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2014-07-30
|
The NuSTAR spectrum of Mrk 335: Extreme relativistic effects within 2 gravitational radii of the event horizon?
|
We present 3-50 keV NuSTAR observations of the AGN Mrk 335 in a very low flux
state. The spectrum is dominated by very strong features at the energies of the
iron line at 5-7 keV and Compton hump from 10-30 keV. The source is variable
during the observation, with the variability concentrated at low energies,
which suggesting either a relativistic reflection or a variable absorption
scenario. In this work we focus on the reflection interpretation, making use of
new relativistic reflection models that self consistently calculate the
reflection fraction, relativistic blurring and angle-dependent reflection
spectrum for different coronal heights to model the spectra. We find that the
spectra can be well fit with relativistic reflection, and that the lowest flux
state spectrum is described by reflection alone, suggesting the effects of
extreme light-bending occurring within ~2 gravitational radii of the event
horizon. The reflection fraction decreases sharply with increasing flux,
consistent with a point source moving up to above 10 Rg as the source
brightens. We constrain the spin parameter to greater than 0.9 at the 3 sigma
confidence level. By adding a spin-dependent upper limit on the reflection
fraction to our models, we demonstrate that this can be a powerful way of
constraining the spin parameter, particularly in reflection dominated states.
We also calculate a detailed emissivity profile for the iron line, and find
that it closely matches theoretical predictions for a compact source within a
few Rg of the black hole.
|
1407.8223v1
|
2014-09-08
|
Chemical tagging of the Ursa Major moving group: A northern selection of FGK stars
|
Stellar kinematic groups are kinematical coherent groups of stars which might
share a common origin.These groups spread through the Galaxy over time due to
tidal effects caused by Galactic rotation and disc heating.However, the
chemical information survives these processes. The information provided by the
analysis of chemical elements can reveal the origin of these kinematic groups.
Here we investigate the origin of the stars that belong to the Ursa Major
Moving Group. We present high-resolution spectroscopic observations obtained
from three different spectrographs of kinematically selected FGK stars of the
Ursa Major moving group. Stellar atmospheric parameters (Teff, log(g), xi, and
[Fe/H]) were determined using our own automatic code (StePar) which makes use
of the sensitivity of iron equivalent widths measured in the spectra. We
critically compare the StePar results with other methods (Teff values derived
using the infrared flux method and log(g) values based on Hipparcos
parallaxes). We derived the chemical abundances of 20 elements, and their
[X/Fe] ratios of all stars in the sample. We perform a differential abundance
analysis with respect to a reference star of the UMa MG (HD115043). We have
also carried out a systematic comparison of the abundance pattern of the Ursa
Major MG and the Hyades SC with the thin disc stellar abundances. Our chemical
tagging analysis indicates that the Ursa Major MG is less affected by field
star contamination than other moving groups (such as the Hyades SC). We find a
roughly solar iron composition for the finally selected stars, whereas the
[X/Fe] ratios are roughly sub-solar except for super-solar Barium abundance.We
conclude that 29 out of 44 (i.e. 66 %) candidate stars share a similar chemical
composition. In addition, we find that the abundance pattern of the Ursa Major
MG might be marginally different from that of the Hyades SC.
|
1409.2348v1
|
2014-09-11
|
XMM-Newton observations reveal the disappearance of the wind in 4U 1630-47
|
We report on XMM-Newton observations of the black hole X-ray binary 4U
1630-47 during its 2012-2013 outburst. The first five observations monitor the
source as its luminosity increases across the high-soft state of accretion. In
the sixth observation the source has made a transition to an "anomalous" state,
characterised by a significant contribution of electron scattering. A
thermally/radiatively driven disc wind is present in the first four
observations, which becomes more photoionised as the luminosity increases with
time. In the fifth observation, the wind is not observed any more as a
consequence of strong photoionisation and the low sensitivity of this
observation. This overall trend is then consistent with a fully ionised wind
causing the electron scattering characteristic of the anomalous state in the
sixth observation. A broad iron emission line co-exists with the absorption
features from the wind in the first four observations but is not visible in the
last two observations. We find that the changes in the state of the wind as
measured from modelling the absorption features with a self-consistent warm
absorber model are correlated to the changes in the broad iron line. When the
latter is modeled with a reflection component we find that the reflection
fraction decreases as the illumination increases. We propose that the changes
in both the absorption and broad emission lines are caused by the increasing
luminosity and temperature of the accretion disc along the soft state. Such
changes ultimately enable the transition to a state where the wind is fully
ionised and consequently Comptonisation plays a significant role.
|
1409.3406v2
|
2014-09-15
|
The origin of the most iron-poor star
|
We investigate the origin of carbon-enhanced metal-poor (CEMP) stars starting
from the recently discovered $\rm [Fe/H]<-7.1$ star SMSS J031300 (Keller et al.
2014). We show that the elemental abundances observed on the surface of SMSS
J031300 can be well fit by the yields of faint, metal free, supernovae. Using
properly calibrated faint supernova explosion models, we study, for the first
time, the formation of dust grains in such carbon-rich, iron-poor supernova
ejecta. Calculations are performed assuming both unmixed and uniformly mixed
ejecta and taking into account the partial destruction by the supernova reverse
shock. We find that, due to the paucity of refractory elements beside carbon,
amorphous carbon is the only grain species to form, with carbon condensation
efficiencies that range between (0.15-0.84), resulting in dust yields in the
range (0.025-2.25)M$_{\odot}$. We follow the collapse and fragmentation of a
star forming cloud enriched by the products of these faint supernova explosions
and we explore the role played by fine structure line cooling and dust cooling.
We show that even if grain growth during the collapse has a minor effect of the
dust-to-gas ratio, due to C depletion into CO molecules at an early stage of
the collapse, the formation of CEMP low-mass stars, such as SMSS J031300, could
be triggered by dust cooling and fragmentation. A comparison between model
predictions and observations of a sample of C-normal and C-rich metal-poor
stars supports the idea that a single common pathway may be responsible for the
formation of the first low-mass stars.
|
1409.4424v2
|
2014-09-26
|
VLT/FLAMES spectroscopy of red giant branch stars in the Fornax dwarf spheroidal galaxy
|
Fornax is one of the most massive dwarf spheroidal galaxies in the Local
Group. The Fornax field star population is dominated by intermediate age stars
but star formation was going on over almost its entire history. It has been
proposed that Fornax experienced a minor merger event. Despite recent progress,
only the high metallicity end of Fornax field stars ([Fe/H]>-1.2 dex) has been
sampled in larger number via high resolution spectroscopy. We want to better
understand the full chemical evolution of this galaxy by better sampling the
whole metallicity range, including more metal poor stars. We use the VLT-FLAMES
multi-fibre spectrograph in high-resolution mode to determine the abundances of
several alpha, iron-peak and neutron-capture elements in a sample of 47
individual Red Giant Branch stars in the Fornax dwarf spheroidal galaxy. We
combine these abundances with accurate age estimates derived from the age
probability distribution from the colour-magnitude diagram of Fornax. Similar
to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are
typically alpha-rich while the young metal-rich stars are alpha-poor. In the
classical scenario of the time delay between SNe II and SNe Ia, we confirm that
SNe Ia started to contribute to the chemical enrichment at [Fe/H] between -2.0
and -1.8 dex. We find that the onset of SNe Ia took place between 12-10 Gyrs
ago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia and
AGB stars in the abundance pattern of the younger stellar population of Fornax.
Our findings of low [alpha/Fe] and enhanced [Eu/Mg] are compatible with an
initial mass function that lacks the most massive stars and with star formation
that kept going on throughout the whole history of Fornax. We find that massive
stars kept enriching the interstellar medium in alpha-elements, although they
were not the main contributor to the iron enrichment.
|
1409.7703v1
|
2014-10-03
|
Orbital occupancy and charge doping in iron-based superconductors
|
Iron-based superconductors (FBS) comprise several families of compounds
having the same atomic building blocks for superconductivity, but large
discrepancies among their physical properties. A longstanding goal in the field
has been to decipher the key underlying factors controlling TC and the various
doping mechanisms. In FBS materials this is complicated immensely by the
different crystal and magnetic structures exhibited by the different families.
