after CAI
for the IAB parent body, and is interpreted to represent a body-wide melting
event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic
break-up and subsequent reassembly of the parent body. Thermal models of the
interior evolution that are consistent with these estimates suggest that the
IAB parent body underwent metal-silicate separation as a result of internal
heating by short-lived radionuclides and accreted at around 1.4 $\pm$ 0.1 Ma
after CAIs with a radius of greater than 60 km.
|
1802.10421v1
|
2018-07-05
|
V902 Monocerotis: a likely disc-accreting intermediate polar
|
Aims: We aim to confirm whether the eclipsing cataclysmic variable V902 Mon
is an Intermediate Polar, to characterise its X-ray spectrum and flux, and to
refine its orbital ephemeris and spin period. Methods: We performed
spectrographic observations of V902 Mon in 2016 with the 2.2m Calar Alto
telescope, and X-ray photometry and spectroscopy with XMM-Newton in October
2017. This data was supplemented by several years of AAVSO visual photometry.
Results: We have confirmed V902 Mon as an IP based on detecting the spin
period, with a value of 2,208s, at multiple epochs. Spectroscopy of the donor
star and Gaia parallax yield a distance of 3.5+1.3-0.9, kpc, suggesting an
X-ray luminosity one or two orders of magnitude lower than the 10^33 erg/s
typical of previously known IPs. The X-ray to optical flux ratio is also very
low. The inclination of the system is more than 79deg, with a most likely value
of around 82deg. We have refined the eclipse ephemeris, stable over 14,000
cycles. The Halpha line is present throughout the orbital cycle and is clearly
present during eclipse, suggesting an origin distant from the white dwarf, and
shows radial velocity variations at the orbital period. The amplitude and
overall recessional velocity seem inconsistent with an origin in the disc. The
\emph{XMM-Newton} observation reveals a partially absorbed plasma model typical
of magnetic CVs, with a fluorescent iron line at 6.4keV showing a large
equivalent width of 1.4keV. Conclusions: V902 Mon is an IP, and probably a
member of the hypothesized X-ray underluminous class of IPs. It is likely to be
a disc accretor, though the radial velocity behaviour of the Halpha line
remains puzzling. The large equivalent width of the fluorescent iron line, the
small FX/Fopt ratio, and the only marginal detection of X-ray eclipses suggests
that the X-ray emission arises from scattering.
|
1807.01981v1
|
2018-07-05
|
Mineralogy, structure and habitability of carbon-enriched rocky exoplanets: A laboratory approach
|
Carbon-enriched rocky exoplanets have been proposed around dwarf stars as
well as around binary stars, white dwarfs and pulsars. However, the
mineralogical make up of such planets is poorly constrained. We performed
high-pressure high-temperature laboratory experiments ($P$ = 1$-$2 GPa, $T$ =
1523$-$1823 K) on carbon-enriched chemical mixtures to investigate the deep
interiors of Pluto- to Mars-size planets the upper mantles of larger planets.
Our results show that these exoplanets, when fully-differentiated, comprise a
metallic core, a silicate mantle and a graphite layer on top of the silicate
mantle. The silicate mineralogy (olivine, orthopyroxene, clinopyroxene and
spinel) is largely unaffected by the amount of carbon. Metals are either two
immiscible iron-rich alloys (S-rich and S-poor) or a single iron-rich alloy in
the Fe-C-S system with immiscibility depending on the S/Fe ratio and core
pressure. Graphite is the dominant carbon-bearing phase at the conditions of
our experiments with no traces of silicon carbide or carbonates. If the bulk
carbon content is higher than needed to saturate the mantle and the core,
graphite would be in the form of an additional layer on top of the silicate
mantle assuming differentiation. For a thick enough graphite layer, diamonds
would form at the bottom of this layer due to high pressures.
We model the interior structure of Kepler-37b and show that a mere 10 wt%
graphite layer would decrease its derived mass by 7%, suggesting future space
missions that determine both radius and mass of rocky exoplanets with
insignificant gaseous envelopes could provide quantitative limits on their
carbon content. Future observations of rocky exoplanets with graphite-rich
surfaces would show low albedos due to the low reflectance of graphite. The
absence of life-bearing elements other than carbon on the surface likely makes
them uninhabitable.
|
1807.02064v2
|
2018-07-10
|
X-shooting GRBs at high redshift: probing dust production history
|
Evolved asymptotic giant branch (AGB) stars and Type Ia supernovae (SNe) are
important contributors to the elements that form dust in the interstellar
medium of galaxies, in particular, carbon and iron. However, they require at
least a Gyr to start producing these elements, therefore, a change in dust
quantity or properties may appear at high redshifts. In this work, we use
extinction of gamma-ray burst (GRB) afterglows as a tool to look for variations
in dust properties at z>3. We use a spectroscopically selected sample of GRB
afterglows observed with the VLT/X-shooter instrument to determine extinction
curves out to high redshifts. We present ten new z>3 X-shooter GRBs of which
six are dusty. Combining these with individual extinction curves of three
previously known z>3 GRBs, we find an average extinction curve consistent with
the SMC-Bar. A comparison with spectroscopically selected GRBs at all redshifts
indicates a drop in visual extinction (A_V) at z>3.5 with no moderate or high
extinction bursts. We check for observational bias using template spectra and
find that GRBs up to z~8 are detectable with X-shooter up to A_V~0.3 mag.
Although other biases are noted, a uniformly low dust content above z>3.5
indicates a real drop, suggesting a transition in dust properties and/or
available dust building blocks. The remarkable increase in dust content at
z<3.5 could arise due to carbon and possibly iron production by the first
carbon-rich AGB and Type Ia SNe, respectively. Alternatively, z>3.5 dust drop
could be the result of low stellar masses of GRB host galaxies.
|
1807.03597v1
|
2018-07-18
|
Optical dimming of RW Aur associated with an iron rich corona and exceptionally high absorbing column density
|
RW Aur is a binary system composed of two young, low-mass stars. The primary,
RW Aur A, has undergone visual dimming events ($\Delta V =2-3$~mag) in 2011,
2014-16, and 2017-2018. Visual and IR observations indicate a gray absorber
that moved into the line-of-sight. This dimming is also associated with changes
in the outflow. In 2017, when the optical brightness was almost 2~mag below the
long-term average we triggered a Chandra observation to measure the absorbing
column density $N_\mathrm{H}$ and to constrain dust properties and the
gas-to-dust ratio of the absorber. In 2017, the X-ray spectrum is more absorbed
than it was in the optically bright state ($N_\mathrm{H} = (4\pm 1) \times
10^{23}\;\mathrm{cm}^{-2}$) and shows significantly more hot plasma than in
X-ray observations taken before. Also, a new emission feature at $6.63\pm0.02$
keV (statistic) $\pm0.02$ keV (systematic) appeared indicating an Fe abundance
an order of magnitude above Solar, in contrast with previous sub-Solar Fe
abundance measurements. Comparing X-ray absorbing column density $N_\mathrm{H}$
and optical extinction $A_V$, we find that either the gas-to-dust ratio in the
absorber is orders of magnitude higher than in the ISM or the absorber has
undergone significant dust evolution. Given the high column density coupled
with changes in the X-ray spectral shape, this absorber is probably located in
the inner disk. We speculate that a break-up of planetesimals or a terrestrial
planet could supply large grains causing gray absorption; some of these grains
would be accreted and enrich the stellar corona with iron which could explain
the inferred high abundance.
|
1807.06995v1
|
2018-07-25
|
APOGEE Data Releases 13 and 14: Stellar Parameter and Abundance Comparisons With Independent Analyses
|
Data from the SDSS-IV / Apache Point Observatory Galactic Evolution
Experiment (APOGEE-2) have been released as part of SDSS Data Releases 13
(DR13) and 14 (DR14). These include high resolution H-band spectra, radial
velocities, and derived stellar parameters and abundances. DR13, released in
August 2016, contained APOGEE data for roughly 150,000 stars, and DR14,
released in August 2017, added about 110,000 more. Stellar parameters and
abundances have been derived with an automated pipeline, the APOGEE Stellar
Parameter and Chemical Abundance Pipeline (ASPCAP). We evaluate the performance
of this pipeline by comparing the derived stellar parameters and abundances to
those inferred from optical spectra and analysis for several hundred stars. For
most elements -- C, Na, Mg, Al, Si, S, Ca, Cr, Mn, Ni -- the DR14 ASPCAP
analysis have systematic differences with the comparisons samples of less than
0.05 dex (median), and random differences of less than 0.15 dex (standard
deviation). These differences are a combination of the uncertainties in both
the comparison samples as well as the ASPCAP-analysis. Compared to the
references, magnesium is the most accurate alpha-element derived by ASPCAP, and
shows a very clear thin/thick disk separation, while nickel is the most
accurate iron-peak element (besides iron).
|
1807.09784v1
|
2018-08-01
|
Nebular-phase spectra of superluminous supernovae: physical insights from observational and statistical properties
|
We study the spectroscopic evolution of superluminous supernovae (SLSNe)
later than 100 days after maximum light. We present new data for Gaia16apd and
SN2017egm, and analyse these with a larger sample comprising 41 spectra of 12
events. The spectra become nebular within 2-4 $e$-folding times after light
curve peak, with the rate of spectroscopic evolution correlated to the light
curve timescale. Emission lines are identified with well-known transitions of
oxygen, calcium, magnesium, sodium and iron. SLSNe are differentiated from
other Type Ic SNe by a prominent O I $\lambda$7774 line and higher-ionisation
states of oxygen. The iron-dominated region around 5000 \AA\ is more similar to
broad-lined SNe Ic than to normal SNe Ic. Principal Component Analysis shows
that 5 `eigenspectra' capture 75% of the variance, while a clustering analysis
shows no clear evidence for multiple SLSN sub-classes. Line velocities are
5000--8000 km/s, and show stratification of the ejecta. O I $\lambda$7774
likely arises in a dense inner region that also produces calcium emission,
while [O I] $\lambda$6300 comes from further out until 300--400 days. The
luminosities of O I $\lambda$7774 and Ca II suggest significant clumping, in
agreement with previous studies. Ratios of [Ca II]$\lambda$7300/[O
I]$\lambda$6300 favour progenitors with relatively massive helium cores, likely
$\gtrsim 6$ M$_\odot$, though more modelling is required here. SLSNe with broad
light curves show the strongest [O I] $\lambda$6300, suggesting larger ejecta
masses. We show how the inferred velocity, density and ionisation structure
point to a central power source.
|
1808.00510v2
|
2018-10-24
|
Fe I in the Beta Pictoris circumstellar gas disk. II. The time variations in the iron circumstellar gas
|
Beta Pictoris is a young planetary system surrounded by a debris disk of dust
and gas. The gas source of this disk could be exocomets (or 'falling and
evaporating bodies', FEBs), which produce refractory elements (Mg, Ca, Fe)
through sublimation of dust grains at several tens of stellar radii. Nearly
1700 high resolution spectra of Beta Pictoris have been obtained from 2003 to
2017 using the HARPS spectrograph. In paper I, we showed that a very high S/N
ratio allows the detection of many weak Fe I lines in more than ten excited
levels, and we derived the physical characteristics of the iron gas in the
disk. The measured temperature of the gas (~1300 K) suggested that it is
produced by evaporation of grains at about 0.3 au (38 Rstar) from the star.
Here we describe the yearly variations of the column densities of all Fe I
components (from both ground and excited levels). The drop in the Fe I ground
level column density after 2011 coincides with a drop in Fe I excited levels
column density, as well as in the Ca II doublet and a ground level Ca I line at
the same epoch. All drops are compatible together with
photoionisation-recombination equilibrium and Beta Pic like relative
abundances, in a medium at 1300 K and at 0.3 au from Beta Pictoris.
Interestingly, this warm medium does not correlate with the numerous exocomets
in the circumstellar environnement of this young star.
|
1810.10421v1
|
2019-01-17
|
Emergence of Superconductivity from Fully Incoherent Normal State in an Iron-Based Superconductor (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$
|
In unconventional superconductors, it is generally believed that
understanding the physical properties of the normal state is a pre-requisite
for understanding the superconductivity mechanism. In conventional
superconductors like niobium or lead, the normal state is a Fermi liquid with a
well-defined Fermi surface and well-defined quasipartcles along the Fermi
surface. Superconductivity is realized in this case by the Fermi surface
instability in the superconducting state and the formation and condensation of
the electron pairs (Cooper pairing). The high temperature cuprate
superconductors, on the other hand, represent another extreme case that
superconductivity can be realized in the underdoped region where there is
neither well-defined Fermi surface due to the pseudogap formation nor
quasiparticles near the antinodal regions in the normal state. Here we report a
novel scenario that superconductivity is realized in a system with well-defined
Fermi surface but without quasiparticles along the Fermi surface in the normal
state. High resolution laser-based angle-resolved photoemission measurements
have been performed on an optimally-doped iron-based superconductor
(Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$. We find that, while sharp superconducting
coherence peaks emerge in the superconducting state on the hole-like Fermi
surface sheets, no quasiparticle peak is present in the normal state. Its
electronic behaviours deviate strongly from a Fermi liquid system. The
superconducting gap of such a system exhibits an unusual temperature dependence
that it is nearly a constant in the superconducting state and abruptly closes
at T$_c$. These observations have provided a new platform to study
unconventional superconductivity in a non-Fermi liquid system.
|
1901.05693v1
|
2019-01-24
|
Anisotropic magnetic excitations and incipient Néel order in Ba(Fe$_{1-x}$Mn$_{x}$)$_{2}$As$_{2}$
|
It is currently understood that high temperature superconductivity (SC) in
the transition metal $(M)$ substituted iron arsenides
Ba(Fe$_{1-x}$$M$$_{x}$)$_{2}$As$_{2}$ is promoted by magnetic excitations with
wave vectors $(\pi,0)$ or $(0,\pi)$. It is known that while a small amount of
Co substitution leads to SC, the same does not occur for Mn for any value of
$x$. In this work, magnetic excitations in the iron arsenides
Ba(Fe$_{1-x}$Mn$_{x}$)$_{2}$As$_{2}$ ($x=0.0$, $0.007$, $0.009$, $0.08$) are
investigated by means of Resonant Inelastic X rays Scattering (RIXS) at the Fe
$L_{3}$-edge, for momentum transfer $\boldsymbol{q}$ along the high symmetry
Brillouin zone $(\pi,0)$ and $(\pi,\pi)$ directions. It is shown that with
increasing Mn content ($x$), the excitations become anisotropic both in
dispersion and lineshape. Both effects are detected even for small values of
$x$, evidencing a cooperative phenomenon between the Mn impurities, that we
ascribe to emerging N\'eel order of the Mn spins. Moreover, for $x=0.08$, the
excitations along $\boldsymbol{q}\parallel(\pi,0)$ are strongly damped and
nearly non dispersive. This result suggests that phases of arsenides containing
local moments at the FeAs layers, as in Mn or Cr substituted phases, do not
support high temperature SC due to absence of the appropriate magnetic
excitations.