In this paper, using aberration-corrected scanning transmission electron
microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), we
observe a universal behavior in the hole concentration and magnetic moment
across different families. All the parent materials have the same total number
of electrons in the Fe 3d bands; however, the local Fe magnetic moment varies
due to different orbital occupancy. Although the common understanding has been
that both long-range and local magnetic moments decrease with doping, we find
that, near the onset of superconductivity, the local magnetic moment increases
and shows a dome-like maximum near optimal doping, where no ordered magnetic
moment is present. In addition, we address a longstanding debate concerning how
Co substitutions induces superconductivity in the 122 arsenide family, showing
that the 3d band filling increases a function of doping. These new microscopic
insights into the properties of FBS demonstrate the importance of spin
fluctuations for the superconducting state, reveal changes in orbital occupancy
among different families of FBS, and confirm charge doping as one of the
mechanisms responsible for superconductivity in 122 arsenides.
|
1410.0971v1
|
2014-12-11
|
The 2D Distribution of Iron Rich Ejecta in the Remnant of SN 1885 in M31
|
We present Hubble Space Telescope (HST) ultraviolet Fe I and Fe II images of
the remnant of Supernova 1885 (S And) which is observed in absorption against
the bulge of the Andromeda galaxy, M31. We compare these Fe I and Fe II
absorption line images to previous HST absorption images of S And, of which the
highest quality and theoretically cleanest is Ca II H & K. Because the remnant
is still in free expansion, these images provide a 2D look at the distribution
of iron synthesized in this probable Type Ia explosion, thus providing insights
and constraints for theoretical SN Ia models. The Fe I images show extended
absorption offset to the east from the remnant's center as defined by Ca II
images and is likely an ionization effect due to self-shielding. More
significant is the remnant's apparent Fe II distribution which consists of four
streams or plumes of Fe-rich material seen in absorption that extend from
remnant center out to about 10,000 km/s. This is in contrast to the remnant's
Ca II absorption, which is concentrated in a clumpy, roughly spherical shell at
1000 to 5000 km/s but which extends out to 12,500 km/s. The observed
distributions of Ca and Fe rich ejecta in the SN 1885 remnant are consistent
with delayed detonation white dwarf models. The largely spherical symmetry of
the Ca-rich layer argues against a highly anisotropic explosion as might result
from a violent merger of two white dwarfs.
|
1412.3815v3
|
2014-12-31
|
Strong Variability of Overlapping Iron Broad Absorption Lines in Five Radio-selected Quasars
|
We present the variability study of broad absorption lines (BALs) in a
uniformly radio-selected sample of 28 BAL quasars using the archival data from
the FIRST Bright Quasar Survey (FBQS) and the Sloan Digital Sky Survey (SDSS),
as well as those obtained by ourselves, covering time scales $\sim 1-10$ years
in the quasar's rest-frame. To our surprise, 5 quasars showing strong
variations are all belong to a special subclass of overlapping iron low
ionization BAL (OFeLoBAL) quasars, however, other 4 non-overlapping FeLoBALs
(non-OFeLoBALs) are invariable except one case with weak optical depth change.
Meanwhile, we also find 6 typical variations of high-ionization and
low-ionization BALs in this BAL quasar sample. Photoionization models suggest
that OFeLoBALs are formed in a relative dense ($n_e>10^6$ cm$^{-3}$) outflows
at a distance from the subparsec to the dozens of parsecs from the continuum
source. They differ from those of non-OFeLoBALs, which are likely produced by
low-density gas, locating at a distance of hundreds to thousands parsecs. Thus,
OFeLoBALs and non-OFeLoBALs, i.e., FeLoBALs with/without strong BAL variations,
perhaps represent the bimodality of Fe II absorption, the former is located in
the active galactic nucleus environment rather than the host galaxy. We suggest
that high density and small distance are the necessary conditions that cause
OFeLoBALs. As suggested in the literature, strong BAL variability is possibly
due to variability of the covering factor of BAL regions caused by clouds
transiting across the line of sight rather than ionization variations.
|
1501.00091v3
|
2015-01-02
|
Determining the Mass of Kepler-78b With Nonparametric Gaussian Process Estimation
|
Kepler-78b is a transiting planet that is 1.2 times the radius of Earth and
orbits a young, active K dwarf every 8 hours. The mass of Kepler-78b has been
independently reported by two teams based on radial velocity measurements using
the HIRES and HARPS-N spectrographs. Due to the active nature of the host star,
a stellar activity model is required to distinguish and isolate the planetary
signal in radial velocity data. Whereas previous studies tested parametric
stellar activity models, we modeled this system using nonparametric Gaussian
process (GP) regression. We produced a GP regression of relevant Kepler
photometry. We then use the posterior parameter distribution for our
photometric fit as a prior for our simultaneous GP + Keplerian orbit models of
the radial velocity datasets. We tested three simple kernel functions for our
GP regressions. Based on a Bayesian likelihood analysis, we selected a
quasi-periodic kernel model with GP hyperparameters coupled between the two RV
datasets, giving a Doppler amplitude of 1.86 $\pm$ 0.25 m s$^{-1}$ and
supporting our belief that the correlated noise we are modeling is
astrophysical. The corresponding mass of 1.87 $^{+0.27}_{-0.26}$ M$_{\oplus}$
is consistent with that measured in previous studies, and more robust due to
our nonparametric signal estimation. Based on our mass and the radius
measurement from transit photometry, Kepler-78b has a bulk density of
6.0$^{+1.9}_{-1.4}$ g cm$^{-3}$. We estimate that Kepler-78b is 32$\pm$26% iron
using a two-component rock-iron model. This is consistent with an Earth-like
composition, with uncertainty spanning Moon-like to Mercury-like compositions.
|
1501.00369v2
|
2015-01-05
|
Relationship between X-ray spectral index and X-ray Eddington ratio for Mrk 335 and Ark 564
|
We present a comprehensive flux resolved spectral analysis of the bright
Narrow line Seyfert I AGNs, Mrk~335 and Ark~564 using observations by
XMM-Newton satellite. The mean and the flux resolved spectra are fitted by an
empirical model consisting of two Comptonization components, one for the low
energy soft excess and the other for the high energy power-law. A broad Iron
line and a couple of low energies edges are required to explain the spectra.
For Mrk~335, the 0.3 - 10 keV luminosity relative to the Eddington value,
L{$_{X}$}/L$_{Edd}$, varied from 0.002 to 0.06. The index variation can be
empirically described as $\Gamma$ = 0.6 log$_{10}$ L{$_{X}$}/L$_{Edd}$ + 3.0
for $0.005 < L{_{X}}/L_{Edd} < 0.04$. At $ L_{{X}}/L_{Edd} \sim 0.04$ the
spectral index changes and then continues to follow $\Gamma$ = 0.6 log$_{10}$
L$_{{X}}$/L$_{Edd}$ + 2.7, i.e. on a parallel track. We confirm that the result
is independent of the specific spectral model used by fitting the data in the 3
- 10 keV band by only a power-law and an Iron line. For Ark~564, the index
variation can be empirically described as $\Gamma$ = 0.2 log$_{10}$
L$_{{X}}$/L$_{Edd}$ + 2.7 with a significantly large scatter as compared to
Mrk~335. Our results indicate that for Mrk~335, there may be accretion disk
geometry changes which lead to different parallel tracks. These changes could
be related to structural changes in the corona or enhanced reflection at high
flux levels. There does not seem to be any homogeneous or universal
relationship for the X-ray index and luminosity for different AGNs or even for
the same AGN.
|
1501.00908v1
|
2015-01-08
|
Neutrino-driven supernova of a low-mass iron-core progenitor boosted by three-dimensional turbulent convection
|
We present the first successful simulation of a neutrino-driven supernova
explosion in three dimensions (3D), using the Prometheus-Vertex code with an
axis-free Yin-Yang grid and a sophisticated treatment of three-flavor,
energy-dependent neutrino transport. The progenitor is a nonrotating,
zero-metallicity 9.6 Msun star with an iron core. While in spherical symmetry
outward shock acceleration sets in later than 300 ms after bounce, a successful
explosion starts at ~130 ms postbounce in two dimensions (2D). The 3D model
explodes at about the same time but with faster shock expansion than in 2D and
a more quickly increasing and roughly 10 percent higher explosion energy of
>10^50 erg. The more favorable explosion conditions in 3D are explained by
lower temperatures and thus reduced neutrino emission in the cooling layer
below the gain radius. This moves the gain radius inward and leads to a bigger
mass in the gain layer, whose larger recombination energy boosts the explosion
energy in 3D. These differences are caused by less coherent, less massive, and
less rapid convective downdrafts associated with postshock convection in 3D.