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1901.08543v3
|
2019-04-09
|
CsMn$_4$As$_3$: A layered tetragonal transition-metal pnictide compound with antiferromagnetic ground state
|
We report the synthesis and properties of a new layered tetragonal ternary
compound CsMn$_4$As$_3$ (structure: KCu$_4$S$_3$-type, space group: $P4/mmm$,
No. 123 and $Z = 2$). The material is a small band-gap semiconductor and
exhibits an antiferromagnetic ground state associated with Mn spins. The
compound exhibits a signature of a distinct magnetic moment canting event at
150(5)~K with a canting angle of $\approx 0.3^{\circ}$. Although, some features
of the magnetic characteristics of this new compound are qualitatively similar
to those of the related BaMn$_2$As$_2$, the underlying Mn sublattices of the
two materials are quite different. While the Mn square-lattice layers in
BaMn$_2$As$_2$ are equally spaced along the $c$-direction with the interlayer
distance $d_{\rm L\,Ba} = 6.7341(4)$ Ang., the Mn sublattice forms bilayers in
CsMn$_4$As$_3$ with the interlayer distance within a bilayer $d_{\rm L\,Cs} =
3.1661(6)$ Ang. and the distance between the two adjacent bilayers $d_{\rm B} =
7.290(6)$ Ang. This difference in the Mn sublattice is bound to significantly
alter the energy balance between the $J_{1}$, $J_{2}$ and $J_{c}$ exchange
interactions within the J1-J2-Jc model compared to that in BaMn$_2$As$_2$ and
the other related 122 compounds including the well-known iron-arsenide
superconductor parent compound BaFe$_2$As$_2$. Owing to the novelty of its
transition metal sublattice, this new addition to the family of tetragonal
materials related to the iron-based superconductors brings prospects for doping
and pressure studies in the search of new superconducting phases as well as
other exciting correlated-electron properties.
|
1904.04598v1
|
2019-04-09
|
Insulating Parent Phase and Distinct Doping Evolution to Superconductivity in Single-Layer FeSe/SrTiO3 Films
|
The single-layer FeSe/SrTiO3 (FeSe/STO) films have attracted much attention
because of their simple crystal structure, distinct electronic structure and
record high superconducting transition temperature (Tc). The origin of the
dramatic Tc enhancement in single-layer FeSe/STO films and the dichotomy of
superconductivity between single-layer and multiple-layer FeSe/STO films are
still under debate. Here we report a comprehensive high resolution
angle-resolved photoemission spectroscopy and scanning tunneling
microscopy/spectroscopy measurements on the electronic structure evolution with
doping in single-layer and multiple-layer FeSe/STO films. We find that the
single-layer FeSe/STO films have a distinct parent phase and a route of doping
evolution to superconductivity that are fundamentally different from
multiple-layer FeSe/STO films. The parent phase of the single-layer FeSe/STO
films is insulating, and its doping evolution is very similar to that of doping
a Mott insulator in cuprate superconductors. In multiple-layer FeSe/STO films,
high-Tc superconductivity occurs by suppressing the nematic order in the parent
compound with electron doping. The single-layer FeSe/STO films represent the
first clear case in the iron-based superconductors that the parent compound is
an insulator. Our observations of the unique parent state and doping evolution
in the single-layer FeSe/STO films provide key insight in understanding its
record high-Tc superconductivity. They also provide a new route of realizing
superconductivity in iron-based superconductors that is common in high
temperature cuprate superconductors.
|
1904.04662v1
|
2019-06-19
|
Iron abundance distribution in the hot gas of merging galaxy clusters
|
We present XMM-Newton/EPIC observations of six merging galaxy clusters and
study the distributions of their temperature, iron (Fe) abundance and
pseudo-entropy along the merging axis. For the first time, we focus
simultaneously, and in a comprehensive way, on the chemical and thermodynamic
properties of the freshly collided intracluster medium (ICM). The Fe
distribution of these clusters along the merging axis is found to be in good
agreement with the azimuthally-averaged Fe abundance profile in typical
non-cool-core clusters out to $r_{500}$. In addition to showing a moderate
central abundance peak, though less pronounced than in relaxed systems, the Fe
abundance flattens at large radii towards $\sim$0.2-0.3 $Z_\odot$. Although
this shallow metal distribution is in line with the idea that disturbed,
non-cool-core clusters originate from the merging of relaxed, cool-core
clusters, we find that in some cases, remnants of metal-rich and low entropy
cool cores can persist after major mergers. While we obtain a mild
anti-correlation between the Fe abundance and the pseudo-entropy in the (lower
entropy, $K$ = 200-500 keV cm$^2$) inner regions, no clear correlation is found
at (higher entropy, $K$ = 500-2300 keV cm$^2$) outer radii. The apparent
spatial abundance uniformity that we find at large radii is difficult to
explain through an efficient mixing of freshly injected metals, particularly in
systems for which the time since the merger is short. Instead, our results
provide important additional evidence in favour of the early enrichment
scenario - in which the bulk of the metals are released outside galaxies at $z$
> 2-3 - and extend it from cool-core and (moderate) non-cool-core clusters to a
few of the most disturbed merging clusters as well. These results constitute a
first step towards a deeper understanding of the chemical history of merging
clusters.
|
1906.08067v1
|
2019-07-02
|
Neutron star mergers and rare core-collapse supernovae as sources of r-process enrichment in simulated galaxies
|
We use cosmological, magnetohydrodynamical simulations of Milky Way-mass
galaxies from the Auriga project to study their enrichment with rapid neutron
capture (r-process) elements. We implement a variety of enrichment models from
both binary neutron star mergers and rare core-collapse supernovae. We focus on
the abundances of (extremely) metal-poor stars, most of which were formed
during the first ~Gyr of the Universe in external galaxies and later accreted
onto the main galaxy. We find that the majority of metal-poor stars are
r-process enriched in all our enrichment models. Neutron star merger models
result in a median r-process abundance ratio which increases with metallicity,
whereas the median trend in rare core-collapse supernova models is
approximately flat. The scatter in r-process abundance increases for models
with longer delay times or lower rates of r-process producing events. Our
results are nearly perfectly converged, in part due to the mixing of gas
between mesh cells in the simulations. Additionally, different Milky Way-mass
galaxies show only small variation in their respective r-process abundance
ratios. Current (sparse and potentially biased) observations of metal-poor
stars in the Milky Way seem to prefer rare core-collapse supernovae over
neutron star mergers as the dominant source of r-process elements at low
metallicity, but we discuss possible caveats to our models. Dwarf galaxies
which experience a single r-process event early in their history show highly
enhanced r-process abundances at low metallicity, which is seen both in
observations and in our simulations. We also find that the elements produced in
a single event are mixed with ~10^8 Msun of gas relatively quickly,
distributing the r-process elements over a large region.
|
1907.01557v2
|
2019-07-17
|
Heavy metals in intermediate He-rich hot subdwarfs: The chemical composition of HZ44 and HD127493
|
Hot subluminous stars can be spectroscopically classified as subdwarf B (sdB)
and O (sdO) stars. While the latter are predominantly hydrogen deficient, the
former are mostly helium deficient. The atmospheres of most sdOs are almost
devoid of hydrogen, whereas a small group of hot subdwarf stars of mixed H/He
composition exists, showing extreme metal abundance anomalies. Whether such
intermediate helium-rich (iHe) subdwarf stars provide an evolutionary link
between the dominant classes is an open question. The presence of strong Ge,
Sn, and Pb lines in the UV spectrum of HZ$\,$44 suggests a strong enrichment of
heavy elements in this iHe-sdO star and calls for a detailed quantitative
spectral analysis focusing on trans-iron elements. Non-LTE model atmospheres
calculated with TLUSTY are combined with high-quality optical, UV and FUV
spectra of HZ$\,$44 and its hotter sibling HD$\,$127493 to determine their
atmospheric parameters and metal abundance patterns. By collecting atomic data
from literature we succeeded to determine abundances of 29 metals in HZ$\,$44,
including the trans-iron elements Ga, Ge, As, Se, Zr, Sn, and Pb and provide
upper limits for 10 other metals. This makes it the best described hot subdwarf
in terms of chemical composition. For HD$\,$127493 the abundance of 15 metals,
including Ga, Ge, and Pb and upper limits for another 16 metals were derived.
Heavy elements turn out to be overabundant by one to four orders of magnitude
with respect to the Sun. Zr and Pb are among the most enriched elements. The C,
N, and O abundance for both stars can be explained by nucleosynthesis of
hydrogen burning in the CNO cycle along with their helium enrichment. On the
other hand, the heavy-element anomalies are unlikely to be caused by
nucleosynthesis. Instead diffusion processes are evoked with radiative
levitation overcoming gravitational settlement of the heavy elements.
|
1907.07781v1
|
2019-07-29
|
Turbulence and Rotation in Solar-Type Stars
|
Stellar spectra with a high resolution of 115000 obtained with the HARPS
spectrograph provide an opportunity to examine turbulence velocities and their
depth distributions in the photosphere of stars. Fourier analysis was performed
for 17 iron lines in the spectra of 13 stars with an effective temperature of
4900--6200 K and a logarithm of surface gravity of 3.9--5.0 as well as in the
spectrum of the Sun as a star. Models of stellar atmospheres were taken from
the MARCS database. The standard concept of isotropic Gaussian microturbulence
was assumed in this study. A satisfactory fit between the synthesized profiles
of spectral lines and observational data verified the reliability of the
Fourier method. The most likely estimates of turbulence velocities, the
rotation velocity, and the iron abundance and their photospheric depth
distribution profiles were obtained as a result. Microturbulence does not vary
to any significant degree with depth, while macroturbulence has a marked depth
dependence. The macroturbulence velocity increases with depth in the stellar
atmosphere. The higher the effective temperature of a star and the stronger the
surface gravity, the steeper the expected macroturbulence gradient. The mean
macroturbulence velocity increases for stars with higher temperatures, weaker
gravity, and faster rotation. The mean macro- and microturbulence velocities
are correlated with each other and with the rotation velocity in the examined
stars. The ratio between the macroturbulence velocity and the rotation velocity
in solar-type stars varies from 1 (the hottest stars) to 1.7 (the coolest
stars). The age dependence of the rotation velocity is more pronounced than
that of the velocity of macroturbulent motions.
|
1907.12241v1
|
2019-10-07
|
Optical and photoemission investigation of structural and magnetic transitions in the iron-based superconductor Sr$_\mathbf{0.67}$Na$_\mathbf{0.33}$Fe$_\mathbf{2}$As$_\mathbf{2}$
|
We report the temperature-dependent optical conductivity and ARPES studies of
the iron-based superconductor (SC) Sr$_{0.67}$Na$_{0.33}$Fe$_2$As$_2$ in the
high-temperature tetragonal paramagnetic phase; below the structural and
magnetic transitions at $T_{\rm N}\simeq$125 K in the orthorhombic
spin-density-wave (SDW)-like phase, and $T_r\simeq$42 K in the reentrant
tetragonal double-Q magnetic phase where both charge and SDW order exist; and
below the SC transition at $T_c\simeq$10 K. The free-carrier component in the
optical conductivity is described by two Drude contributions; one strong and
broad, the other weak and narrow. The broad Drude component decreases
dramatically below $T_{\rm N}$ and $T_r$, with much of its strength being
transferred to a bound excitation in the mid-infrared, while the narrow Drude
component shows no anomalies at either of the transitions, actually increasing
in strength at low temperature while narrowing dramatically. The behavior of an
infrared-active mode suggests zone-folding below $T_r$. Below $T_c$ the
dramatic decrease in the low-frequency optical conductivity signals the
formation of a SC energy gap. ARPES reveals hole-like bands at the center of
the Brillouin zone (BZ), with both electron- and hole-like bands at the
corners. Below $T_{\rm N}$, the hole pockets at the center of the BZ decrease
in size, consistent with the behavior of the broad Drude component; while below
$T_r$ the electron-like bands shift and split, giving rise to a low-energy
excitation in the optical conductivity at ~20 meV. The magnetic states, with
resulting SDW and charge-SDW order, respectively, lead to a significant
reconstruction of the Fermi surface that has profound implications for the
transport originating from the electron and hole pockets, but appears to have
relatively little impact on the SC in this material.
|
1910.02927v2
|
2019-10-28
|
Type Ia Supernova Explosions from Hybrid Carbon-Oxygen-Neon White Dwarf Progenitors That Have Mixed During Cooling
|
The creation of "hybrid" white dwarfs, made of a C-O core within a O-Ne shell
has been proposed, and studies indicate that ignition in the C-rich central
region makes these viable progenitors for thermonuclear (type Ia) supernovae.
Recent work found that the C-O core is mixed with the surrounding O-Ne as the
white dwarf cools prior to accretion, which results in lower central C
fractions in the massive progenitor than previously assumed. To further
investigate the efficacy of hybrid white dwarfs as progenitors of thermonuclear
supernovae, we performed simulations of thermonuclear supernovae from a new
series of hybrid progenitors that include the effects of mixing during cooling.
The progenitor white dwarf model was constructed with the one-dimensional
stellar evolution code MESA and represented a star evolved through the phase of
unstable interior mixing followed by accretion until it reached conditions for
the ignition of carbon burning. This MESA model was then mapped to a
two-dimensional initial condition for explosions simulated with FLASH. For
comparison, similar simulations were performed for a traditional C-O progenitor
white dwarf. By comparing the yields of the explosions, we find that, as with
earlier studies, the lower C abundance in the hybrid progenitor compared to the
traditional C-O progenitor leads to a lower average yield of 56Ni. Although the
unmixed hybrid WD showed a similar decrement also in total iron group yield,
the mixed case does not and produces a smaller fraction of iron group elements
in the form of 56Ni. We attribute this to the higher central density required
for ignition and the location, center or off-center, of deflagration ignition.
|
1910.12403v1
|
2019-12-15
|
The influence of NLTE effects in Fe I lines on an inverted atmosphere I. 6301 A and 6302 A lines formed in 1D NLTE
|
Ultraviolet over-ionisation of iron atoms in the solar atmosphere leads to
deviations in their level populations from the Saha-Boltzmann statistics. This
causes their line profiles to form in Non-Local Thermodynamic Equilibrium
(NLTE) conditions. While inverting such profiles to determine atmospheric
parameters, the NLTE effects are often neglected and deviations from LTE are
compensated for by tweaking other quantities. We investigate how the routinely
employed LTE inversion of iron lines formed in NLTE under- or over-estimates
atmospheric quantities such as temperature (T), line-of-sight velocity (v_LOS),
magnetic field strength (B) and inclination (gamma) while the previous papers
have focused mainly on T. We synthesize the Stokes profiles of Fe I 6301.5 A
and 6302.5 A lines in both LTE and NLTE using a snapshot of a 3D MHD
simulation. The profiles are then inverted in LTE. By considering the
atmosphere inferred from inversion of LTE profiles to be the fiducial model, we
compare atmosphere from the inversion of NLTE profiles with it. Any differences
observed are attributed to NLTE effects. Neglecting the NLTE effects introduces
errors in the inverted atmosphere. While the errors in T can go up to 13%, in
v_LOS and B the errors can be as high as 50% or more. We find these errors to
be present at all three inversion nodes. Importantly, they survive degradation
from spatial averaging of the profiles. We give an overview of how the neglect
of NLTE effects influences the values of T, v_LOS, B and gamma determined by
inverting Fe I 6300 A line pair, as observed, e.g., by Hinode. Errors are found
at the sites of granules, intergranular lanes, magnetic elements, basically in
every region susceptible to NLTE effects. For an accurate determination of
atmospheric quantities and their stratification, it is therefore important to
take account of NLTE effects.
|
1912.07007v2
|
2020-03-06
|
Studying the Reflection Spectra of the New Black Hole X-ray Binary Candidate MAXI J1631-479 Observed by NuSTAR: A Variable Broad Iron Line Profile
|
We present results from the Nuclear Spectroscopic Telescope Array (NuSTAR)
observations of the new black hole X-ray binary candidate MAXI J1631-479 at two
epochs during its 2018-2019 outburst, which caught the source in a disk
dominant state and a power-law dominant state. Strong relativistic disk
reflection features are clearly detected, displaying significant variations in
the shape and strength of the broad iron emission line between the two states.
Spectral modeling of the reflection spectra reveals that the inner radius of
the optically-thick accretion disk evolves from $<1.9$ $r_{\rm g}$ to $12\pm1$
$r_{\rm g}$ (statistical errors at 90% confidence level) from the disk dominant
to the power-law dominant state. Assuming in the former case that the inner
disk radius is consistent with being at the ISCO, we estimate a black hole spin
of $a^*>0.94$. Given that the bolometric luminosity is similar in the two
states, our results indicate that the disk truncation observed in MAXI
J1631-479 in the power-law dominant state is unlikely to be driven by a global
variation in the accretion rate. We propose that it may instead arise from
local instabilities in the inner edge of the accretion disk at high accretion
rates. In addition, we find an absorption feature in the spectra centered at
$7.33\pm0.03$ keV during the disk dominant state, which is evidence for a rare
case that an extremely fast disk wind ($v_{\rm out}=0.067^{+0.001}_{-0.004}~c$)
is observed in a low-inclination black hole binary, with the viewing angle of
$29\pm1^{\circ}$ as determined by the reflection modeling.
|
2003.03465v1
|
2020-08-04
|
The infancy of core-collapse supernova remnants
|
We present 3D hydrodynamic simulations of neutrino-driven supernovae (SNe)
with the PROMETHEUS-HOTB code, evolving the asymmetrically expanding ejecta
from shock breakout until they reach the homologous expansion phase after
roughly one year. Our calculations continue the simulations for two red
supergiant (RSG) and two blue supergiant (BSG) progenitors by Wongwathanarat et
al., who investigated the growth of explosion asymmetries produced by
hydrodynamic instabilities during the first second of the explosion and their
later fragmentation by Rayleigh-Taylor instabilities. We focus on the late time
acceleration and inflation of the ejecta caused by the heating due to the
radioactive decay of $^{56}$Ni to $^{56}$Fe and by a new outward-moving shock,
which forms when the reverse shock from the He/H-shell interface compresses the
central part of the ejecta. The mean velocities of the iron-rich ejecta
increase between 100 km/s and 350 km/s ($\sim$8-30\%), and the fastest one
percent of the iron accelerates by up to $\sim$1000 km/s ($\sim$20-25\%). This
'Ni-bubble effect', known from 1D models, accelerates the bulk of the nickel in
our 3D models and causes an inflation of the initially overdense Ni-rich
clumps, which leads to underdense, extended fingers, enveloped by overdense
skins of compressed surrounding matter. We also provide volume and surface
filling factors as well as a spherical harmonics analysis to characterize the
spectrum of Ni-clump sizes quantitatively. Three of our four models give volume
filling factors larger than 0.3, consistent with what is suggested for SN 1987A
by observations.
|
2008.01763v2
|
2020-08-13
|
On the detection of surface spin freezing in iron oxide nanoparticles and its long-term evolution under ambient oxidation
|
Exchange bias effects linked to surface spin freezing (SSF) are commonly
found in iron oxide nanoparticles, while signatures of SSF in low-field
temperature-dependent magnetization curves have been much less frequently
reported. Here, we present magnetic properties of dense assemblies of
similar-sized (~ 8 nm diameter) particles synthesized by a magnetite (sample
S1) and a maghemite (sample S2) method, and the influence of long-term (4-year)
sample aging under ambient conditions on these properties. The size of the
exchange bias field of the different sample (fresh or aged) states is found to
correlate with (a) whether a low-temperature hump feature signaling the SSF
transition is detected in out-of-phase ac susceptibility or zero-field-cooled
(ZFC) dc magnetization recorded at low field and with (b) the prominence of
irreversibility between FC and ZFC curves recorded at high field. Sample S1
displays a lower magnetization than S2, and it is in S1 where the largest SSF
effects are found. These effects are significantly weakened by aging but remain
larger than the SSF effects in S2, where the influence of aging is considerably
smaller. A non-saturating component due to spin disorder in S1 also weakens
with aging, accompanied by, we infer, an increase in the superspin and the
radius of the ordered nanoparticle cores. X-ray diffraction and M\"ossbauer
spectroscopy provide indication of maghemite-like stoichiometry in both aged
samples as well as thicker disordered particle shells in aged-S1 relative to
aged-S2 (crystallographically-disordered and spin-disordered according to
diffraction and M\"ossbauer, respectively). The pronounced diminution in SSF
effects with aging in S1 is attributed to a (long-term) transition, caused by
ambient oxidation, from magnetite-like to maghemite-like stoichiometry, and a
concomitant softening of the spin-disordered shell anisotropy...
|
2008.05874v1
|
2020-08-31
|
Structural, magnetic and electronic properties of CaBaCo4-xMxO7 (M= Fe, Zn)
|
The effect of substituting iron and zinc for cobalt in CaBaCo$_4$O$_7$ has
been investigated using neutron diffraction and x-ray absorption spectroscopy.
The orthorhombic distortion present in the parent compound CaBaCo$_4$O$_7$
decreases with increasing the content of either Fe or Zn. The samples
CaBaCo$_3$ZnO$_7$ and CaBaCo$_{4-x}$Fe$_x$O$_7$ with $x \leq 1.5$ are
metrically hexagonal but much better refinements in the neutron diffraction
patterns are obtained using an orthorhombic unit cell. The two types of
substitution have opposite effects on the structural and magnetic properties.
Fe atoms preferentially occupy the sites at the triangular layer. Thus, the
replacement of Co by Fe supresses the ferrimagnetic ordering and
CaBaCo$_{4-x}$Fe$_x$O$_7$ samples are antiferromagnetically ordered with a new
propagation vector k=(1/3,0,0). However, the Zn atoms prefer occupying the
Kagome layer, which is very detrimental for the long range magnetic
interactions giving rise to a magnetic glass. The oxidation state of iron and
zinc is found to be 3+ and 2+, respectively, independently of the content.
Therefore, the average Co oxidation state changes accordingly with the
Fe$^{3+}$ or Zn$^{2+}$ doping. Also, x-ray absorption spectroscopy data
confirms the different preferential occupation for both Fe and Zn cations. The
combined information obtained by neutron diffraction and x-ray absorption
spectroscopy indicates that cobalt atoms can be either in a fluctuating
Co$^{2+}$/Co$^{3+}$ valence state or, alternatively, Co$^{2+}$ and Co$^{3+}$
ions being randomly distributed in the lattice. These results explain the
occurrence of local disorder in the CoO$_4$ tetrahedra obtained by EXAFS. An
anomaly in the lattice parameters and an increase in the local disorder is
observed only at the ferrimagnetic transition for CaBaCo$_4$O$_7$ revealing the
occurrence of local magneto-elastic coupling.
|
2008.13649v1
|
2020-10-14
|
The X-SHOOTER/ALMA sample of Quasars in the Epoch of Reionization. I. NIR spectral modeling, iron enrichment and broad emission line properties
|
We present X-SHOOTER near-infrared spectroscopy of a large sample of 38
luminous ($M_{1450}=-29.0$ to $-24.4$) quasars at $5.78<z<7.54$, which have
complementary CII observations from ALMA. This X-SHOOTER/ALMA sample provides
us with the most comprehensive view of reionization-era quasars to date,
allowing us to connect the quasar properties with those of its host galaxy. In
this work we introduce the sample, discuss data reduction and spectral fitting,
and present an analysis of the broad emission line properties. The measured
FeII/MgII flux ratio suggests that the broad line regions of all quasars in the
sample are already enriched in iron. We also find the MgII line to be on
average blueshifted with respect to the CII redshift with a median of
$-391\,\rm{km}\,\rm{s}^{-1}$. A significant correlation between the MgII-CII
and CIV-CII velocity shifts indicates a common physical origin. Furthermore, we
frequently detect large CIV-MgII emission line velocity blueshifts in our
sample with a median value of $-1848\,\rm{km}\,\rm{s}^{-1}$. While we find all
other broad emission line properties not to be evolving with redshift, the
median CIV-MgII blueshift is much larger than found in low-redshift,
luminosity-matched quasars ($-800\,\rm{km}\,\rm{s}^{-1}$). Dividing our sample
into two redshift bins, we confirm an increase of the average CIV-MgII
blueshift with increasing redshift. Future observations of the rest-frame
optical spectrum with the James Webb Space Telescope will be instrumental in
further constraining the possible evolution of quasar properties in the epoch
of reionization.
|
2010.06902v1
|
2020-10-21
|
Composition and origin of L5 Trojan asteroids of Mars: Insights from spectroscopy
|
We investigate the mineralogy of L5 Martian Trojan asteroids via reflectance
spectroscopy, in particular (101429) 1998 $\mbox{VF}_{31}$, the only L5 Trojan
that does not belong to the Eureka family (Christou, 2013). We find that this
asteroid most likely belongs to the Bus-Demeo S-complex, in agreement with
Rivkin et al. (2007) and obtain good spectral matches with Sq- or S-type
asteroids, the lunar surface and of Martian and lunar meteorites. Mixture
fitting to spectral endmembers suggests a surface abundance of Mg-rich
orthopyroxene and iron metal or, alternatively, a mixture of plagioclase and
metal with a small amount of Mg-poor orthopyroxene. The metallic component may
be part of the intrinsic mineral makeup of the asteroid or an indication of
extreme space weathering.
We discuss several origin scenarios for (101429). The asteroid could be
related to iron-rich primitive achondrites (Rivkin et al.), may have originated
as impact ejecta from Mars - as proposed recently for the Eureka family
asteroids (Polishook et al., 2017) - or could be a relic fragment of the Moon's
original solid crust. If, on the other hand, (101429) is a relatively recent
addition to the Martian Trojan clouds (Christou et al., 2020), its origin is
probably traced to high-inclination asteroid families in the Inner Main Belt.
For the olivine-dominated Eureka family, we find that the two smaller
asteroids are more spectrally similar to one another than to (5261) Eureka.
Spectral profiles of all three asteroids are closely similar shortward of
$\sim$0.7$\mu$m but diverge at longer wavelengths. For the two smaller
asteroids in particular, we find the spectra are virtually identical up to
$0.8$$\mu$m. We attribute spectral differences in the near-IR region to
differences in either: degree of space weathering, olivine chemical composition
and/or regolith grain size.
|
2010.10947v1
|
2020-10-29
|
Spectroscopic Evidence of Superconductivity Pairing at 83 K in Single-Layer FeSe/SrTiO3 Films
|
Single-layer FeSe films grown on the SrTiO3 substrate (FeSe/STO) have
attracted much attention because of their possible record-high superconducting
critical temperature Tc and distinct electronic structures in iron-based
superconductors. However, it has been under debate on how high its Tc can
really reach due to the inconsistency of the results obtained from the
transport, magnetic and spectroscopic measurements. Here we report
spectroscopic evidence of superconductivity pairing at 83 K in single-layer
FeSe/STO films. By preparing high-quality single-layer FeSe/STO films, we
observe for the first time strong superconductivity-induced Bogoliubov
back-bending bands that extend to rather high binding energy ~100 meV by
high-resolution angle-resolved photoemission measurements. The Bogoliubov
back-bending band provides a new definitive benchmark of superconductivity
pairing that is directly observed up to 83 K in the single-layer FeSe/STO
films. Moreover, we find that the superconductivity pairing state can be
further divided into two temperature regions of 64-83 K and below 64 K. We
propose the 64-83 K region may be attributed to superconductivity fluctuation
while the region below 64 K corresponds to the realization of long-range
superconducting phase coherence. These results indicate that either Tc as high
as 83 K is achievable in iron-based superconductors, or there is a pseudogap
formation from superconductivity fluctuation in single-layer FeSe/STO films.
|
2010.15362v1
|
2020-11-03
|
Discovery of a Fast Iron Low-ionization Outflow in the Early Evolution of the Nearby Tidal Disruption Event AT2019qiz
|
We report the results of ultraviolet (UV) and optical photometric and
spectroscopic analysis of the tidal disruption event (TDE) AT2019qiz. Our
follow-up observations started $<$10 days after the source began to brighten in
the optical and lasted for a period of six months. Our late-time host-dominated
spectrum indicates that the host galaxy likely harbors a weak active galactic
nucleus. The initial {\it Hubble Space Telescope (HST)} spectrum of AT2019qiz
exhibits an iron and low-ionization broad absorption line (FeLoBAL) system that
is seen for the first time in a TDE. This spectrum also bears a striking
resemblance to that of Gaia16apd, a superluminous supernova. Our observations
provide insights into the outflow properties in TDEs and show evidence for a
connection between TDEs and engine-powered supernovae at early phase, as
originally suggested in Metzger & Stone (2016). In a time frame of 50 days, the
UV spectra of AT2019qiz started to resemble previous TDEs with only
high-ionization BALs. The change in UV spectral signatures is accompanied by a
decrease in the outflow velocity, which began at $15,000$ km s$^{-1}$ and
decelerated to $\sim10,000$ km s$^{-1}$. A similar evolution in the H$\alpha$
emission line width further supports the speculation that the broad Balmer
emission lines are formed in TDE outflows. In addition, we detect narrow
absorption features on top of the FeLoBAL signatures in the early HST UV
spectrum of AT2019qiz. The measured HI column density corresponds to a
Lyman-limit system whereas the metal absorption lines, such as NV, CIV, FeII,
and MgII, are likely probing the circumnuclear gas and interstellar medium in
the host galaxy.
|
2011.01593v2
|
2020-11-15
|
Three-dimensional Supernova Models Provide New Insights into the Origins of Stardust
|
We present the isotope yields of two post-explosion, three-dimensional 15
$M_\odot$ core-collapse supernova models, 15S and 15A, and compare them to the
carbon, nitrogen, silicon, aluminum, sulfur, calcium, titanium, iron, and
nickel isotopic compositions of SiC stardust. We find that these core-collapse
supernova models predict similar carbon and nitrogen compositions to SiC X
grains and grains with $^{12}$C/$^{13}$C $<$ 20 and $^{14}$N/$^{15}$N $<$ 60,
which we will hereafter refer to as SiC 'D' grains. Material from the interior
of a 15 $M_\odot$ explosion reaches high enough temperatures shortly after core
collapse to produce the large enrichments of $^{13}$C and $^{15}$N necessary to
replicate the compositions of SiC D grains. The innermost ejecta in a
core-collapse supernova is operating in the neutrino-driven regime and
undergoes fast proton capture after being heated by the supernova shockwave.
Both 3-D models predict 0.3 $<$ $^{26}$Al/$^{27}$Al $<$ 1.5, comparable to the
ratios seen in SiC X, C, and D grains. Models 15S and 15A, in general, predict
very large anomalies in calcium isotopes but do compare qualitatively with the
SiC X grain measurements that show $^{44}$Ca and $^{43}$Ca excesses. The
titanium isotopic compositions of SiC X grains are well reproduced. The models
predict $^{57}$Fe excesses and depletions that are observed in SiC X grains,
and in addition predict accurately the $^{60}$Ni/$^{58}$Ni,
$^{61}$Ni/$^{58}$Ni, and $^{62}$Ni/$^{58}$Ni ratios in SiC X grains, as a
result of fast neutron captures initiated by the propagation of the supernova
shockwave. Finally, symmetry has a noticeable effect on the production of
silicon, sulfur, and iron isotopes in the SN ejecta.
|
2011.07459v2
|
2020-11-25
|
Light Quantum Control of Persisting Higgs Modes in Iron-Based Superconductors
|
The Higgs mechanism, i.e., spontaneous symmetry breaking of the quantum
vacuum, is a cross-disciplinary principle, universal for understanding dark
energy, antimatter and quantum materials, from superconductivity to magnetism.