The less violent impact of these accretion downflows in the cooling layer
produces less shock heating and therefore diminishes energy losses by neutrino
emission. We thus have, for the first time, identified a reduced mass accretion
rate, lower infall velocities, and a smaller surface filling factor of
convective downdrafts as consequences of 3D postshock turbulence that
facilitate neutrino-driven explosions and strengthen them compared to the 2D
case.
|
1501.01961v2
|
2015-01-20
|
Spectroscopic Abundances in the Open Cluster, NGC 6819
|
High-dispersion spectra of 333 stars in the open cluster NGC 6819, obtained
using the HYDRA spectrograph on the WIYN 3.5m telescope, have been analyzed to
determine the abundances of iron and other metals from lines in the 400 A
region surrounding the Li 6708 A line. Our spectra, with signal-to-noise per
pixel (SNR) ranging from 60 to 300, span the luminosity range from the tip of
the red giant branch to a point two magnitudes below the top of the cluster
turnoff. We derive radial and rotational velocities for all stars, as well as
[Fe/H] based on 17 iron lines, [Ca/H], [Si/H], and [Ni/H] in the 247 most
probable, single members of the cluster. Input temperature estimates for model
atmosphere analysis are provided by (B-V) colors merged from several sources,
with individual reddening corrections applied to each star relative to a
cluster mean of E(B-V) = 0.16. Extensive use is made of ROBOSPECT, an automatic
equivalent width measurement program; its effectiveness on large spectroscopic
samples is discussed. From the sample of likely single members, [Fe/H] = -0.03
+/- 0.06, where the error describes the median absolute deviation about the
sample median value, leading to an internal precision for the cluster below
0.01 dex. The final uncertainty in the cluster abundance is therefore dominated
by external systematics due to the temperature scale, surface gravity, and
microturbulent velocity, leading to [Fe/H] = -0.02 +/- 0.02 for a sub-sample
restricted to main sequence and turnoff stars. This result is consistent with
our recent intermediate-band photometric determination of a slightly subsolar
abundance for this cluster. [Ca/Fe], [Si/Fe], and [Ni/Fe] are determined to be
solar within the uncertainties. NGC 6819 has an abundance distribution typical
of solar metallicity thin disk stars in the solar neighborhood.
|
1501.04973v1
|
2015-01-26
|
A one-dimensional Chandrasekhar-mass delayed-detonation model for the broad-lined Type Ia supernova 2002bo
|
We present 1D non-local thermodynamic equilibrium (non-LTE) time-dependent
radiative-transfer simulations of a Chandrasekhar-mass delayed-detonation model
which synthesizes 0.51 Msun of 56Ni, and confront our results to the Type Ia
supernova (SN Ia) 2002bo over the first 100 days of its evolution. Assuming
only homologous expansion, this same model reproduces the bolometric and
multi-band light curves, the secondary near-infrared (NIR) maxima, and the
optical and NIR spectra. The chemical stratification of our model qualitatively
agrees with previous inferences by Stehle et al., but reveals significant
quantitative differences for both iron-group and intermediate-mass elements. We
show that +/-0.1 Msun (i.e., +/-20 per cent) variations in 56Ni mass have a
modest impact on the bolometric and colour evolution of our model. One notable
exception is the U-band, where a larger abundance of iron-group elements
results in less opaque ejecta through ionization effects, our model with more
56Ni displaying a higher near-UV flux level. In the NIR range, such variations
in 56Ni mass affect the timing of the secondary maxima but not their magnitude,
in agreement with observational results. Moreover, the variation in the I, J,
and K_s magnitudes is less than 0.1 mag within ~10 days from bolometric
maximum, confirming the potential of NIR photometry of SNe Ia for cosmology.
Overall, the delayed-detonation mechanism in single Chandrasekhar-mass white
dwarf progenitors seems well suited for SN 2002bo and similar SNe Ia displaying
a broad Si II 6355 A line. Whatever multidimensional processes are at play
during the explosion leading to these events, they must conspire to produce an
ejecta comparable to our spherically-symmetric model.
|
1501.06583v2
|
2015-02-26
|
Suzaku broad-band spectrum of 4U 1705-44: Probing the Reflection component in the hard state
|
Iron emission lines at 6.4-6.97 keV, identified with Kalpha radiative
transitions, are among the strongest discrete features in the X-ray band. These
are one of the most powerful probes to infer the properties of the plasma in
the innermost part of the accretion disk around a compact object. In this paper
we present a recent Suzaku observation, 100-ks effective exposure, of the atoll
source and X-ray burster 4U 1705-44, where we clearly detect signatures of a
reflection component which is distorted by the high-velocity motion in the
accretion disk. The reflection component consists of a broad iron line at about
6.4 keV and a Compton bump at high X-ray energies, around 20 keV. All these
features are consistently fitted with a reflection model, and we find that in
the hard state the smearing parameters are remarkably similar to those found in
a previous XMM-Newton observation performed in the soft state. In particular,
we find that the inner disk radius is Rin = 17 +/- 5 Rg (where Rg is the
Gravitational radius, GM/c^2), the emissivity dependence from the disk radius
is -2.5 +/- 0.5, the inclination angle with respect to the line of sight is i =
43 +/- 5 degrees, and the outer radius of the emitting region in the disk is
Rout > 200 Rg. We note that the accretion disk does not appear to be truncated
at large radii, although the source is in a hard state at about 3 % of the
Eddington luminosity for a neutron star. We also find evidence of a broad
emission line at low energies, at 3.03 +/- 0.03 keV, compatible with emission
from mildly ionized Argon (Ar XVI-XVII). Argon transitions are not included in
the self-consistent reflection models that we used and we therefore added an
extra component to our model to fit this feature. The low energy line appears
compatible with being smeared by the same inner disk parameters found for the
reflection component.
|
1502.07669v1
|
2015-02-26
|
The spin rate of pre-collapse stellar cores: wave-driven angular momentum transport in massive stars
|
The core rotation rates of massive stars have a substantial impact on the
nature of core-collapse supernovae and their compact remnants. We demonstrate
that internal gravity waves (IGW), excited via envelope convection during a red
supergiant phase or during vigorous late time burning phases, can have a
significant impact on the rotation rate of the pre-SN core. In typical ($10 \,
M_\odot \lesssim M \lesssim 20 \, M_\odot$) supernova progenitors, IGW may
substantially spin down the core, leading to iron core rotation periods $P_{\rm
min,Fe} \gtrsim 30 \, {\rm s}$. Angular momentum (AM) conservation during the
supernova would entail minimum NS rotation periods of $P_{\rm min,NS} \gtrsim 3
\, {\rm ms}$. In most cases, the combined effects of magnetic torques and IGW
AM transport likely lead to substantially longer rotation periods. However, the
stochastic influx of AM delivered by IGW during shell burning phases inevitably
spin up a slowly rotating stellar core, leading to a maximum possible core
rotation period. We estimate maximum iron core rotation periods of $P_{\rm
max,Fe} \lesssim 5 \times 10^3 \, {\rm s}$ in typical core-collapse supernova
progenitors, and a corresponding spin period of $P_{\rm max, NS} \lesssim 500
\, {\rm ms}$ for newborn neutron stars. This is comparable to the typical birth
spin periods of most radio pulsars. Stochastic spin-up via IGW during shell
O/Si burning may thus determine the initial rotation rate of most neutron
stars. For a given progenitor, this theory predicts a Maxwellian distribution
in pre-collapse core rotation frequency that is uncorrelated with the spin of
the overlying envelope.
|
1502.07779v3
|
2015-03-09
|
Gate-tuned Superconductor-Insulator transition in (Li,Fe)OHFeSe
|
The antiferromagnetic(AFM) insulator-superconductor transition has been
always a center of interest in the underlying physics of unconventional
superconductors. The quantum phase transition between Mott insulator with AFM
and superconductor can be induced by doping charge carriers in high-Tc cuprate
superconductors. For the best characterized organic superconductors of
k-(BEDT-TTF)2X (X=anion), a first order transition between AFM insulator and
superconductor can be tuned by applied external pressure or chemical pressure.