Yet, Higgs modes in one-band superconductors (SCs) are currently under debate
due to their competition with charge-density fluctuations. A distinct Higgs
mode, controllable by terahertz (THz) laser pulses, can arise in multi-band,
unconventional SCs via strong {\em interband} Coulomb interaction, but is yet
to be accessed. Here we both discover and demonstrate quantum control of such
collective mode in iron-based high-temperature superconductors. Using
two-pulse, phase coherent THz spectroscopy, we observe a tunable and coherent
2$\Delta_{\mathrm{SC}}$ amplitude oscillation of the complex order parameter in
such SC with coupled lower and upper bands. The nonlinear dependence of the
amplitude mode oscillations on the THz driving fields is distinct from any
one-band and conventional SC results: we observe a large nonlinear change of
resonance strength, yet with a persisting mode frequency. We argue that this
result provides compelling evidence for a transient coupling between the
electron and hole amplitude modes via strong interband coherent interaction. To
support this scenario, we perform quantum kinetic modeling of a hybrid Higgs
mechanism without invoking extra disorder or phonons. In addition to
distinguishing between collective modes and charge fluctuations, the light
quantum control of multiband SCs can be extended to probe and manipulate
many-body entanglement and hidden symmetries in different quantum materials.
|
2011.13036v2
|
2020-11-27
|
XMM-Newton observations of the extremely X-ray luminous quasar CFHQS J142952+544717=SRGE J142952.1+544716 at redshift z=6.18
|
We present results from a 20 ks XMM-Newton DDT observation of the radio-load
quasar CFHQS J142952+544717 at z=6.18, whose extreme X-ray luminosity was
recently revealed by the SRG/eROSITA telescope in the course of its first
all-sky survey. The quasar has been confidently detected with a total of $\sim
1400$ net counts in the 0.2-10 keV energy band (1.4 to 72 keV in the object's
rest frame). Its measured spectrum is unusually soft and can be described by an
absorbed power-law model with a photon index of $\Gamma = 2.5\pm0.2$. There are
no signs of a high-energy cutoff or reflected component, with an 90 % upper
limit on the fluorescence iron K$\alpha$ equivalent width of $\approx 290$ eV
and the corresponding upper limit on the iron K-edge absorption depth of 0.6.
We have detected, at the $> 95\%$ confidence level, an excess absorption above
the Galactic value, corresponding to a column density $N_H= 3\pm2 \times
10^{22}$ cm$^{-2}$ of material located at z=6.18. The intrinsic luminosity of
CFHQS J142952+544717 in the 1.4 to 72 keV energy band is found to be
$5.5_{-0.6}^{+0.8} \times 10^{46}$ erg s$^{-1}$. We did not detect
statistically significant flux changes between two SRG scans and the XMM-Newton
observation, spanning over $\sim 7.5$ months, implying that the quasar remained
at this extremely high luminosity level for at least a month in its rest frame.
We put forward the hypothesis that the extreme X-ray properties of CFHQS
J142952+544717 are associated with inverse Compton scattering of cosmic
microwave background photons (at z=6.18) in its relativistic jets.
|
2011.13724v2
|
2020-12-02
|
Synthesis and Characterization of Sodium Iron Antimonate Na2FeSbO5 One-Dimensional Antiferromagnetic Chain Compound with a Spin Glass Ground State
|
A new oxide, sodium iron antimonate, Na2FeSbO5, was synthesized and
structurally characterized, and its static and dynamic magnetic properties were
comprehensively studied both experimentally by dc and ac magnetic
susceptibility, magnetization, specific heat, electron spin resonance (ESR) and
Moessbauer measurements, and theoretically by density functional calculations.
The resulting single-crystal structure (a = 15.6991(9) A; b = 5.3323 (4) A; c =
10.8875(6) A; S.G. Pbna) consists of edge-shared SbO6 octahedral chains, which
alternate with vertex-linked, magnetically active FeO4 tetrahedral chains. The
57Fe Moessbauer spectra confirmed the presence of high-spin Fe3+ (3d5) ions in
a distorted tetrahedral oxygen coordination. The magnetic susceptibility and
specific heat data show the absence of a long-range magnetic ordering in
Na2FeSbO5 down to 2 K, but ac magnetic susceptibility unambigously demonstrates
spin-glass-type behavior with a unique two-step freezing at Tf1 about 80 K and
Tf2 about 35 K. Magnetic hyperfine splitting of 57Fe Moessbauer spectra was
observed below T* about 104 K (Tf1 < T*). The spectra just below T* (Tf1 < T <
T*) exhibit a relaxation behavior caused by critical spin fluctuations,
indicating the existence of short-range correlations. The stochastic model of
ionic spin relaxation was used to account for the shape of the Moessbauer
spectra below the freezing temperature. A complex slow dynamics is further
supported by ESR data revealing two different absorption modes presumably
related to ordered and disordered segments of spin chains. The data imply a
spin-cluster ground state for Na2FeSbO5.
|
2012.01106v1
|
2021-01-02
|
Influence of NLTE effects in Fe I lines on inverted atmosphere II. 6301 A and 6302 A lines formed in 3DNLTE
|
This paper forms the second part of our study on how the neglect of NLTE
conditions in the formation of Fe I 6301.5 A and the 6302.5 A lines influences
the atmosphere obtained by inverting their profiles in LTE. The main cause of
NLTE effects is the line opacity deficit due to the excess ionization of the Fe
I atoms by the UV photons in the Sun. In the first paper, the above
photospheric lines were assumed to have formed in 1DNLTE and the effects of
horizontal radiation transfer (RT) were neglected. In the present paper, the
iron lines are computed in 3DNLTE. We investigate the influence of horizontal
RT on the inverted atmosphere and how it can enhance or reduce the errors due
to the neglect of 1DNLTE effects. The iron lines are computed in LTE, 1DNLTE
and 3DNLTE. They all are inverted using an LTE inversion code. The atmosphere
from the inversion of LTE profiles is taken as the reference model. The test
atmospheres from the inversion of 1DNLTE and 3DNLTE profiles are compared with
it. The differences between models are analysed and correspondingly attributed
to NLTE and 3D effects. The effects of horizontal RT are evident in regions
surrounded by strong horizontal gradients in temperature. In some regions, the
3D effects enhance the 1DNLTE effects while in some, they weaken. The errors
due to neglecting the 3D effects are less than 5% in temperature while the
errors are mostly less than 20% in both velocity and magnetic field strength.
These errors are found to survive spatial and spectral degradation. The neglect
of horizontal RT is found to introduce errors in the derived atmosphere. How
large the errors are depends on how strong the local horizontal gradients are
in temperature. Compared to the 1DNLTE effect, the 3D effects are more
localised to specific regions in the atmosphere and overall less dominant.
|
2101.00506v1
|
2021-01-24
|
Exploring the diversity of double detonation explosions for type Ia supernovae: Effects of the post-explosion helium shell composition
|
The detonation of a helium shell on top of a carbon-oxygen white dwarf has
been argued as a potential explosion mechanism for type Ia supernovae (SNe~Ia).
The ash produced during helium shell burning can lead to light curves and
spectra that are inconsistent with normal SNe~Ia, but may be viable for some
objects showing a light curve bump within the days following explosion. We
present a series of radiative transfer models designed to mimic predictions
from double detonation explosion models. We consider a range of core and shell
masses, and systematically explore multiple post-explosion compositions for the
helium shell. We find that a variety of luminosities and timescales for early
light curve bumps result from those models with shells containing $^{56}$Ni,
$^{52}$Fe, or $^{48}$Cr. Comparing our models to SNe~Ia with light curve bumps,
we find that these models can reproduce the shapes of almost all of the bumps
observed, but only those objects with red colours around maximum light ($B-V
\gtrsim 1$) are well matched throughout their evolution. Consistent with
previous works, we also show that those models in which the shell does not
contain iron-group elements provide good agreement with normal SNe~Ia of
different luminosities from shortly after explosion up to maximum light. While
our models do not amount to positive evidence in favour of the double
detonation scenario, we show that provided the helium shell ash does not
contain iron-group elements, it may be viable for a wide range of normal
SNe~Ia.
|
2101.09792v1
|
2021-03-12
|
Chemical analysis of the Bulge Globular Cluster NGC 6553
|
Globular Clusters are among the oldest objects in the Galaxy, thus their
researchers are key to understanding the processes of evolution and formation
that the galaxy has experienced in early stages. Spectroscopic studies allow us
to carry out detailed analyzes on the chemical composition of Globular
Clusters. The aim of our research is to perform a detailed analysis of chemical
abundances to a sample of stars of the Bulge Globular Cluster NGC 6553, in
order to determine chemical patterns that allow us to appreciate the phenomenon
of Multiple Population in one of the most metal-rich Globular Clusters in the
Galaxy. This analysis is being carried out with data obtained by FLAMES/GIRAFFE
spectrograph, VVV Survey and DR2 of Gaia Mission. We analyzed 20 Red Horizontal
Branch Stars, being the first extensive spectroscopic abundance analysis for
this cluster and measured 8 chemical elements (O, Na, Mg, Si, Ca, Ti, Cr and
Ni), deriving a mean iron content of $[Fe/H] = -0.10\pm0.01$ and a mean of
$[\alpha/Fe] = 0.21\pm0.02$, considering Mg, Si, Ca and Ti (errors on the
mean). We found a significant spread in the content of Na but a small or
negligible in O. We did not find an intrinsic variation in the content of
$\alpha$ and iron-peak elements, showing a good agreement with the trend of the
Bulge field stars, suggesting a similar origin and evolution.
|
2103.07014v1
|
2021-04-03
|
Genuine Electronic Structure and Superconducting Gap Structure in (Ba$_{0.6}$K$_{0.4}$)Fe$_{2}$As$_{2}$ Superconductor
|
The electronic structure and superconducting gap structure are prerequisites
to establish microscopic theories in understanding the superconductivity
mechanism of iron-based superconductors. However, even for the most extensively
studied optimally-doped (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$, there remain
outstanding controversies on its electronic structure and superconducting gap
structure. Here we resolve these issues by carrying out high-resolution
angle-resolved photoemission spectroscopy (ARPES) measurements on the
optimally-doped (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ superconductor using both
Helium lamp and laser light sources. Our results indicate the "flat band"
feature observed around the Brillouin zone center in the superconducting state
originates from the combined effect of the superconductivity-induced band
back-bending and the folding of a band from the zone corner to the center. We
found direct evidence of the band folding between the zone corner and the
center in both the normal and superconducting state. Our resolution of the
origin of the flat band makes it possible to assign the three hole-like bands
around the zone center and determine their superconducting gap correctly.
Around the zone corner, we observe a tiny electron-like band and an M-shaped
band simultaneously in both the normal and superconducting states. The obtained
gap size for the bands around the zone corner ($\sim$5.5 meV) is significantly
smaller than all the previous ARPES measurements. Our results establish a new
superconducting gap structure around the zone corner and resolve a number of
prominent controversies concerning the electronic structure and superconducting
gap structure in the optimally-doped (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$. They
provide new insights in examining and establishing theories in understanding
superconductivity mechanism in iron-based superconductors.
|
2104.01407v1
|
2021-05-04
|
The Effect of Geometry, Spin and Orbital Optimization in Achieving Accurate, Fully-Correlated Results for Iron-Sulfur Cubanes
|
Iron-sulfur clusters comprise an important functional motif of the catalytic
centers of biological systems, capable of enabling important chemical
transformations at ambient conditions. This remarkable capability derives from
a notoriously complex electronic structure that is characterized by a high
density of states that is sensitive to geometric changes. The spectral
sensitivity to subtle geometric changes has received little attention from
fully-correlated calculations, owing partly to the exceptional computational
complexity for treating these large and correlated systems accurately. To
provide insight into this aspect, we report the first Complete Active Space
Self Consistent Field (CASSCF) calculations for different geometries of
cubane-based clusters using two complementary, fully-correlated solvers:
spin-pure Adaptive Sampling Configuration Interaction (ASCI) and Density Matrix
Renormalization Group (DMRG). We find that the previously established picture
of a double-exchange driven magnetic structure, with minute energy gaps (< 1
mHa) between consecutive spin states, has a weak dependence on the underlying
geometry. However, the spin gap between the lowest singlet and the highest spin
states is strongly geometry dependent, changing by an order of magnitude upon
slight deformations that are still within biologically relevant parameters. The
CASSCF orbital optimization procedure, using active spaces as large as 86
electrons in 52 orbitals, was found to reduce this gap by a factor of two
compared to typical mean-field orbital approaches. Our results clearly
demonstrate the need for performing highly correlated calculations to unveil
the challenging electronic structure of these complex catalytic centers.
|
2105.01754v2
|
2021-05-10
|
Cloud busting: enstatite and quartz clouds in the atmosphere of 2M2224-0158
|
We present the most detailed data-driven exploration of cloud opacity in a
substellar object to-date. We have tested over 60 combinations of cloud
composition and structure, particle size distribution, scattering model, and
gas phase composition assumptions against archival $1-15 {\rm \mu m}$
spectroscopy for the unusually red L4.5~dwarf 2MASSW~J2224438-015852 using the
Brewster retrieval framework. We find that, within our framework, a model that
includes enstatite and quartz cloud layers at shallow pressures, combined with
a deep iron cloud deck fits the data best. This models assumes a Hansen
distribution for particle sizes for each cloud, and Mie scattering. We
retrieved particle effective radii of $\log_{10} a {\rm (\mu m)} =
-1.41^{+0.18}_{-0.17}$ for enstatite, $-0.44^{+0.04}_{-0.20}$ for quartz, and
$-0.77^{+0.05}_{-0.06}$ for iron. Our inferred cloud column densities suggest
${\rm (Mg/Si)} = 0.69^{+0.06}_{-0.08}$ if there are no other sinks for
magnesium or silicon. Models that include forsterite alongside, or in place of,
these cloud species are strongly rejected in favour of the above combination.
We estimate a radius of $0.75 \pm 0.02$ Rjup, which is considerably smaller
than predicted by evolutionary models for a field age object with the
luminosity of 2M2224-0158. Models which assume vertically constant gas
fractions are consistently preferred over models that assume thermochemical
equilibrium. From our retrieved gas fractions we infer ${\rm [M/H]} =
+0.38^{+0.07}_{-0.06}$ and ${\rm C/O} = 0.83^{+0.06}_{-0.07}$. Both these
values are towards the upper end of the stellar distribution in the Solar
neighbourhood, and are mutually consistent in this context. A composition
toward the extremes of the local distribution is consistent with this target
being an outlier in the ultracool dwarf population.
|
2105.04268v1
|
2021-05-13
|
Near-Room-Temperature Ferromagnetic Behavior of Single-Atom-Thick 2D Iron in Nanolaminated Ternary MAX Phases
|
Two dimensional (2D) ferromagnetic materials have attracted much attention in
the fields of condensed matter physics and materials science, but their
synthesis is still a challenge given their limitations on structural stability
and susceptibility to oxidization. MAX phases nanolaminated ternary carbides or
nitrides possess a unique crystal structure in which single-atom-thick A
sublayers are interleaved by two dimensional MX slabs, providing nanostructured
templates for designing 2D ferromagnetic materials if the non-magnetic A
sublayers can be substituted replaced by magnetic elements. Here, we report
three new ternary magnetic MAX phases (Ta2FeC, Ti2FeN and Nb2FeC) with A
sublayers of single-atom-thick 2D iron through an isomorphous replacement
reaction of MAX precursors (Ta2AlC, Ti2AlN and Nb2AlC) with a Lewis acid salts
(FeCl2). All these MAX phases exhibit ferromagnetic (FM) behavior. The Curie
temperature (Tc) of Ta2FeC and Nb2FeC MAX phase are 281 K and 291 K,
respectively, i.e. close to room temperature. The saturation magnetization of
these ternary magnetic MAX phases is almost two orders of magnitude higher than
that of V2(Sn,Fe)C MAX phase whose A-site is partial substituted by Fe.