Also, the superconducting state can be directly developed from AFM insulator by
application of pressure in Cs3C60. The resemblance of these phase diagrams
hints a universal mechanism governing the unconventional superconductivity in
close proximity to AFM insulators. However, the superconductivity in iron-based
high-Tc superconductors evolves from an AFM bad metal by doping charge
carriers, and no superconductor-insulator transition has been observed so far.
Here, we report a first-order transition from superconductor to insulator with
a strong charge doping induced by ionic gating in the thin flakes of single
crystal (Li,Fe)OHFeSe. The Tc is continuously enhanced with electron doping by
ionic gating up to a maximum Tc of 43 K, and a striking
superconductor-insulator transition occurs just at the verge of optimal doping
with highest Tc. A novel phase diagram of temperature-gating voltage with the
superconductor-insulator transition is mapped out, indicating that the
superconductor -insulator transition is a common feature for unconventional
superconductivity. These results help to uncover the underlying physics of
iron-based superconductivity as well as the universal mechanism of high-Tc
superconductivity. Our finding also suggests that the gate-controlled strong
charge doping makes it possible to explore novel states of matter in a way
beyond traditional methods.
|
1503.02457v1
|
2015-03-12
|
On the alpha-element gradients of the Galactic thin disk using Cepheids
|
We present new homogeneous measurements of Na, Al and three alpha-elements
(Mg, Si, Ca) for 75 Galactic Cepheids. The abundances are based on high
spectral resolution (R ~ 38,000) and high signal-to-noise ratio (S/N ~ 50-300)
spectra collected with UVES at ESO VLT. The current measurements were
complemented with Cepheid abundances either provided by our group (75) or
available in the literature, for a total of 439 Galactic Cepheids. Special
attention was given in providing a homogeneous abundance scale for these five
elements plus iron (Genovali et al. 2013, 2014). In addition, accurate
Galactocentric distances (RG) based on near-infrared photometry are also
available for all the Cepheids in the sample (Genovali et al. 2014). They cover
a large fraction of the Galactic thin disk (4.1 <= RG <= 18.4 kpc). We found
that the above five elements display well defined linear radial gradients and
modest standard deviations over the entire range of RG. Moreover, the
[element/Fe] abundance ratios are constant across the entire thin disk; only
the Ca radial distribution shows marginal evidence of a positive slope. These
results indicate that the chemical enrichment history of iron and of the quoted
five elements has been quite similar across the four quadrants of the Galactic
thin disk. The [element/Fe] ratios are also constant over the entire period
range. This empirical evidence indicates that the chemical enrichment of
Galactic Cepheids has also been very homogenous during the range in age that
they cover (~10-300 Myr). Once again, [Ca/Fe] vs. log(P) shows a (negative)
gradient, being underabundant among youngest Cepheids. Finally, we also found
that Cepheid abundances agree quite well with similar abundances for thin and
thick disk dwarf stars and they follow the typical Mg-Al and Na-O correlations.
|
1503.03758v1
|
2015-03-30
|
Electrochemical synthesis of highly ordered nanowires with a rectangular cross-section using an in-plane nanochannel array
|
Rapid and reproducible assembly of aligned nanostructures on a wafer-scale is
a crucial, yet one of the most challenging tasks in the incorporation of
nanowires into integrated circuits. We present the synthesis of a periodic
nanochannel template designed for electrochemical growth of perfectly aligned,
rectangular nanowires over large areas. The nanowires can be electrically
contacted and characterized in situ using a pre-patterned multi-point
measurement platform. During the measurement the wires remain within a thick
oxide matrix providing protection against breaking and oxidation. We use laser
interference lithography, reactive ion etching and atomic layer deposition to
create cm-long parallel nanochannels with characteristic dimensions as small as
40 nm. In a showcase study pulsed electrodeposition of iron is carried out
creating rectangular shaped iron nanowires within the nanochannels. By design
of the device, the grown wires are in contact with an integrated electrode
system on both ends directly after the deposition. No further processing steps
are required for electrical characterization, minimizing the risk of damage and
oxidation. The developed nanowire measurement device allows for multi-probe
resistance measurements and can easily be adopted for transistor applications.
The guided, in-plane growth of electrodeposited nanowire arrays which are
tunable in size and density paves the way for the incorporation of nanowires
into a large variety of multifunctional devices.
|
1503.08597v1
|
2015-04-17
|
Electronic Structure and Superconductivity of FeSe-Related Superconductors
|
The FeSe superconductor and its related systems have attracted much attention
in the iron-based superconductors owing to their simple crystal structure and
peculiar electronic and physical properties. The bulk FeSe superconductor has a
superconducting transition temperature (Tc) of ~8 K; it can be dramatically
enhanced to 37 K at high pressure. On the other hand, its cousin system, FeTe,
possesses a unique antiferromagnetic ground state but is non-superconducting.
Substitution of Se by Te in the FeSe superconductor results in an enhancement
of Tc up to 14.5 K and superconductivity can persist over a large composition
range in the Fe(Se,Te) system. Intercalation of the FeSe superconductor leads
to the discovery of the AxFe2-ySe2 (A=K, Cs and Tl) system that exhibits a Tc
higher than 30 K and a unique electronic structure of the superconducting
phase. The latest report of possible high temperature superconductivity in the
single-layer FeSe/SrTiO3 films with a Tc above 65 K has generated much
excitement in the community. This pioneering work opens a door for interface
superconductivity to explore for high Tc superconductors. The distinct
electronic structure and superconducting gap, layer-dependent behavior and
insulator-superconductor transition of the FeSe/SrTiO3 films provide critical
information in understanding the superconductivity mechanism of the iron-based
superconductors. In this paper, we present a brief review on the investigation
of the electronic structure and superconductivity of the FeSe superconductor
and related systems, with a particular focus on the FeSe films.
|
1504.04436v2
|
2015-05-20
|
Constraints on explosive silicon burning in core-collapse supernovae from measured Ni/Fe ratios
|
Measurements of explosive nucleosynthesis yields in core-collapse supernovae
provide tests for explosion models. We investigate constraints on explosive
conditions derivable from measured amounts of nickel and iron after radioactive
decays using nucleosynthesis networks with parameterized thermodynamic
trajectories. The Ni/Fe ratio is for most regimes dominated by the production
ratio of 58Ni/(54Fe + 56Ni), which tends to grow with higher neutron excess and
with higher entropy. For SN 2012ec, a supernova that produced a Ni/Fe ratio of
$3.4\pm1.2$ times solar, we find that burning of a fuel with neutron excess
$\eta \approx 6\times 10^{-3}$ is required. Unless the progenitor metallicity
is over 5 times solar, the only layer in the progenitor with such a neutron
excess is the silicon shell. Supernovae producing large amounts of stable
nickel thus suggest that this deep-lying layer can be, at least partially,
ejected in the explosion. We find that common spherically symmetric models of
$M_{\rm ZAMS} \lesssim 13$ Msun stars exploding with a delay time of less than
one second ($M_{\rm cut} < 1.5$ Msun) are able to achieve such silicon-shell
ejection. Supernovae that produce solar or sub-solar Ni/Fe ratios, such as SN
1987A, must instead have burnt and ejected only oxygen-shell material, which
allows a lower limit to the mass cut to be set. Finally, we find that the
extreme Ni/Fe value of 60-75 times solar derived for the Crab cannot be
reproduced by any realistic-entropy burning outside the iron core, and
neutrino-neutronization obtained in electron-capture models remains the only
viable explanation.
|
1505.05323v1
|
2015-05-25
|
The Carina Project. VIII. The α-element abundances
|
We have performed a new abundance analysis of Carina Red Giant (RG) stars
from spectroscopic data collected with UVES (high resolution) and
FLAMES/GIRAFFE (high and medium resolution) at ESO/VLT. The former sample
includes 44 RGs, while the latter consists of 65 (high) and ~800 (medium
resolution) RGs, covering a significant fraction of the galaxy's RG branch
(RGB), and red clump stars. To improve the abundance analysis at the faint
magnitude limit, the FLAMES/GIRAFFE data were divided into ten surface gravity
and effective temperature bins. The spectra of the stars belonging to the same
gravity/temperature bin were stacked. This approach allowed us to increase by
at least a factor of five the signal-to-noise ratio in the faint limit
(V>20.5mag). We took advantage of the new photometry index cU,B,I introduced by
Monelli et al. (2014), as an age and probably a metallicity indicator, to split
stars along the RGB. These two stellar populations display distinct [Fe/H] and
[Mg/H] distributions: their mean Fe abundances are -2.15$\pm$0.06dex
(sig=0.28), and -1.75$\pm$0.03dex (sig=0.21), respectively. The two iron
distributions differ at the 75% level. This supports preliminary results by
Lemasle et al. (2012) and by Monelli et al. (2014). Moreover, we found that the
old and intermediate-age stellar populations have mean [Mg/H] abundances of
-1.91$\pm$0.05dex (sig=0.22) and -1.35$\pm$0.03dex (sig=0.22); these differ at
the 83% level. Carina's {\alpha}-element abundances agree, within 1sigma, with
similar abundances for field Halo stars and for cluster (Galactic, Magellanic)
stars. The same outcome applies to nearby dwarf spheroidals and ultra-faint
dwarf galaxies, in the iron range covered by Carina stars. Finally, we found
evidence of a clear correlation between Na and O abundances, thus suggesting
that Carina's chemical enrichment history is quite different than in the
globular clusters.
|
1505.06597v2
|
2015-05-25
|
Twin peak HF QPOs as a spectral imprint of dual oscillation modes of accretion tori
|
High frequency (millisecond) quasi-periodic oscillations (HF QPOs) are
observed in the X-ray power-density spectra of several microquasars and low
mass X-ray binaries. Two distinct QPO peaks, so-called twin peak QPOs, are
often detected simultaneously exhibiting their frequency ratio close or equal
to 3/2. Following the analytic theory and previous studies of observable
spectral signatures, we aim to model the twin peak QPOs as a spectral imprint
of specific dual oscillation regime defined by a combination of the lowest
radial and vertical oscillation mode of optically thick slender tori with
constant specific angular momentum. We examined power spectra and fluorescent
K$\alpha$ iron line profiles for two different simulation setups with the mode
frequency relations corresponding to the epicyclic resonance HF QPOs model and
modified relativistic precession QPOs model. We use relativistic ray-tracing
implemented in parallel simulation code LSDplus. In the background of the Kerr
spacetime geometry, we analyze the influence of the distant observer
inclination and the spin of the central compact object. Relativistic optical
projection of the oscillating slender torus is illustrated by images in false
colours related to the frequency shift. We show that performed simulations
yield power spectra with the pair of dominant peaks corresponding to the
frequencies of radial and vertical oscillation modes with the proper ratio
equal to 3/2 on a wide range of inclinations and spin values. We also discuss
exceptional cases of a very small and very high inclination as well as unstable
high spin relativistic precession-like configuration predicting constant
frequency ratio equal to 1/2. We demonstrate signifiant dependency of broadened
K$\alpha$ iron line profiles on the inclination of the distant observer.
|
1505.06673v1
|
2015-06-04
|
Systematic NLTE study of the -2.6 < [Fe/H] < 0.2 F and G dwarfs in the solar neighbourhood. I. Stellar atmosphere parameters
|
We present atmospheric parameters for 51 nearby FG dwarfs uniformly
distributed over the -2.60 < [Fe/H] < +0.20 metallicity range that is suitable
for the Galactic chemical evolution research. Lines of iron, Fe I and Fe II,
were used to derive a homogeneous set of effective temperatures, surface
gravities, iron abundances, and microturbulence velocities. We used
high-resolution (R>60000) Shane/Hamilton and CFHT/ESPaDOnS observed spectra and
non-local thermodynamic equilibrium (NLTE) line formation for Fe I and Fe II in
the classical 1D model atmospheres. The spectroscopic method was tested with
the 20 benchmark stars, for which there are multiple measurements of the
infrared flux method (IRFM) Teff and their Hipparcos parallax error is < 10%.
We found NLTE abundances from lines of Fe I and Fe II to be consistent within
0.06 dex for every benchmark star, when applying a scaling factor of S_H = 0.5
to the Drawinian rates of inelastic Fe+H collisions. The obtained atmospheric
parameters were checked for each program star by comparing its position in the
log g-Teff plane with the theoretical evolutionary track in the Yi et al.
(2004) grid. Our final effective temperatures lie in between the T_IRFM scales
of Alonso et al. (1996) and Casagrande et al. (2011), with a mean difference of
+46 K and -51 K, respectively. NLTE leads to higher surface gravity compared
with that for LTE. The shift in log g is smaller than 0.1 dex for stars with
either [Fe/H] > -0.75, or Teff < 5750 K, or log g > 4.20. NLTE analysis is
crucial for the VMP turn-off and subgiant stars, for which the shift in log g
between NLTE and LTE can be up to 0.5 dex. The obtained atmospheric parameters
will be used in the forthcoming papers to determine NLTE abundances of
important astrophysical elements from lithium to europium and to improve
observational constraints on the chemo-dynamical models of the Galaxy
evolution.
|
1506.01621v1
|
2015-07-02
|
A Spectroscopic Analysis of the Galactic Globular Cluster NGC 6273 (M19)
|
A combined effort utilizing spectroscopy and photometry has revealed the
existence of a new globular cluster class. These "anomalous" clusters, which we
refer to as "iron-complex" clusters, are differentiated from normal clusters by
exhibiting large (>0.10 dex) intrinsic metallicity dispersions, complex
sub-giant branches, and correlated [Fe/H] and s-process enhancements. In order
to further investigate this phenomenon, we have measured radial velocities and
chemical abundances for red giant branch stars in the massive, but scarcely
studied, globular cluster NGC 6273. The velocities and abundances were
determined using high resolution (R~27,000) spectra obtained with the
Michigan/Magellan Fiber System (M2FS) and MSpec spectrograph on the
Magellan-Clay 6.5m telescope at Las Campanas Observatory. We find that NGC 6273
has an average heliocentric radial velocity of +144.49 km s^-1 (sigma=9.64 km
s^-1) and an extended metallicity distribution ([Fe/H]=-1.80 to -1.30) composed
of at least two distinct stellar populations. Although the two dominant
populations have similar [Na/Fe], [Al/Fe], and [alpha/Fe] abundance patterns,
the more metal-rich stars exhibit significant [La/Fe] enhancements. The [La/Eu]
data indicate that the increase in [La/Fe] is due to almost pure s-process
enrichment. A third more metal-rich population with low [X/Fe] ratios may also
be present. Therefore, NGC 6273 joins clusters such as omega centauri, M 2, M
22, and NGC 5286 as a new class of iron-complex clusters exhibiting complicated
star formation histories.