Theoretical calculations on magnetic orderings of spin moments of Fe atoms in
these nanolaminated magnetic MAX phases reveal that the magnetism can be mainly
ascribed to intralayer exchange interaction of the 2D Fe atomic layers. Owning
to the richness in composition of MAX phases, there is a large compositional
space for constructing functional single-atom-thick 2D layers in materials
using these nanolaminated templates.
|
2105.06139v1
|
2021-05-26
|
Magnetic Particle Spectroscopy (MPS) with One-stage Lock-in Implementation for Magnetic Bioassays with Improved Sensitivities
|
In recent years, magnetic particle spectroscopy (MPS) has become a highly
sensitive and versatile sensing technique for quantitative bioassays. It relies
on the dynamic magnetic responses of magnetic nanoparticles (MNPs) for the
detection of target analytes in liquid phase. There are many research studies
reporting the application of MPS for detecting a variety of analytes including
viruses, toxins, and nucleic acids, etc. Herein, we report a modified version
of MPS platform with the addition of a one-stage lock-in design to remove the
feedthrough signals induced by external driving magnetic fields, thus capturing
only MNP responses for improved system sensitivity. This one-stage lock-in MPS
system is able to detect as low as 781 ng multi-core Nanomag50 iron oxide MNPs
(micromod Partikeltechnologie GmbH) and 78 ng single-core SHB30 iron oxide MNPs
(Ocean NanoTech). In addition, using a streptavidin-biotin binding system as a
proof-of-concept, we show that these single-core SHB30 MNPs can be used for
Brownian relaxation-based bioassays while the multi-core Nanomag50 cannot be
used. The effects of MNP amount on the concentration dependent response
profiles for detecting streptavidin was also investigated. Results show that by
using lower concentration/amount of MNPs, concentration-response curves shift
to lower concentration/amount of target analytes. This lower
concentrationresponse indicates the possibility of improved bioassay
sensitivities by using lower amounts of MNPs.
|
2105.12718v1
|
2021-09-13
|
The stellar mass versus stellar metallicity relation of star-forming galaxies at $1.6\le z\le3.0$ and implications for the evolution of the $α$-enhancement
|
We measure the relationship between stellar mass and stellar metallicity, the
stellar mass--metallicity relation (MZR), for 1336 star-forming galaxies at
$1.6\le z\le3.0$ (<z>=2.2) using rest-frame far-ultraviolet spectra from the
zCOSMOS-deep survey. High signal-to-noise composite spectra containing stellar
absorption features are fit with population synthesis model spectra of a range
of metallicity. We find stellar metallicities, which mostly reflect iron
abundances, scaling as
$(Z_{Fe,\ast}/Z_{Fe,\odot})=-(0.81\pm0.01)+(0.32+0.03)\log(M_\ast/10^{10}M_\odot)$
across the mass range of $10^9\lesssim M_\ast/M_\odot\lesssim10^{11}$, being
$\approx6\times$ lower than seen locally at the same masses. The instantaneous
oxygen-to-iron ratio ($\alpha$-enhancement) inferred using the gas-phase oxygen
MZRs, is on average found to be [O/Fe]$\approx0.47$, being higher than the
local [O/Fe]$\approx0$. The observed changes in [O/Fe] and [Fe/H] are
reproduced in simple flow-through gas-regulator models with steady
star-formation histories (SFHs) that follow the evolving main sequence. Our
models show that the [O/Fe] is determined almost entirely by the instantaneous
specific star formation rate alone while being independent of the SFHs, mass,
and the gas-regulation characteristics of the systems. We find that the
locations of $\sim10^{10}M_\odot$ galaxies at z~2 in the [O/Fe]--metallicity
planes are in remarkable agreement with the sequence of low-metallicity
thick-disk stars in our Galaxy. This manifests a beautiful concordance between
the results of Galactic archaeology and observations of high-redshift Milky Way
progenitors. However, there remains a question of how and when the old
metal-rich, low-$\alpha$/Fe stars seen in the bulge had formed by z~2 because
such a stellar population is not seen in our data and difficult to explain in
the context of our models.
|
2109.06044v1
|
2021-09-28
|
Stable nickel production in Type Ia supernovae: A smoking gun for the progenitor mass?
|
At present, there are strong indications that white dwarf (WD) stars with
masses well below the Chandrasekhar limit (MCh ~ 1.4 Msun) contribute a
significant fraction of SN Ia progenitors. The relative fraction of stable
iron-group elements synthesized in the explosion has been suggested as a
possible discriminant between MCh and sub-MCh events. In particular, it is
thought that the higher-density ejecta of MCh WDs, which favours the synthesis
of stable isotopes of nickel, results in prominent [Ni II] lines in late-time
spectra. We study the explosive nucleosynthesis of stable nickel in SNe Ia
resulting from MCh and sub-MCh progenitors. We explore the potential for lines
of [Ni II] at 7378 \AA\ and 1.94 microns in late-time spectra to serve as a
diagnostic of the exploding WD mass, using nonlocal thermodynamic equilibrium
radiative-transfer simulations with the CMFGEN code. We find that the radiative
proton-capture reaction 57Co(p,gamma)58Ni is the dominant production mode for
58Ni in both MCh and sub-MCh models, while the alpha-capture reaction on 54Fe
has a negligible impact on the final 58Ni yield. More importantly, we
demonstrate that the lack of [Ni II] lines in late-time spectra of sub-MCh
events is not always due to an under-abundance of stable Ni; rather, it results
from the higher ionization of Ni in the inner ejecta. Conversely, the strong
[Ni II] lines predicted in our 1D MCh models are completely suppressed when
56Ni is sufficiently mixed with the innermost layers, which are rich in stable
iron-group elements. [Ni II] lines in late-time SN Ia spectra have a complex
dependency on the abundance of stable Ni, which limits their use in
distinguishing among MCh and sub-MCh progenitors. However, we argue that a
low-luminosity SN Ia displaying strong [Ni II] lines would most likely result
from a Chandrasekhar-mass progenitor. [Abridged]
|
2109.13840v3
|
2021-10-17
|
The iron and oxygen content of LMC Classical Cepheids and its implications for the Extragalactic Distance Scale and Hubble constant
|
Classical Cepheids are primary distance indicators and a crucial stepping
stone to determining the present-day Hubble constant Ho to the precision and
accuracy required to constrain apparent deviations from the LCDM Concordance
Cosmological Model. We have measured the iron and oxygen abundances of of 89
Cepheids in the LMC, one of the anchors of the local Distance Scale,
quadrupling the prior sample and including 68 of the 70 Cepheids used to
constrain Ho by the SH0ES program. The goal is to constrain the extent to which
the Cepheid luminosity is influenced by their chemical composition, an
important contributor to the uncertainty on the determination of the Ho itself
and a critical factor in the internal consistency of the distance ladder. We
have derived stellar parameters and abundances from a self-consistent
spectroscopic analysis based on Equivalent Width of absorption lines. The
[Fe/H] distribution of LMC Cepheids is a single Gaussian with a mean of
-0.4079+-0.003 dex (0.1 dex systematic uncertainty) and sigma 0.076+-0.003 dex.
The latter is fully compatible with the measurement error and supports the low
dispersion of 0.069 mag seen in the NIR HST LMC period-luminosity relation. The
uniformity of the abundance has the important consequence that the LMC Cepheids
alone cannot provide any meaningful constraint on the dependence of the Cepheid
Period-Luminosity relation on chemical composition at any wavelength. This
revises a prior claim based on a small sample of 22 LMC Cepheids that there was
little dependence (or uncertainty) between composition and NIR luminosity, a
conclusion which would produce a conflict between anchors of the distance
ladder with different mean abundance. The chemical homogeneity of the LMC
Cepheid population makes it an ideal environment to calibrate the metallicity
dependence between the more metal poor SMC and metal rich Milky Way and
NGC4258.
|
2110.08860v2
|
2022-01-05
|
Phase-resolved spectroscopy of a quasi-periodic oscillation in the black hole X-ray binary GRS 1915+105 with NICER and NuSTAR
|
Quasi-periodic oscillations (QPOs) are often present in the X-ray flux from
accreting stellar-mass black holes (BHs). If they are due to relativistic
(Lense-Thirring) precession of an inner accretion flow which is misaligned with
the disc, the iron emission line caused by irradiation of the disc by the inner
flow will rock systematically between red and blue shifted during each QPO
cycle. Here we conduct phase-resolved spectroscopy of a $\sim2.2$ Hz type-C QPO
from the BH X-ray binary GRS 1915+105, observed simultaneously with NICER and
NuSTAR. We apply a tomographic model in order to constrain the QPO
phase-dependent illumination profile of the disc. We detect the predicted QPO
phase-dependent shifts of the iron line centroid energy, with our best fit
featuring an asymmetric illumination profile ($>2{\sigma}$ confidence). The
observed line energy shifts can alternatively be explained by the spiral
density waves of the accretion-ejection instability model. However we
additionally measure a significant ($>3{\sigma}$) modulation in reflection
fraction, strongly favouring a geometric QPO origin. We infer that the disc is
misaligned with previously observed jet ejections, which is consistent with the
model of a truncated disc with an inner precessing hot flow. However our
inferred disc inner radius is small ($r_\text{in}{\sim} 1.4 GM/c^2$). For this
disc inner radius, Lense-Thirring precession cannot reproduce the observed QPO
frequency. In fact, this disc inner radius is incompatible with the predictions
of all well-studied QPO models in the literature.
|
2201.01765v1
|
2022-01-24
|
Formation of Zn and Pb sulfides in a redox-sensitive modern system due to high atmospheric fallout
|
The study shows that the air-derived metal enrichment (up to 2.3 g Zn kg-1,
1.1 g Pb kg-1, and 62 mg Cd kg-1) is retained in a thin layer (~30 cm) around
10-15 cm below the peat surface. A combination of focused ion beam (FIB)
technology and scanning (SEM) and transmission (TEM) electron microscopy
reveals that micrometric spheroids are most characteristic for ZnS and
(Zn,Cd)S, although the sulfides readily form pseudomorphs after different plant
tissues resulting in much larger aggregates. The aggregates have a complex
polycrystalline sphalerite structure much more advanced than typically obtained
during low-temperature synthesis or observed in other modern occurrences. Platy
highly-disordered radially-aggregated submicrometre crystals develop within the
time constraints of several decades in the cold (~15{\deg}C) and acid (pH
3.4-4.4) peat. The less abundant Pb sulfides occur as submicron cube-like
crystals between ZnS or as flat irregular or square patches on plant root
macrofossils. All PbS are crystalline and defect-free. Pb ion complexation with
dissolved and solid organic matter is probably responsible for the low number
and equilibrium shape of PbS crystals. Iron is absent in the authigenic sulfide
mineralization and occurs entirely as organically bound ferric iron (Fe3+), as
revealed by Mossbauer spectroscopy. The different affinity of metals to organic
matter enhances the precipitation of Zn and Cd as sulfides over Pb and Fe. Our
findings demonstrate that human activities lead to the formation of
near-surface stratiform metal sulfide accumulations in peat, and the polluted
sites can be of use to understand and reconstruct ancient ore deposits' genesis
and mechanisms of formation.
|
2201.09552v1
|
2022-02-14
|
Giant Impact onto a Vesta-Like Asteroid and Formation of Mesosiderites through Mixing of Metallic Core and Surface Crust
|
Mesosiderites are a type of stony-iron meteorites composed of a mixture of
silicates and Fe-Ni metals. The mesosiderite silicates and metals are
considered to have originated from the crust and metal core, respectively, of a
differentiated asteroid. In contrast, mesosiderites rarely contain the olivine
that is mainly included in a mantle. Although a giant impact onto a
differentiated asteroid is considered to be a probable mechanism to mix crust
and metal materials to form mesosiderites, it is not obvious how such a giant
impact can form mesosiderite-like materials without including mantle materials.
We conducted numerical simulations of giant impacts onto differentiated
asteroids, using the smoothed particle hydrodynamics method, to investigate the
detailed distribution of mixed materials on the resultant bodies. For the
internal structure of a target body, we used a thin-crust model derived from
the magma ocean crystallization model of the asteroid Vesta and a thick-crust
and a large-core model suggested from the proximity observation of Vesta by the
Dawn probe. In the simulations with the former model, excavation of the metal
core requires nearly catastrophic impacts and mantle is exposed over large
surface areas. Thus, stony-iron materials produced on its surface are likely to
include mantle materials and it is difficult to produce mesosiderite-like
materials. Conversely, in the simulations with the latter model, mantle
materials are exposed only at impact sites, even when the impacts excavate the
metal core, and the formation of a surface with little mantle material and the
formation of mesosiderite-like materials are possible. Therefore, our
simulations suggest that an internal structure with a thick crust and a large
core is more likely as a mesosiderite parent body rather than the thin-crust
internal structure inferred from the conventional magma ocean model.
|
2202.06486v1
|
2022-02-16
|
Spin Seebeck effect in iron oxide thin films: Effects of phase transition, phase coexistence, and surface magnetism
|
Understanding impacts of phase transition, phase coexistence, and surface
magnetism on the longitudinal spin Seebeck effect (LSSE) in a magnetic system
is essential to manipulate the spin to charge current conversion efficiency for
spincaloritronic applications. We aim to elucidate these effects by performing
a comprehensive study of the temperature dependence of LSSE in biphase iron
oxide (BPIO = alpha-Fe2O3 + Fe3O4) thin films grown on Si (100) and Al2O3 (111)
substrates. A combination of temperature-dependent anomalous Nernst effect
(ANE) and electrical resistivity measurements show that the contribution of ANE
from the BPIO layer is negligible compared to the intrinsic LSSE in the
Si/BPIO/Pt heterostructure even at room temperature. Below the Verwey
transition of the Fe3O4 phase, the total signal across BPIO/Pt is dominated by
the LSSE. Noticeable changes in the intrinsic LSSE signal for both Si/BPIO/Pt
and Al2O3/BPIO/Pt heterostructures around the Verwey transition of the Fe3O4
phase and the antiferromagnetic (AFM) Morin transition of the alpha-Fe2O3 phase
are observed. The LSSE signal for Si/BPIO/Pt is found to be almost two times
greater than that for Al2O3/BPIO/Pt, an opposite trend is observed for the
saturation magnetization though. Magnetic force microscopy reveals the higher
density of surface magnetic moments of the Si/BPIO film compared to the
Al2O3/BPIO film, which underscores a dominant role of interfacial magnetism on
the LSSE signal and thereby explains the larger LSSE for Si/BPIO/Pt.
|
2202.07910v1
|
2022-02-16
|
A new and homogeneous metallicity scale for Galactic classical Cepheids II. The abundance of iron and alpha elements
|
Classical Cepheids are the most popular distance indicators and tracers of
young stellar populations. The key advantage is that they are bright and they
can be easily identified in Local Group and Local Volume galaxies. Their
evolutionary and pulsation properties depend on their chemical abundances. The
main aim of this investigation is to perform a new and accurate abundance
analysis of two tens of calibrating Galactic Cepheids using high spectral
resolution (R$\sim$40,000-115,000) and high S/N spectra ($\sim$400) covering
the entire pulsation cycle. We focus our attention on possible systematics
affecting the estimate of atmospheric parameters and elemental abundances along
the pulsation cycle. We cleaned the line list by using atomic transition
parameters based on laboratory measurements and by removing lines that are
either blended or display abundance variations along the pulsation cycle. The
spectroscopic approach that we developed brings forward small dispersions in
the variation of the atmospheric parameters ($\sigma$($T_{\rm eff}$)$\sim$50 K,
$\sigma$($\log{g}$)$\sim$0.2 dex, and $\sigma$($\xi$)$\sim$0.2 km/s) and in the
abundance of both iron ($\lesssim$ 0.05 dex) and alpha elements ($\lesssim$0.10
dex) over the entire pulsation cycle. We also provide new and accurate
effective temperature templates by splitting the calibrating Cepheids into four
different period bins, ranging from short to long periods. For each period bin,
we performed an analytical fit with Fourier series providing $\theta =
5040/{T_{\rm eff}}$ as a function of the pulsation phase. The current findings
are a good viaticum to trace the chemical enrichment of the Galactic thin disk
by using classical Cepheids and a fundamental stepping stone for further
investigations into the more metal-poor regime typical of Magellanic Cepheids.
|
2202.07945v1
|
2022-03-01
|
Low-luminosity type IIP supernovae: SN 2005cs and SN 2020cxd as very low-energy iron core-collapse explosions
|
SN 2020cxd is a representative of the family of low-energy, underluminous
Type IIP supernovae (SNe), whose observations and analysis were recently
reported by Yang et al. (2021). Here we re-evaluate the observational data for
the diagnostic SN properties by employing the hydrodynamic explosion model of a
9 MSun red supergiant progenitor with an iron core and a pre-collapse mass of
8.75 Msun. The explosion of the star was obtained by the neutrino-driven
mechanism in a fully self-consistent simulation in three dimensions (3D).
Multi-band light curves and photospheric velocities for the plateau phase are
computed with the one-dimensional radiation-hydrodynamics code STELLA, applied
to the spherically averaged 3D explosion model as well as spherisized radial
profiles in different directions of the 3D model. We find that the overall
evolution of the bolometric light curve, duration of the plateau phase, and
basic properties of the multi-band emission can be well reproduced by our SN
model with its explosion energy of only 0.7x10^50 erg and an ejecta mass of 7.4
Msun. These values are considerably lower than the previously reported numbers,
but they are compatible with those needed to explain the fundamental
observational properties of the prototype low-luminosity SN 2005cs. Because of
the good compatibility of our photospheric velocities with line velocities
determined for SN 2005cs, we conclude that the line velocities of SN 2020cxd
are probably overestimated by up to a factor of about 3. The evolution of the
line velocities of SN 2005cs compared to photospheric velocities in different
explosion directions might point to intrinsic asymmetries in the SN ejecta.
|
2203.00473v2
|
2022-03-06
|
Coupling between the accreting corona and the relativistic jet in the micro quasar GRS 1915+105
|
GRS 1915+105 was the first stellar-mass black-hole in our Galaxy to display a
superluminal radio jet, similar to those observed in active galactic nuclei
with a supermassive black hole at the centre. It has been proposed that the
radio emission in GRS 1915+105 is fed by instabilities in the accretion disc by
which the inner parts of the accretion flow is ejected in the jet. Here we show
that there is a significant correlation between: (i) the radio flux, coming
from the jet, and the flux of the iron emission line, coming from the disc and,
(ii) the temperature of the corona that produces the high-energy part of the
X-ray spectrum via inverse Compton scattering and the amplitude of a
high-frequency variability component coming from the innermost part of the
accretion flow. At the same time, the radio flux and the flux of the iron line
are strongly anti-correlated with the temperature of the X-ray corona and the
amplitude of the high-frequency variability component. These correlations
persist over ~10 years, despite the highly variable X-ray and radio properties
of the source in that period. Our findings provide, for the first time,
incontrovertible evidence that the energy that powers this black-hole system
can be directed either to the X-ray corona or the jet. When this energy is used
to power the corona, raising its temperature, there is less energy left to fuel
the jet and the radio flux drops, and vice versa. These facts, plus the
modelling of the variability in this source show conclusively that in GRS
1915+105 the X-ray corona morphs into the jet.
|
2203.02963v1
|
2022-03-07
|
The in situ origin of the globular cluster NGC 6388 from abundances of Sc, V, and Zn of a large sample of stars
|
Chemical tagging of globular clusters (GCs) is often done using abundances of
alpha-elements. The iron-peak elements Sc, V, and in particular Zn were
proposed as an alternative to alpha-elements to tag accreted GCs in the
metal-rich regime, where the dwarf galaxy Sagittarius and its GCs show
peculiarly marked under-abundances of these heavier species with respect to
Milky Way stars. A handful of stars in NGC 6388 was used to suggest an accreted
origin for this GC, contradicting the results from dynamics. We tested the
efficiency of the iron-peak method by using large samples of stars in NGC 6388,
compared to thousands of field stars in the disc and the bulge of the Milky
Way. Our abundance ratios of Sc (185 stars) and V (35 stars) for NGC 6388 are
within about 1.5 sigma from the average for the field stars with a similar
metallicity, and they are in perfect agreement for Zn (31 stars), claimed to be
the most sensitive element concerning the accretion pattern. Moreover, the
chemo-dynamical plots, coupled to the bifurcated age-metallicity relation of
GCs in the Galaxy, clearly rule out any association of NGC 6388 to the groups
of accreted GCs. Using a large set of GC abundances from the literature, we
also show that the new method with Sc, V, and Zn seems to be efficient in
picking up GCs related to the Sagittarius dwarf galaxy. Whether this is also
generally true for accreted GCs seems to be less evident, and it should be
verified with larger and homogeneous samples of stars both in the field and in
GCs.
|
2203.03645v1
|
2022-03-22
|
Co-loading of doxorubicin and iron oxide nanocubes in polycaprolactone fibers for combining Magneto-Thermal and chemotherapeutic effects on cancer cells
|
Among the strategies to fight cancer, multi-therapeutic approaches are
considered as a wise choice to put in place multiple weapons to suppress
tumors. In this work, to combine chemotherapeutic effects to magnetic
hyperthermia when using biocompatible scaffolds, we have established an
electrospinning method to produce nanofibers of polycaprolactone loaded with
magnetic nanoparticles as heat mediators to be selectively activated under
alternating magnetic field and doxorubicin as a chemotherapeutic drug.
Production of the fibers was investigated with iron oxide nanoparticles of
peculiar cubic shape (at 15 and 23 nm in cube edges) as they provide benchmark
heat performance under clinical magnetic hyperthermia conditions. With 23 nm
nanocubes when included into the fibers, an arrangement in chains was obtained.
This linear configuration of magnetic nanoparticles resemble that of the
magnetosomes, produced by magnetotactic bacteria, and our magnetic fibers
exhibited remarkable heating effects as the magnetosomes. Magnetic fiber
scaffolds showed excellent biocompatibility on fibroblast cells when missing
the chemotherapeutic agent and when not exposed to magnetic hyperthermia as
shown by viability assays. On the contrary, the fibers containing both magnetic
nanocubes and doxorubicin showed significant cytotoxic effects on cervical
cancer cells following the exposure to magnetic hyperthermia. Notably, these
tests were conducted at magnetic hyperthermia field conditions of clinical use.
As here shown, on the doxorubicin sensitive cervical cancer cells, the
combination of heat damage by magnetic hyperthermia with enhanced diffusion of
doxorubicin at therapeutic temperature are responsible for a more effective
oncotherapy.
|
2203.12380v1
|
2022-03-23
|
Prospects of measuring a metallicity trend and spread in globular clusters from low-resolution spectroscopy
|
The metallicity spread, or the metallicity trend along the evolutionary
sequence of a globular cluster, is a rich source of information to help
understand the cluster physics (e.g. multiple populations) and stellar physics
(e.g. atomic diffusion). Low-resolution integral-field-unit spectroscopy in the
optical with the MUSE is an attractive prospect if it can provide these
diagnostics because it allows us to extract spectra of a large fraction of the
cluster stars. We investigate the possibilities of full-spectrum fitting to
derive stellar parameters and chemical abundances at low spectral resolution
(R~2000). We reanalysed 1584 MUSE spectra of 1061 stars above the turn-off of
NGC 6397 using FERRE and employing two different synthetic libraries. We derive
the equivalent iron abundance \fehe for fixed values of \afe. We find that (i)
the interpolation schema and grid mesh are not critical for the precision,
metallicity spread, and trend; (ii) with the two grids, \fehe increases by ~0.2
dex along the sub-giant branch, starting from the turn-off of the main
sequence; (iii) restricting the wavelength range to the optical decreases the
precision significantly; and (iv) the precision obtained with the synthetic
libraries is lower than the precision obtained previously with empirical
libraries. Full-spectrum fitting provides reproducible results that are robust
to the choice of the reference grid of synthetic spectra and to the details of
the analysis. The \fehe increase along the sub-giant branch is in stark
contrast with the nearly constant iron abundance previously found with
empirical libraries. The precision of the measurements (0.05 dex on \fehe) is
currently not sufficient to assess the intrinsic chemical abundance spreads,
but this may change with deeper observations. Improvements of the synthetic
spectra are still needed to deliver the full possibilities of full-spectrum
fitting.
|
2203.12685v1
|
2022-03-30
|
The p-process in exploding rotating massive stars
|
The p-process nucleosynthesis can explain proton-rich isotopes that are
heavier than iron, which are observed in the Solar System, but discrepancies
still persist and important questions concerning the astrophysical site(s) of
the p-process remain unanswered. We investigate how the p-process operates in
exploding rotating massive stars that have experienced an enhanced s-process
nucleosynthesis during their life through rotational mixing. We computed 25
$M_{\odot}$ stellar models at a metallicity of $Z=10^{-3}$ with different
initial rotation velocities and rates for the uncertain
$^{17}$O($\alpha$,$\gamma$)$^{21}$Ne reaction. The nucleosynthesis calculation,
followed with a network of 737 isotopes, was coupled to stellar evolution, and
the p-process nucleosynthesis was calculated in post-processing during both the
final evolutionary stages and spherical explosions of various energies. In our
models, the p-nuclides are mainly synthesized during the explosion, but not
much during the ultimate hydrostatic burning stages. The p-process yields
mostly depend on the initial number of trans-iron seeds, which in turn depend
on the initial rotation. We found that the impact of rotation on the p-process
is comparable to the impact of rotation on the s-process. From no to fast
rotation, the s-process yields of nuclides with mass number $A<140$ increase by
$3-4$ dex, and so do the p-process yields. Fast rotation with a lower
$^{17}$O($\alpha,\gamma$) rate significantly produces s- and p-nuclides with
$A\geq140$. Our results suggest that the contribution of core-collapse
supernovae from massive stars to the solar (and Galactic) p-nuclei has been
underestimated in the past, and more specifically, that the contribution from
massive stars with sub-solar metallicities may even dominate. A more detailed
study including stellar models with a wide range of masses and metallicities
remains to be performed.
|
2203.16380v1
|
2022-04-11
|
The GAPS Programme at TNG XXXIII. HARPS-N detects multiple atomic species in emission from the dayside of KELT-20b
|
The detection of lines in emission in planetary atmospheres provides direct
evidence of temperature inversion. We confirm the trend of ultra-hot Jupiters
orbiting A-type stars showing temperature inversions on their daysides, by
detecting metals emission lines in the dayside of KELT-20b. We first detect the
planetary emission by using the G2 stellar mask of the HARPS-N pipeline, which
is mainly composed of neutral iron lines, as a template. Using neutral iron
templates, we perform a retrieval of the atmospheric temperature-pressure
profile of the planet, confirming a thermal inversion. Then we create models of
planetary emission of different species using the retrieved inverted
temperature-pressure profile. By using the cross-correlation technique, we
detect FeI, FeII and CrI at signal-to-noise ratio levels of 7.1, 3.9 and 3.6,
respectively. The latter is detected for the first time in emission in the
atmosphere of an exoplanet. Contrary to FeI, FeII and CrI are detected only
after the occultation and not before, hinting for different atmospheric
properties in view on the pre- and post- occultation orbital phases. A further
retrieval of the temperature-pressure profile performed independently on the
pre- and post- occultation phases, while not highly significant, points to a
steeper thermal inversion in the post-occultation.
|
2204.04948v1
|
2022-05-19
|
Nuclear Weak Rates and Nuclear Weak Processes in Stars
|
Nuclear weak rates in stellar environments are obtained by shell-model
calculations including Gamow-Teller (GT) and spin-dipole transitions, and
applied to nuclear weak processes in stars. The important roles of accurate
weak rates for the study of astrophysical processes are pointed out. The weak
rates in $sd$-shell are used to study the evolution of ONeMg cores in stars
with 8-10 M$_{\odot}$. Cooling of the core by nuclear Urca processes, and the
heating by double e-captures on $^{20}$Ne are studied. Especially, the
e-capture rates for a second-forbidden transition in $^{20}$Ne are evaluated
with the multipole expansion method of Walecka and
Behrens-B$\ddot{\mbox{u}}$hring, and the final fate of the cores, core-collapse
or thermonuclear explosion, are discussed. The weak rates in $pf$-shell are
applied to nucleosynthesis of iron-group elements in Type Ia supernovae. The
over-production problem of neutron-rich iron isotopes compared with the solar
abundances is now reduced to be within a factor of two. The weak rates for
nuclear Urca pair with $A$=31 in the island of inversion are evaluated with the
effective interaction obtained by the extended Kuo-Krenciglowa method. The
transition strengths and e-capture rates in $^{78}$Ni, important for
core-collapse processes, are evaluated with the $pf$-$sdg$ shell, and compared
with those obtained by the random-phase-approximation and an effective rate
formula. $\beta$-decay rates of $N$ =126 isotones are evaluated with both the
GT and first-forbidden transitions. The half-lives are found to be shorter than
those obtained by standard models. Neutrino-nucleus reaction cross sections on
$^{13}$C, $^{16}$O and $^{40}$Ar are obtained with new shell-model
Hamiltonians. Implications on nucleosynthesis, neutrino detection, neutrino
oscillations and neutrino mass hierarchy are discussed.
|
2205.09262v2
|
2022-06-29
|
Does the Fe L-shell blend bias abundance measurements in intermediate-temperature clusters?
|
In intermediate-temperature (T = 2.5 - 4.5 keV) galaxy clusters, abundance
measurements are almost-equally driven by Fe K and L transitions, at $\sim$ 6.7
keV and 0.9 - 1.3 keV, respectively. While K-shell-derived measurements are
considered reliable, the resolution of the currently available instrumentation,
as well as our current knowledge of the atomic processes, makes the modelling
of the L-line complex challenging, resulting in potential biases for abundance
measurements. In this work, we study systematics related to the modelling of
the Fe L-line complex that may influence iron-abundance measurements in the
intermediate-temperature range. To this aim, we select a sample of three bright
galaxy clusters, with long XMM-Newton observations available and temperature in
the 2.5 - 4.5 keV range. We fit spectra extracted from concentric rings with
APEC and APEC+APEC models, by alternatively excluding the L and K bands, and
derive the fractional difference of the metal abundances, $\Delta Z/Z$, as
indication of the consistency between K- and L-shell-derived measurements. The
$\Delta Z/Z$ distribution is then studied as a function of the cluster radius,
ring temperature and X-ray flux. The L-induced systematics, measured through an
individual fit of each MOS and pn spectrum, remain constant at a 5 - 6% value
in the whole 2.5 - 4.5 keV temperature range. Conversely, a joint fit of MOS
and pn spectra leads to a slight excess of 1 - 2% in the above estimate. No
significant dependence on the ring X-ray flux is highlighted. The measured 5 -
8% value indicates a modest contribution of the systematics to the derived iron
abundances, giving confidence for future measurements. To date, these findings
represent the best-achievable estimate of the systematics in analysis, while
future microcalorimeters will significantly improve our understanding of the
atomic processes underlying the Fe L emissions.
|
2206.14827v1
|
2022-08-08
|
The chemical abundance pattern of the extremely metal-poor thin disk star 2MASS J1808-5104 and its origins
|
We present a high-resolution ($R\sim35,000$), high signal-to-noise
($S/N=350$) Magellan/MIKE spectrum of the bright extremely metal-poor star
2MASS~J1808$-$5104. We find [Fe/H] = $-$4.01 (spectroscopic LTE stellar
parameters), [Fe/H] = $-$3.8 (photometric stellar parameters), [Fe/H] = $-$3.7
(spectroscopic NLTE stellar parameters). We measured a carbon-to-iron ratio of
$\mbox{[C/Fe]}= 0.38$ from the CH G-band. J1808$-$5104 is thus not
carbon-enhanced, contrary to many other stars with similarly low iron
abundances. We also determine, for the first time, a barium abundance
($\mbox{[Ba/Fe]} =-0.78$), and obtain a significantly reduced upper limit for
the nitrogen abundance ([N/Fe]$ < - 0.2$). J1808$-$5104 has low ratio of
$\mbox{[Sr/Ba]}=-0.17$, which is consistent with that of stars in ultra-faint
dwarf galaxies. We also fit the abundance pattern of J1808$-$5104 with
nucleosynthesis yields from a grid of Population\,III supernova models. There
is a good fit to the abundance pattern which suggests J1808$-$5104 originated
from gas enriched by a single massive supernova with a high explosion energy of
E $=10\times10^{51}$\,erg and a progenitor stellar mass of
M$=29.5$\,M$_{\odot}$. Interestingly, J1808$-$5104 is a member of the Galactic
thin disk, as confirmed by our detailed kinematic analysis and calculated
stellar actions and velocities. Finally, we also established the orbital
history of J1808$-$5104 using our time-dependent Galactic potential the
\texttt{ORIENT}. J1808$-$5104 appears to have a stable quasi-circular orbit and
been largely confined to the thin disk. This unique orbital history, the star's
very old age ($\sim13.5$\,Gyr), and the low [C/Fe] and [Sr/Ba] ratios suggest
that J1808$-$5104 may have formed at the earliest epoch of the hierarchical
assembly of the Milky Way, and it is most likely associated with the primordial
thin disk.
|
2208.03891v1
|
2022-09-02
|
An intermediate polar candidate toward the Galactic plane
|
For the past decade, it has been suggested that intermediate polars (IPs), a
subclass of magnetic cataclysmic variables (CVs), are one of the main
contributors to the hard diffuse X-ray emission from the Galactic center (GC)
and Galactic ridge. In our ongoing \emph{XMM-Newton} survey of the central
region of the Galactic disk ($20^\circ\times2^\circ$), we detected a persistent
IP candidate, $1.7^\circ$ away from the GC. In this work, we better
characterize the behavior of this source by looking at the new and archival
XMM-Newton data. We performed a detailed X-ray spectral modeling of the source.