|
1507.00756v1
|
2015-08-07
|
Spectrophotometry of asteroids 32 Pomona, 145 Adeona, 704 Interamnia, 779 Nina, 330825, and 2012 QG42 and laboratory study of possible analog samples
|
Six asteroids including two NEAs, one of which is PHA, accessible for
observation in September 2012 were investigated using a low-resolution
spectrophotometry in the range 0.35-0.90 um with the aim to study features of
their reflectance spectra. For the first time we discovered likely spectral
signs (as a maximum at 0.4-0.6 um in reflectance spectra) of simultaneous
sublimation activity and presence of a temporal coma on three primitive-type
main-belt asteroids, Adeona, Interamnia, and Nina, being at perihelion
distances or approaching to it. We suggest that such a cometary-like activity
may be a common phenomenon at the highest subsolar surface temperatures for C
and close type asteroids including considerable amounts of H2O and CO2 ices
beneath the surface. However, excavation of fresh ice at recent impact event(s)
could be an alternative explanation of the phenomenon. Similar absorption bands
centered at 0.38, 0.44 and 0.67-0.71 um registered in the reflectance spectra
of Adeona, Interamnia, and Nina clearly point to predominantly silicate surface
matter. To specify its content, we performed laboratory investigations of
ground samples of known carbonaceous chondrites (Orguel, Mighei, Murchison, and
Boriskino) and seven samples of low-iron Mg serpentines as possible analogs of
the asteroids. In particular, we found that the equivalent width of the band
centered at 0.44 um in reflectance spectra of the low-Fe serpentine samples has
a high correlation with content of Fe3+ (octahedral and tetrahedral). It means
that the absorption feature can be used as an indicator of ferric iron in
oxidized and hydrated low-Fe silicate compounds on asteroids (abridged).
|
1508.01690v1
|
2015-09-08
|
Identifying the best iron-peak and $α$-capture elements for chemical tagging: The impact of the number of lines on measured scatter
|
The main goal of this work is to explore which elements carry the most
information about the birth origin of stars and as such that are best suited
for chemical tagging. We explored different techniques to minimize the effect
of outlier value lines in the abundances by using Ni abundances derived for
1111 FGK type stars.We evaluated how the limited number of spectral lines can
affect the final chemical abundance. Then we were able to make an efficient
even footing comparison of the [X/Fe] scatter between the elements that have
different number of observable spectral lines in the studied spectra. We found
that the most efficient way of calculating the average abundance of elements
when several spectral lines are available is to use a weighted mean (WM) where
as a weight we considered the distance from the median abundance. This method
can be effectively used without removing suspected outlier lines.We showed that
when the same number of lines is used to determine chemical abundances, the
[X/Fe] star-to-star scatter for iron group and $\alpha$-capture elements is
almost the same. On top of this, but at a lower level the largest scatter was
observed for Al and the smallest for Cr and Ni. We recommend caution when
comparing [X/Fe] scatters among elements that have a different number of
spectral lines available. A meaningful comparison is necessary to identify
elements that show the largest intrinsic scatter and can be thus used for
chemical tagging.
|
1509.02419v1
|
2015-09-08
|
Metallicity of Ca2Cu6P5 with Single and Double Copper-Pnictide Layers
|
We report thermodynamic and transport properties, and also theoretical
calculations, for Cu-based compound Ca2Cu6P5 and compare with CaCu(2-x)P2. Both
materials have layers of edge-sharing copper pnictide tetrahedral CuP4, similar
to Fe-As and Fe-Se layers (with FeAs4, FeSe4) in the iron-based
superconductors. Despite the presence of this similar transition-metal pnictide
layer, we find that both Ca2Cu6P5 and CaCu(2-x)P2 have temperature-independent
magnetic susceptibility and show metallic behavior with no evidence of either
magnetic ordering or superconductivity down to 1.8 K. CaCu(2-x)P2 is slightly
off-stoichiometric, with delta = 0.14. Theoretical calculations suggest that
unlike Fe 3d-based magnetic materials with a large density of states (DOS) at
the Fermi surface, Cu have comparatively low DOS, with the majority of the 3d
spectral weight located well below Fermi level. The room-temperature
resistivity value of Ca2Cu6P5 is only 9 micro ohm-cm, due to a substantial
plasma frequency and an inferred electron-phonon coupling lambda of 0.073
(significantly smaller than that of metallic Cu). Also, microscopy result shows
that Cu-Cu distance along the c-axis within the double layers can be very short
(2.5 A), even shorter than metallic elemental copper bond (2.56 A). The value
of dp over dT for CaCu(2-x)P2 at 300 K is approximately three times larger than
in Ca2Cu6P5, which suggests the likelihood of stronger electron-phonon
coupling. This study shows that the details of Cu-P layers and bonding are
important for their transport characteristics. In addition, it emphasizes the
remarkable character of the DOS of '122' iron-based materials, despite much
structural similarities.
|
1509.02508v1
|
2015-09-14
|
Cool Core Clusters from Cosmological Simulations
|
We present results obtained from a set of cosmological hydrodynamic
simulations of galaxy clusters, aimed at comparing predictions with
observational data on the diversity between cool-core (CC) and non-cool-core
(NCC) clusters. Our simulations include the effects of stellar and AGN feedback
and are based on an improved version of the smoothed particle hydrodynamics
code GADGET-3, which ameliorates gas mixing and better captures gas-dynamical
instabilities by including a suitable artificial thermal diffusion. In this
Letter, we focus our analysis on the entropy profiles, the primary diagnostic
we used to classify the degree of cool-coreness of clusters, and on the iron
profiles. In keeping with observations, our simulated clusters display a
variety of behaviors in entropy profiles: they range from steadily decreasing
profiles at small radii, characteristic of cool-core systems, to nearly flat
core isentropic profiles, characteristic of non-cool-core systems. Using
observational criteria to distinguish between the two classes of objects, we
find that they occur in similar proportions in both simulations and in
observations. Furthermore, we also find that simulated cool-core clusters have
profiles of iron abundance that are steeper than those of NCC clusters, which
is also in agreement with observational results. We show that the capability of
our simulations to generate a realistic cool-core structure in the cluster
population is due to AGN feedback and artificial thermal diffusion: their
combined action allows us to naturally distribute the energy extracted from
super-massive black holes and to compensate for the radiative losses of
low-entropy gas with short cooling time residing in the cluster core.
|
1509.04247v2
|
2015-09-29
|
Enrichment of r-process elements in dwarf spheroidal galaxies in chemo-dynamical evolution model
|
The rapid neutron-capture process (r-process) is a major process to
synthesize elements heavier than iron, but the astrophysical site(s) of
r-process is not identified yet. Neutron star mergers (NSMs) are suggested to
be a major r-process site from nucleosynthesis studies. Previous chemical
evolution studies however require unlikely short merger time of NSMs to
reproduce the observed large star-to-star scatters in the abundance ratios of
r-process elements relative to iron, [Eu/Fe], of extremely metal-poor stars in
the Milky Way (MW) halo. This problem can be solved by considering chemical
evolution in dwarf spheroidal galaxies (dSphs) which would be building blocks
of the MW and have lower star formation efficiencies than the MW halo. We
demonstrate that enrichment of r-process elements in dSphs by NSMs using an
N-body/smoothed particle hydrodynamics code. Our high-resolution model
reproduces the observed [Eu/Fe] by NSMs with a merger time of 100 Myr when the
effect of metal mixing is taken into account. This is because metallicity is
not correlated with time up to ~ 300 Myr from the start of the simulation due
to low star formation efficiency in dSphs. We also confirm that this model is
consistent with observed properties of dSphs such as radial profiles and
metallicity distribution. The merger time and the Galactic rate of NSMs are
suggested to be <~ 300 Myr and ~ $10^{-4}$ yr$^{-1}$, which are consistent with
the values suggested by population synthesis and nucleosynthesis studies. This
study supports that NSMs are the major astrophysical site of r-process.