Furthermore, we searched for X-ray pulsations in the light curve as well as its
counterpart at other wavelengths. The XMM-Newton spectrum (0.8--10 keV) of the
source is described by a partial covering collisionally ionized diffuse gas
with plasma temperature $kT=15.7^{+20.9}_{-3.6}$ keV. In addition, the spectrum
shows the presence of iron lines at $E=6.44$, 6.65, and 6.92 keV with
equivalent widths of $194^{+89}_{-70}$, $115^{+79}_{-75}$, and $98^{+93}_{-74}$
eV, respectively. The X-ray light curve shows a coherent modulation with a
period of $P=432.44\pm0.36$ s, which we infer is the spin period of the white
dwarf. The white dwarf mass estimated from fitting a physical model to the
spectrum results in $M_{\rm WD}=1.05^{+0.16}_{-0.21}\ M_{\odot}$. We were able
to find a likely optical counterpart in the Gaia catalog with a G magnitude of
19.26, and the distance to the source derived from the measured Gaia parallax
is $\sim$4.3 kpc. We provide an improved source localization with subarcsec
accuracy. The spectral modeling of the source indicates the presence of
intervening circumstellar gas, which absorbs the soft X-ray photons. The
measured equivalent width of the iron lines and the detection of the spin
period in the light curve are consistent with those from IPs.
|
2209.00970v1
|
2022-09-12
|
53Mn-53Cr chronology and ε54Cr-Δ17O genealogy of Erg Chech 002: the oldest andesite in the Solar System
|
The meteorite sample Erg Chech (EC) 002 is the oldest felsic igneous rock
from the Solar System analysed to date and provides a unique opportunity to
study the formation of felsic crusts on differentiated protoplanets immediately
after metal-silicate equilibration or core formation. The extinct 53Mn-53Cr
chronometer provides chronological constraints on the formation of EC 002 by
applying the isochron approach using chromite, metal-silicate-sulphide and
whole-rock fractions as well as "leachates" obtained by sequential digestion of
a bulk sample. Assuming a chondritic evolution of its parent body, a 53Cr/52Cr
model age is also obtained from the chromite fraction. The 53Mn-53Cr isochron
age of 1.73 (+/-) 0.96 Ma (anchored to D'Orbigny angirte) and the chromite
model age constrained between 1.46 (-0.68/+0.78) and 2.18 (-1.06/+1.32) Ma
after the formation of calcium-aluminium-rich inclusions (CAIs) agree with the
26Al-26Mg ages (anchored to CAIs) reported in previous studies. This indicates
rapid cooling of EC 002 that allowed near-contemporaneous closure of multiple
isotope systems. Additionally, excess in the neutron-rich 54Cr (nucleosynthetic
anomalies) combined with mass-independent isotope variations of 17O provide
genealogical constraints on the accretion region of the EC 002 parent body. The
54Cr and 17O isotope compositions of EC 002 confirm its origin in the
"non-carbonaceous" reservoir and overlap with the vestoid material NWA 12217
and anomalous eucrite EET 92023. This indicates a common feeding zone during
accretion in the protoplanetary disk between the source of EC 002 and vestoids.
The enigmatic origin of iron meteorites remains still unresolved as EC 002,
which is more like a differentiated crust, has an isotope composition that does
not match known irons meteorite groups that were once planetesimal cores.
|
2209.05381v1
|
2022-09-13
|
The Puzzle of Meteoritic Minerals Heideite and Brezinaite; Are they Iron-based Superconductors? Are they Technosignatures?
|
Transition metal sulfides (Fe, V)3S4 and (Fe, Ti)3S4, with the monoclinic
Cr3S4 type structure have been studied for a long time, itinerant magnetism in
form of Spin density waves (SDW) have been found in these systems with
different features as evidenced by 57Fe Mossbauer Spectroscopy, there is an
intricate relationship between the proportion of Fe, V and Ti atoms, the degree
of commensurability of the SDW and the magnetic transition temperature. These
sulfides have no natural occurrence on Earth and some of these phases were
detected as minerals in meteorites; the mineral Heideite in the Bustee and
Kaidun meteorites, with minor proportion of Cr atoms leading to the general
formula (Fe, Cr)1+x (Ti, Fe)2S4, and the mineral Brezinaite in the Tucson
meteorite, with minor proportion of Fe atoms and traces of V, Ti and Mn atoms,
leading to the formula (Cr2.65Fe0.20V0.09Ti0.06Mn0.04)3.04S4. In this critical
review of the experimental literature, we discuss the issues that these
meteoritic minerals are structurally sensitive to the method of synthesis, and
so is their magnetic behavior, especially in the presence of minor and trace
elements. This discussion could shed light on our knowledge in Solid State
Physics and Planetary Science; these meteoritic minerals are promising
candidates for iron-based superconductors because of three clues: they are
layered structures, they undergo a transition to SDW with variable degree of
commensurability and the minor and trace elements could act as dopants and
hence suppress the SDW giving rise to superconductivity. On the other side, the
genesis of these meteoritic minerals could require controlled and sophisticated
process not easily found in nature. So, it is important to be open-minded and
even provocative to consider the following question: Are these meteoritic
minerals samples of Extraterrestrial Technosignatures?
|
2209.05679v1
|
2022-10-11
|
Ironing the folds: The phase space chevrons of a GSE-like merger as a dark matter subhalo detector
|
Recent work uncovered features in the phase space of the Milky Way's stellar
halo which may be attributed to the last major merger. When stellar material
from a satellite is accreted onto its host, it phase mixes and appears finely
substructured in phase space. For a high-eccentricity merger, this substructure
most clearly manifests as numerous wrapping chevrons in $(v_r, r)$ space,
corresponding to stripes in $(E, \theta_r)$ space. We introduce the idea of
using this substructure as an alternative subhalo detector to cold stellar
streams. We simulate an N-body merger akin to the GSE and assess the impact of
subhaloes on these chevrons. We examine how their deformation depends on the
mass, pericentre, and number of subhaloes. To quantify the impact of
perturbers, we utilise the appearance of chevrons in $(E, \theta_r)$ space to
introduce a new quantity -- the ironing parameter. We show that: (1) a single
flyby of a massive ($\sim 10^{10}$ M$_{\odot}$) subhalo with pericentre
comparable to, or within, the shell's apocentre smooths out the substructure,
(2) a single flyby of a low mass ($\lesssim 10^8$ M$_{\odot}$) has negligible
effect, (3) multiple flybys of subhalos derived from a subhalo mass function
between $10^7-10^{10}$ M$_{\odot}$ cause significant damage if deep within the
potential, (4) the effects of known perturbers (e.g. Sagittarius) should be
detectable and offer constraints on their initial mass. The sensitivity to the
populations of subhaloes suggests that we should be able to place an upper
limit on the Milky Way's subhalo mass function.
|
2210.05679v2
|
2022-10-13
|
TOI-561 b: A Low Density Ultra-Short Period "Rocky" Planet around a Metal-Poor Star
|
TOI-561 is a galactic thick disk star hosting an ultra-short period (0.45 day
orbit) planet with a radius of 1.37 R$_{\oplus}$, making it one of the most
metal-poor ([Fe/H] = -0.41) and oldest ($\sim$10 Gyr) sites where an
Earth-sized planet has been found. We present new simultaneous radial velocity
measurements (RVs) from Gemini-N/MAROON-X and Keck/HIRES, which we combined
with literature RVs to derive a mass of M$_{b}$=2.24 $\pm$ 0.20 M$_{\oplus}$.
We also used two new Sectors of TESS photometry to improve the radius
determination, finding R$_{b}$=$1.37 \pm 0.04 R_\oplus$, and confirming that
TOI-561 b is one of the lowest-density super-Earths measured to date ($\rho_b$=
4.8 $\pm$ 0.5 g/cm$^{3}$). This density is consistent with an iron-poor rocky
composition reflective of the host star's iron and rock-building element
abundances; however, it is also consistent with a low-density planet with a
volatile envelope. The equilibrium temperature of the planet ($\sim$2300 K)
suggests that this envelope would likely be composed of high mean molecular
weight species, such as water vapor, carbon dioxide, or silicate vapor, and is
likely not primordial. We also demonstrate that the composition determination
is sensitive to the choice of stellar parameters, and that further measurements
are needed to determine if TOI-561 b is a bare rocky planet, a rocky planet
with an optically thin atmosphere, or a rare example of a non-primordial
envelope on a planet with a radius smaller than 1.5 R$_{\oplus}$.
|
2210.06665v3
|
2022-10-14
|
The homogeneity of chemical abundances in H II regions of the Magellanic Clouds
|
We use very deep spectra obtained with the Ultraviolet-Visual Echelle
Spectrograph at the Very Large Telescope to derive physical conditions and
chemical abundances of four H II regions of the Large Magellanic Cloud (LMC)
and four H II regions of the Small Magellanic Cloud (SMC). The observations
cover the spectral range 3100-10400 \A with a spectral resolution of
$\Delta\lambda\ge\lambda/11600$, and we measure 95-225 emission lines in each
object. We derive ionic and total abundances of O, N, S, Ne, Ar, Cl, and Fe
using collisionally excited lines. We find average values of
$12+\log(\mbox{O/H})=8.37$ in the LMC and $8.01$ in the SMC, with standard
deviations of $\sigma=0.03$ and 0.02~dex, respectively. The S/O, Ne/O, Ar/O,
and Cl/O abundance ratios are very similar in both clouds, with
$\sigma=0.02$-0.03~dex, which indicates that the chemical elements are well
mixed in the interstellar medium of each galaxy. The LMC is enhanced in N/O by
$\sim0.20$~dex with respect to the SMC, and the dispersions in N/O,
$\sigma=0.05$~dex in each cloud, are larger than those found for the other
elements. The derived standard deviations would be much larger for all the
abundance ratios, up to 0.20~dex for N/O, if previous spectra of these objects
were used to perform the analysis. Finally, we find a wide range of iron
depletions in both clouds, with more than 90 per cent of the iron atoms
deposited onto dust grains in most objects.
|
2210.07460v1
|
2022-11-11
|
The unaltered pulsar: GRO J1750-27, a super-critical X-ray neutron star that does not blink an eye
|
When accreting X-ray pulsars (XRPs) undergo bright X-ray outbursts, their
luminosity-dependent spectral and timing features can be analysed in detail.
The XRP GRO J1750-27 recently underwent one of such episodes, during which it
was observed with $NuSTAR$ and monitored with $NICER$. Such a data set is
rarely available, as it samples the outburst over more than a month at a
luminosity that is always exceeding ${\sim}5\times10^{37}\,$erg/s. This value
is larger than the typical critical luminosity value, where a radiative shock
is formed above the neutron star's surface. Our data analysis of the joint
spectra returns a highly ($N_H\sim(5-8)\times10^{22}\,$cm$^{-2}$) absorbed
spectrum showing a K$\alpha$ iron line, a soft blackbody component likely
originating from the inner edge of the accretion disk, and confirms the
discovery of one of the deepest cyclotron lines, at a centroid energy of
${\sim}44\,$keV corresponding to a magnetic field strength of
$4.7\times10^{12}\,$G. This value is independently supported by the best-fit
physical model for spectral formation in accreting XRPs which, in agreement
with recent findings, favours a distance of $14$ kpc and also reflects a
bulk-Comptonization dominated accretion flow. Contrary to theoretical
expectations and observational evidence from other similar sources, the pulse
profiles as observed by $NICER$ through the outburst raise, peak and decay
remain remarkably steady. The $NICER$ spectrum, including the iron K$\alpha$
line best-fit parameters, also remain almost unchanged at all probed outburst
stages, similar to the pulsed fraction behaviour. We argue that all these
phenomena are linked and interpret them as resulting from a saturation effect
of the accretion column's emission, which occurs in the high-luminosity regime.
|
2211.06367v2
|
2022-12-01
|
Three dimensional magnetorotational core-collapse supernova explosions of a 39 solar mass progenitor star
|
We perform three-dimensional simulations of magnetorotational supernovae
using a $39\,M_{\odot}$ progenitor star with two different initial magnetic
field strengths of $10^{10}$ G and $10^{12}$ G in the core. Both models rapidly
undergo shock revival and their explosion energies asymptote within a few
hundred milliseconds to values of $\gtrsim 2\times10^{51}$ erg after
conservatively correcting for the binding energy of the envelope. Magnetically
collimated, non-relativistic jets form in both models, though the jets are
subject to non-axisymmetric instabilities. The jets do not appear crucial for
driving the explosion, as they only emerge once the shock has already expanded
considerably. Our simulations predict moderate neutron star kicks of about
$150\, \mathrm{km}\,\mathrm{s}^{-1}$, no spin-kick alignment, and rapid early
spin-down that would result in birth periods of about $20\, \mathrm{ms}$, too
slow to power an energetic gamma-ray burst jet. More than $0.2\,M_\odot$ of
iron-group material are ejected, but we estimate that the mass of ejected
$^{56}\mathrm{Ni}$ will be considerably smaller as the bulk of this material is
neutron-rich. Explosive burning does not contribute appreciable amounts of
$^{56}\mathrm{Ni}$ because the burned material originates from the slightly
neutron-rich silicon shell. The iron-group ejecta also show no pronounced
bipolar geometry by the end of the simulations. The models thus do not
immediately fit the characteristics of observed hypernovae, but may be
representative of other transients with moderately high explosion energies. The
gravitational-wave emission reaches high frequencies of up to 2000 Hz and
amplitudes of over 100 cm. The gravitational-wave emission is detectable out to
distances of $\sim4$ Mpc in the planned Cosmic Explorer detector.
|
2212.00200v3
|
2022-12-12
|
Systematic Broad-band X-ray Study of super-Eddington Accretion onto Supermassive Black Holes. I. X-ray Continuum
|
We present the first systematic broad-band X-ray study of super-Eddington
accretion onto SMBHs with simultaneous {\it NuSTAR} and {\it XMM-Newton} or
{\it Swift}/XRT observations of a sample of 8 super-Eddington accreting AGN
with Eddington ratio $1<\lambda_{\rm Edd}<426$. We find that the SEAMBHs show a
steep primary continuum slope as expected for sources accreting in the super
Eddington regime, mostly dominated by relativistic reflection. The Iron
K$\alpha$ emission lines of the sources of our sample show relativistic
broadening. In addition the equivalent widths of the narrow components of the
Iron K$\alpha$ lines follow the 'X-ray Baldwin' effect, also known as the
'Iwasawa-Taniguchi' effect. We found a statistically significant correlation
between the photon-index of the primary power-law and the Eddington ratio,
consistent with past studies. Moreover, as expected for super-Eddington
sources, the median value of the reflection fraction of the sources we analysed
is a factor $\sim 2$ higher than the median reflection fraction value of the
type\,1 AGN from the BASS sample. We are able to estimate the coronal
temperature for three sources of our sample: Mrk\,382 ($kT_e=7.8$\,keV),
PG\,0026+129 ($kT_e=39$\,keV) and IRAS\,04416+1215 ($kT_e=3$\,keV). Looking at
the position of the SEAMBHs sources of our sample in the
compactness-temperature diagram it appears that in super-Eddington AGN, as for
lower Eddington ratio AGN, the X-ray corona is controlled by pair production
and annihilation.
|
2212.06183v1
|
2022-12-15
|
Non-trivial band topology and orbital-selective electronic nematicity in a new titanium-based kagome superconductor
|
Electronic nematicity that spontaneously breaks rotational symmetry has been
shown as a generic phenomenon in correlated quantum systems including
high-temperature superconductors and the AV3Sb5 (A = K, Rb, Cs) family with a
kagome network. Identifying the driving force has been a central challenge for
understanding nematicity. In iron-based superconductors, the problem is
complicated because the spin, orbital and lattice degrees of freedom are
intimately coupled. In vanadium-based kagome superconductors AV3Sb5, the
electronic nematicity exhibits an intriguing entanglement with the charge
density wave order (CDW), making understanding its origin difficult. Recently,
a new family of titanium-based kagome superconductors ATi3Bi5 has been
synthesized. In sharp contrast to its vanadium-based counterpart, the
electronic nematicity occurs in the absence of CDW. ATi3Bi5 provides a new
window to explore the mechanism of electronic nematicity and its interplay with
the orbital degree of freedom. Here, we combine polarization-dependent
angle-resolved photoemission spectroscopy with density functional theory to
directly reveal the band topology and orbital characters of the multi-orbital
RbTi3Bi5. The promising coexistence of flat bands, type-II Dirac nodal line and
nontrivial Z2 topological states is identified in RbTi3Bi5. Remarkably, our
study clearly unveils the orbital character change along the G-M and G-K
directions, implying a strong intrinsic inter-orbital coupling in the Ti-based
kagome metals, reminiscent of iron-based superconductors. Furthermore,
doping-dependent measurements directly uncover the orbital-selective features
in the kagome bands, which can be well explained by the d-p hybridization. The
suggested d-p hybridization, in collaboration with the inter-orbital coupling,
could account for the electronic nematicity in ATi3Bi5.
|
2212.07958v1
|
2023-01-20
|
The chemical DNA of the Magellanic Clouds -- I. The chemical composition of 206 Small Magellanic Cloud red giant stars
|
We present the chemical composition of 206 red giant branch stars members of
the Small Magellanic Cloud (SMC) using optical, high-resolution spectra
collected with the multi-object spectrograph FLAMES-GIRAFFE at the ESO Very
Large Telescope. This sample includes stars in three fields located in
different positions within the parent galaxy. We analysed the main groups of
elements, namely light- (Na), alpha- (O, Mg, Si, Ca, Ti), iron-peak (Sc, V, Fe,
Ni, Cu) and s-process elements (Zr, Ba, La). The metallicity distribution of
the sample displays a main peak around [Fe/H] ~ -1 dex and a weak metal-poor
tail. However, the three fields display [Fe/H] distributions different with
each other, in particular a difference of 0.2 dex is found between the mean
metallicities of the two most internal fields.The fraction of metal-poor stars
increases significantly (from ~1 to ~20%) from the innermost fields to the most
external one, likely reflecting an age gradient in the SMC. Also, we found a
hint of possible chemically/kinematic distinct substructures. The SMC stars
have abundance ratios clearly distinct with respect to the Milky Way stars, in
particular for the elements produced by massive stars (like Na, $\alpha$ and
most iron-peak elements) that have abundance ratios systematically lower than
those measured in our Galaxy. This points out that the massive stars
contributed less to the chemical enrichment of the SMC with respect to the
Milky Way, according to the low star formation rate expected for this galaxy.
Finally, we identified small systematic differences in the abundances of some
elements (Na, Ti, V and Zr) in the two innermost fields, suggesting that the
chemical enrichment history in the SMC has been not uniform.
|
2301.08758v1
|
2023-01-30
|
The Hubble/STIS Near-ultraviolet Transmission Spectrum of HD 189733b
|
The benchmark hot Jupiter HD 189733b has been a key target to lay out the
foundations of comparative planetology for giant exoplanets. As such, HD
189733b has been extensively studied across the electromagnetic spectrum. Here,
we report the observation and analysis of three transit light curves of HD
189733b obtained with {\Hubble}/STIS in the near ultraviolet, the last
remaining unexplored spectral window to be probed with present-day
instrumentation for this planet. The NUV is a unique window for atmospheric
mass-loss studies owing to the strong resonance lines and large photospheric
flux. Overall, from a low-resolution analysis ($R=50$) we found that the
planet's near-ultraviolet spectrum is well characterized by a relatively flat
baseline, consistent with the optical-infrared transmission, plus two regions
at $\sim$2350 and $\sim$2600 {\AA} that exhibit a broad and significant excess
absorption above the continuum. From an analysis at a higher resolution
($R=4700$), we found that the transit depths at the core of the magnesium
resonance lines are consistent with the surrounding continuum. We discarded the
presence of \ion{Mg}{ii} absorption in the upper atmosphere at a
$\sim$2--4$\sigma$ confidence level, whereas we could place no significant
constraint for \ion{Mg}{i} absorption. These broad absorption features coincide
with the expected location of \ion{Fe}{ii} bands; however, solar-abundance
hydrodynamic models of the upper atmosphere are not able to reproduce the
amplitude of these features with iron absorption. Such scenario would require a
combination of little to no iron condensation in the lower-atmosphere,
super-solar metallicities, and a mechanism to enhance the absorption features
(such as zonal wind broadening). The true nature of this feature remains to be
confirmed.
|
2301.13025v1
|
2023-01-30
|
A Catalog of 71 Coronal Line Galaxies in MaNGA: [NeV] is an Effective AGN Tracer
|
Despite the importance of AGN in galaxy evolution, accurate AGN
identification is often challenging, as common AGN diagnostics can be confused
by contributions from star formation and other effects (e.g.,
Baldwin-Phillips-Terlevich diagrams). However, one promising avenue for
identifying AGNs are ``coronal emission lines" (``CLs"), which are highly
ionized species of gas with ionization potentials $\ge$ 100 eV. These CLs may
serve as excellent signatures for the strong ionizing continuum of AGN. To
determine if CLs are in fact strong AGN tracers, we assemble and analyze the
largest catalog of optical CL galaxies using the Sloan Digital Sky Survey's
Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) catalog. We detect
CL emission in 71 MaNGA galaxies, out of the 10,010 unique galaxies from the
final MaNGA catalog, with $\ge$ 5$\sigma$ confidence. In our sample, we measure
[NeV]$\lambda$3347, $\lambda$3427, [FeVII]$\lambda$3586, $\lambda$3760,
$\lambda$6086, and [FeX]$\lambda$6374 emission and crossmatch the CL galaxies
with a catalog of AGNs that were confirmed with broad line, X-ray, IR, and
radio observations. We find that [NeV] emission, compared to [FeVII] and [FeX]
emission, is best at identifying high luminosity AGN. Moreover, we find that
the CL galaxies with the least dust extinction yield the most iron CL
detections. We posit that the bulk of the iron CLs are destroyed by dust grains
in the galaxies with the highest [OIII] luminosities in our sample, and that
AGN in the galaxies with low [OIII] luminosities are possibly too weak to be
detected using traditional techniques.
|
2301.13322v1
|
2023-02-09
|
Chaotic winds from a dying world: a one-dimensional map for evolving atmospheres
|
Planets which are smaller than Mercury and heated to sublimation temperatures
of $\sim$2000 K lose mass catastrophically in dusty evaporative winds. The
winds are observed to gust and recede largely without pattern; transit depths
from the Kepler mission vary randomly from orbit to orbit by up to a factor of
10 or more. We explain how chaotic outflows may arise by constructing a map for
the wind mass-loss rate as a function of time. The map is built on three
statements: (1) The wind mass-loss rate scales in proportion to the surface
equilibrium vapor pressure, rising exponentially with ground temperature. (2)
Because the wind takes a finite time to escape the planet's gravity well, the
surface mass-loss rate at any time determines the wind optical depth at a later
time -- the atmosphere has hysteresis. (3) The ground temperature increases
with optical depth (greenhouse effect) when the atmosphere is optically thin,
and decreases with optical depth when the atmosphere is optically thick
(nuclear winter). Statement (3) follows from how dust condenses in the face of
intense stellar irradiation. As discussed recently, condensates initially naked
before the star must be silicate-rich and iron-poor, staying cool enough for
condensation by absorbing weakly in the visible and emitting strongly in the
infrared. Later, when grains are numerous enough to self-shield from starlight,
they may accrete more iron and reverse their visible-to-infrared opacity ratio.
Depending on parameters, the map for the wind can regularly boom and bust
between a greenhouse and a nuclear winter, or erupt into chaos. Lyapunov times
are measured in orbital periods, the time for the wind to turn by Coriolis
forces away from the planet's dayside, out of the Hill sphere.
|
2302.04898v2
|
2023-03-02
|
Concurrent Ferromagnetism and Superconductivity in Fe(Te,Se) van der Waals Josephson Junctions
|
Ferromagnetism and superconductivity are two key ingredients to create
non-Abelian quasiparticle excitations that are expected as building blocks to
construct topological quantum computers. Adversely, ferromagnetism and
superconductivity are typically also two hostile orderings competing to align
spins in different configurations, making the material design and experimental
implementation extremely challenging. Recently, iron-based superconductor
Fe(Te,Se) has emerged as a connate topological superconductor (TSC), which
differentiates itself from other hybrid TSCs by self-proximitizing its Dirac
surface states with bulk superconductivity. So far, the efforts to search for
Majorana states in this material are prevalently focused on spectroscopy
techniques. In this paper, we present the global transport signature of
interfacial magnetism coexisting with superconductivity. Time-reversal symmetry
breaking superconducting states are confirmed through device level transport
measurements for the first time in a van der Waals (vdW) Josephson junction
structure. Magnetic hysteresis is observed in this device scheme, which only
appears below the superconducting critical temperature, leading to potential
Fulde-Ferrell (FF) superconducting pairing mechanisms. The 0-{\pi} phase mixing
in the Fraunhofer patterns pinpoints the ferromagnetic state dwelling on the
surface. Furthermore, a stochastic field-free superconducting diode effect also
confirms the spontaneous time-reversal symmetry breaking which reflects the
behavior of the ferromagnetism. Our work paves a new way to explore topological
superconductivity in iron-based superconductors for future high Tc
fault-tolerant qubit implementations from a device perspective.
|
2303.00966v2
|
2023-03-19
|
$\rm Li_x(C_5H_5N)_yFe_{2-z}Se_2$: a defect resilient expanded-lattice high-temperature superconductor
|
Two-dimensional iron-chalcogenide intercalates display a remarkable
correlation of the interlayer spacing with the enhancement of the
superconducting critical temperature ($T_c$). In this work, synchrotron x-ray
absorption ($XAS$, at Fe and Se K edges) and emission ($XES$) spectroscopies,
allow to discuss how the important rise of $T_c$ (44 K) in the molecule
intercalated $\rm Li_x(C_5H_5N)_yFe_{2-z}Se_2$ relates to the electronic and
local structure changes felt by the inorganic host upon doping ($x$). $XES$
shows that widely-separated layers of edge-sharing $\rm FeSe_4$ tetrahedra,
carry low-spin moieties with a local Fe magnetic moment slightly reduced
compared to the parent $\beta$-$\rm Fe_{2-z}Se_2$. Pre-edge $XAS$ advises on
the progressively reduced mixing of metal $3d-4p$ states upon lithiation.
Doping-mediated local lattice modifications, probed by conventional
$T_c$-optimization measures (cf. anion height and $FeSe_4$ tetrahedra
regularity), become less relevant when layers are spaced far away. On the basis
of extended x-ray absorption fine structure, such distortions are compensated
by a softer Fe-network that relates to Fe-site vacancies, alleviating
electron-lattice correlations and superconductivity. Density functional theory
($DFT$) guided modification of isolated $\rm Fe_{2-z}Se_2$ ($z$, vacant sites)
planes, resembling the host layers, identify that Fe-site deficiency occurs at
low energy cost, giving rise to stretched Fe-sheets, in accord with
experiments. The robust high-$T_c$ in $\rm Li_x(C_5H_5N)_yFe_{2-z}Se_2$, arises
from the interplay of electron donating spacers and the iron-selenide layers
tolerance to defect chemistry, a tool to favorably tune its Fermi surface
properties.
|
2303.10716v1
|
2023-05-02
|
Lunar Mantle Structure and Composition Inferred From Apollo 12 -- Explorer 35 Electromagnetic Sounding
|
Constraints on the interior of the Moon have been derived from its inductive
response, principally as measured by the magnetic transfer function (TF)
between the distantly orbiting Explorer 35 satellite and the Apollo 12 surface
station. The most successful prior studies used a dataset spanning 0.01-1 mHz,
so the lunar response could be modeled as a simple dipole. However, earlier
efforts also produced transfer functions up to 40 mHz. The smaller
electromagnetic skin depth at higher frequency would better resolve the
uppermost mantle-where key information about primitive lunar evolution may
still be preserved-but requires a multipole treatment. I compute new profiles
of electrical conductivity vs depth using both low- and high-frequency ranges
of published Apollo-Explorer TFs. Using the low-frequency data, I derive
temperature profiles at depths >400 km (<1 mHz) consistent with conductive heat
loss and expectations of the iron (and possibly water) content of the mantle.
The near-constant iron fraction (Mg# 81 +/- 10) could imply efficient mixing
due to now-defunct convection. Alternatively, incomplete overturn of
gravitationally unstable magma-ocean cumulates could have left a heterogeneous
distribution of minerals at hundred-km scales that are not resolved by
electromagnetic sounding. A third explanation is that the electromagnetically
probed region may be the initial equilibrium crystallization in a mantle that
did not buoyantly overturn. In contrast, the high-frequency data produced
higher conductivities than expected, requiring unrealistically low Mg# or high
water content. Either the published transfer functions >> 1 mHz are incorrect,
or the TF multipole method at the Moon is unreliable. Future electromagnetic
sounding using the magnetotelluric method can operate up to 100s Hz and is
largely insensitive to multipole effects, resolving structure to 100 km or
less.
|
2305.01462v2
|
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