|
1509.08934v1
|
2015-10-07
|
Nanoanalytical TEM studies and micromagnetic modelling of Nd-Fe-B magnets
|
We have analysed the influence of the microstructural features, such as
intergranular grain boundary (GB) phases and misalignment of the hard magnetic
grains, on the optimization of magnetization reversal processes in order to
improve the coercive field of Nd-Fe-B magnets. The microstructural model of the
grains and intergranular phases, which is used for theoretical simulations, has
been derived from a detailed nanoanalytical TEM/STEM study of a Dy/Tb free
magnet and a high coercive (Nd,Tb)-Fe-B magnet. Special attention is laid on
the EELS analysis of GB with a thickness ranging from 2 - 30 nm. This analysis
identified the majority of the GB phases to have about 50 -70 at.% of iron and
only a few GBs, which are connecting two nearby grain boundary junctions (GBj),
possess a similar chemical composition as the adjacent GBj with a low iron
content (< 10 at. %) and a high rare earth and oxygen content. Finite element
micromagnetic simulations have been carried out in order to study the influence
of internal demagnetizing fields determined by the microstructure on the
magnetization switching behaviour. Special emphasis was put on the influence of
the GB and their magnetic properties, due to their substantial influence on the
nucleation of reverse magnetic domains and the pinning of domain walls. The
strongest reduction of the coercive field is caused by GB with soft
ferromagnetic properties. Shielding the Nd-Fe-B grains from the nucleation
sites at the GBj with Dy or Tb shells, leads to an increase of the coercivity
from 2.5 to 3.6 T and 2.5 to 4.3 T, respectively.
|
1510.01958v2
|
2015-10-23
|
GIANO-TNG spectroscopy of red supergiants in the young star cluster RSGC3
|
The Scutum complex in the inner disk of the Galaxy has a number of young star
clusters dominated by red supergiants that are heavily obscured by dust
extinction and observable only at infrared wavelengths. These clusters are
important tracers of the recent star formation and chemical enrichment history
in the inner Galaxy. During the technical commissioning and as a first science
verification of the GIANO spectrograph at the Telescopio Nazionale Galileo, we
secured high-resolution (R=50,000) near-infrared spectra of five red
supergiants in the young Scutum cluster RSGC3. Taking advantage of the full
YJHK spectral coverage of GIANO in a single exposure, we were able to measure
several tens of atomic and molecular lines that were suitable for determining
chemical abundances. By means of spectral synthesis and line equivalent width
measurements, we obtained abundances of Fe and iron-peak elements such as Ni,
Cr, and Cu, alpha (O, Mg, Si, Ca, Ti), other light elements (C, N, F, Na, Al,
and Sc), and some s-process elements (Y, Sr). We found average half-solar iron
abundances and solar-scaled [X/Fe] abundance patterns for most of the elements,
consistent with a thin-disk chemistry. We found depletion of [C/Fe] and
enhancement of [N/Fe], consistent with standard CN burning, and low 12C/13C
abundance ratios (between 9 and 11), which require extra-mixing processes in
the stellar interiors during the post-main sequence evolution. We also found
local standard of rest V(LSR)=106 km/s and heliocentric V(HEL)=90 km/s radial
velocities with a dispersion of 2.3 km/s. The inferred radial velocities,
abundances, and abundance patterns of RSGC3 are very similar to those
previously measured in the other two young clusters of the Scutum complex,
RSGC1 and RSGC2, suggesting a common kinematics and chemistry within the Scutum
complex.
|
1510.06870v1
|
2015-10-27
|
Subaru/HDS study of CH stars: elemental abundances for stellar neutron-capture process studies
|
A comprehensive abundance analysis providing rare insight into the chemical
history of lead stars is still lacking. We present results from high resolution
(R ~ 50000), spectral analyses of three CH stars, HD 26, HD 198269, HD 224959,
and, a carbon star with a dusty envelope, HD 100764. Previous studies on these
objects are limited by both resolution and wavelength regions and the results
differ significantly from each other. We have undertaken to re-analyse the
chemical composition of these objects based on high resolution Subaru spectra
covering the wavelength regions 4020 to 6775 A,. Considering local
thermodynamic equilibrium and using model atmospheres, we have derived the
stellar parameters, the effective temperatures Teff, surface gravities log g,
and metallicities [Fe/H] for these objects. The derived parameters for HD 26,
HD 100764, HD 198269 and HD 224959 are (5000, 1.6, -1.13), (4750, 2.0 -0.86),
(4500, 1.5, -2.06) and (5050, 2.1, -2.44) respectively. The stars are found to
exhibit large enhancements of heavy elements relative to iron in conformity to
previous studies. Large enhancement of Pb with respect to iron is also
confirmed. Updates on the elemental abundances for several s-process elements
(Y, Zr, La, Ce, Nd, Sm, Pb) along with the first-time estimates of abundances
for a number of other heavy elements (Sr, Ba, Pr, Eu, Er, W) are reported. Our
analysis suggests that neutron-capture elements in HD 26 primarily originate in
s-process while the major contributions to the abundances of neutron-capture
elements in the more metal-poor objects HD 224959 and HD 198269 are from
r-process, possibly formed from materials that are pre-enriched with products
of r-process.
|
1510.07814v1
|
2015-11-11
|
Polarization of Magnetic Dipole Emission and Spinning Dust Emission from Magnetic Nanoparticles
|
Magnetic dipole emission (MDE) from interstellar magnetic nanoparticles is an
important Galactic foreground in the microwave frequencies, and its
polarization level may pose great challenges for achieving reliable
measurements of cosmic microwave background (CMB) B-mode signal. To obtain
theoretical constraints on the polarization of MDE, we first compute the degree
of alignment of big silicate grains incorporated with magnetic inclusions. We
find that, in realistic conditions of the interstellar medium, thermally
rotating big grains with magnetic inclusions are weakly aligned and achieve
{\it alignment saturation} when the magnetic alignment rate becomes much faster
than the rotational damping rate. We then compute the degree of alignment for
free-flying magnetic nanoparticles, taking into account various interaction
processes of grains with the ambient gas and radiation field, including neutral
collisions, ion collisions, and infrared emission. We find that the rotational
damping by infrared emission can significantly decrease the degree of alignment
of small particles from the saturation level, whereas the excitation by ion
collisions can enhance the alignment of ultrasmall particles. Using the
computed degrees of alignment, we predict the polarization level of MDE from
free-flying magnetic nanoparticles to be rather low. Such a polarization level
is within the upper limits measured for anomalous microwave emission (AME),
which indicates that MDE from free-flying iron particles may not be ruled out
as a source of AME. We also quantify spinning dust emission from free-flying
iron nanoparticles with permanent magnetic moments and find that its emissivity
is one order of magnitude lower than that from spinning polycyclic aromatic
hydrocarbons (PAHs). Finally, we compute the polarization spectra of spinning
dust emission from PAHs for the different interstellar magnetic fields.
|
1511.03691v1
|
2015-12-25
|
Thermal evolution of antiferromagnetic correlations and tetrahedral bond angles in superconducting FeTe$_{1-x}$Se$_x$
|
It has recently been demonstrated that dynamical magnetic correlations
measured by neutron scattering in iron chalcogenides can be described with
models of short-range correlations characterized by particular {choices of
four-spin plaquettes, where the appropriate choice changes as the} parent
material is doped towards superconductivity. Here we apply such models to
describe measured maps of magnetic scattering as a function of two-dimensional
wave vectors obtained for optimally superconducting crystals of
FeTe$_{1-x}$Se$_x$. We show that the characteristic antiferromagnetic wave
vector evolves from that of the bicollinear structure found in underdoped
chalcogenides (at high temperature) to that associated with the stripe
structure of antiferromagnetic iron arsenides (at low temperature); {these can
both be described with the same local plaquette, but with different
inter-plaquette correlations}. While the magnitude of the low-energy magnetic
spectral weight is substantial at all temperatures, it actually weakens
somewhat at low temperature, where the charge carriers become more itinerant.
The observed change in spin correlations is correlated with the dramatic drop
in the electronic scattering rate and the growth of the bulk nematic response
on cooling. Finally, we also present powder neutron diffraction results for
lattice parameters in FeTe$_{1-x}$Se$_x$ indicating that the tetrahedral bond
angle tends to increase towards the ideal value on cooling, in agreement with
the increased screening of the crystal field by more itinerant electrons and
the correspondingly smaller splitting of the Fe $3d$ orbitals.
|
1512.08036v2
|
2015-12-26
|
CALET: a high energy astroparticle physics experiment on the ISS
|
CALET (CALorimetric Electron Telescope) is a high energy astroparticle
physics experiment planned for a long exposure mission aboard the International
Space Station (ISS) by the Japanese Aerospace Exploration Agency, in
collaboration with the Italian Space Agency (ASI) and NASA. The main science
goal is high precision measurements of the inclusive electron (+positron)
spectrum below 1 TeV and the exploration of the energy region above 1 TeV,
where the shape of the high end of the spectrum might unveil the presence of
nearby sources of acceleration. CALET has been designed to achieve a large
proton rejection capability (>10$^5$) with a fine grained imaging calorimeter
(IMC) followed by a total absorption calorimeter (TASC), for a total thickness
of 30 X$_{0}$ and 1.3 proton interaction length. With an excellent energy
resolution and a lower background contamination with respect to previous
experiments, CALET will search for possible spectral signatures of dark matter
with both electrons and gamma rays. CALET will also measure the high energy
spectra and relative abundance of cosmic nuclei from proton to iron and detect
trans-iron elements up to Z$\sim$40. The charge identification of individual
nuclear species is performed by a dedicated module (CHD) and by multiple dE/dx
measurements in the IMC. With a large exposure and high energy resolution,
CALET will be able to verify and complement the observations of CREAM, PAMELA
and AMS-02 on a possible deviation from a pure power-law of proton and He
spectra in the few hundred GeV region and to extend the study to the multi-TeV
region. CALET will also contribute to clarify the present experimental picture
on the energy dependence of the boron/carbon ratio, below and above 1 TeV/n,
thereby providing valuable information on cosmic-ray propagation in the galaxy.
Gamma-ray transients will be studied with a dedicated Gamma-ray Burst Monitor
(GBM).
|
1512.08059v1
|
2016-01-17
|
COSMIC-LAB: Unexpected Results from High-resolution Spectra of AGB Stars in Globular Clusters
|
This thesis is aimed at clarifying one of the least studied phases of stellar
evolution: the asymptotic giant branch (AGB). Recent results obtained for
Galactic globular clusters (GCs) suggest that the AGB stage may contain crucial
information about the evolutionary history of exotic stars (Beccari et al.
2006) and multiple-populations (Campbell et al. 2013) in the parent cluster.
The thesis presents the analysis of a large sample of high-resolution spectra
of AGB stars in four Galactic GCs, acquired at the Very Large Telescope (ESO)
and the 2.2 meter telescope (MPG). The obtained results provide evidence of a
previously unknown physical mechanism affecting the neutral species of some
chemical elements in the atmosphere of most AGB stars: because of it, the
abundances derived from neutral lines are systematically underestimated, while
those measured from ionized lines remain unaffected. Such a behaviour exactly
corresponds to what expected in the case of non-local thermodynamic equilibrium
(NLTE) conditions in the star atmosphere. For this reason, in this work we
refer to it as "NLTE effect", with the caveat that this could be not the case.
In fact, the current NLTE models are unable to account for the observed effect,
thus demonstrating that either our comprehension of NLTE is not adequate
enough, or that some more complex physical phenomena are occurring in AGB
atmospheres. This effect has been found in all the investigated GCs. It affects
most (but not all) AGB stars and, in some cases, also some RGB stars. It is
particularly evident for iron and titanium lines (i.e., the elements providing
the largest number of both neutral and ionized lines). The deep understanding
of the detected phenomenon is of paramount importance since it has a huge
impact on the proper determination of GC chemistry and enrichment history (for
instance, it can mimic spurious iron spreads).
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1601.04346v1
|
2016-01-21
|
High resolution spectroscopic analysis of seven giants in the bulge globular cluster NGC 6723
|
Globular clusters associated with the Galactic bulge are important tracers of
stellar populations in the inner Galaxy. High resolution analysis of stars in
these clusters allows us to characterize them in terms of kinematics,
metallicity, and individual abundances, and to compare these fingerprints with
those characterizing field populations. We present iron and element ratios for
seven red giant stars in the globular cluster NGC~6723, based on high
resolution spectroscopy. High resolution spectra ($R\sim48~000$) of seven K
giants belonging to NGC 6723 were obtained with the FEROS spectrograph at the
MPG/ESO 2.2m telescope. Photospheric parameters were derived from $\sim130$ FeI
and FeII transitions. Abundance ratios were obtained from line-to-line spectrum
synthesis calculations on clean selected features. An intermediate metallicity
of [Fe/H]$=-0.98\pm0.08$ dex and a heliocentric radial velocity of
$v_{hel}=-96.6\pm1.3~km s^{-1}$ were found for NGC 6723. Alpha-element
abundances present enhancements of $[O/Fe]=0.29\pm0.18$ dex,
$[Mg/Fe]=0.23\pm0.10$ dex, $[Si/Fe]=0.36\pm0.05$ dex, and $[Ca/Fe]=0.30\pm0.07$
dex. Similar overabundance is found for the iron-peak Ti with
$[Ti/Fe]=0.24\pm0.09$ dex. Odd-Z elements Na and Al present abundances of
$[Na/Fe]=0.00\pm0.21$ dex and $[Al/Fe]=0.31\pm0.21$ dex, respectively. Finally,
the s-element Ba is also enhanced by $[Ba/Fe]=0.22\pm0.21$ dex. The enhancement
levels of NGC 6723 are comparable to those of other metal-intermediate bulge
globular clusters. In turn, these enhancement levels are compatible with the
abundance profiles displayed by bulge field stars at that metallicity. This
hints at a possible similar chemical evolution with globular clusters and the
metal-poor of the bulge going through an early prompt chemical enrichment.
|
1601.05736v1
|
2016-02-02
|
The initial abundance and distribution of 92Nb in the Solar System
|
Niobium-92 is an extinct proton-rich nuclide, which decays to 92Zr with a
half-life of 37 Ma. This radionuclide potentially offers a unique opportunity
to determine the timescales of early Solar System processes and the site(s) of
nucleosynthesis for p-nuclei, once its initial abundance and distribution in
the Solar System are well established. Here we present internal Nb-Zr isochrons
for three basaltic achondrites with known U-Pb ages: the angrite NWA 4590, the
eucrite Agoult, and the ungrouped achondrite Ibitira. Our results show that the
relative Nb-Zr isochron ages of the three meteorites are consistent with the
time intervals obtained from the Pb-Pb chronometer for pyroxene and
plagioclase, indicating that 92Nb was homogeneously distributed among their
source regions. The Nb-Zr and Pb-Pb data for NWA 4590 yield the most reliable
and precise reference point for anchoring the Nb-Zr chronometer to the absolute
timescale: an initial 92Nb/93Nb ratio of $(1.4 \pm 0.5) \times 10^{-5}$ at
$4557.93 \pm 0.36$ Ma, which corresponds to a 92Nb/93Nb ratio of $(1.7 \pm 0.6)
\times 10^{-5}$ at the time of the Solar System formation. On the basis of this
new initial ratio, we demonstrate the capability of the Nb-Zr chronometer to
date early Solar System objects including troilite and rutile, such as iron and
stony-iron meteorites. Furthermore, we estimate a nucleosynthetic production
ratio of 92Nb to the p-nucleus 92Mo between 0.0015 and 0.035. This production
ratio, together with the solar abundances of other p-nuclei with similar
masses, can be best explained if these light p-nuclei were primarily
synthesized by photodisintegration reactions in Type Ia supernovae.
|
1602.00966v1
|
2016-03-02
|
Optimal Auctions for Negatively Correlated Items
|
We consider the problem of designing revenue-optimal auctions for selling two
items and bidders' valuations are independent among bidders but negatively
correlated among items.
In this paper, we obtain the closed-form optimal auction for this setting, by
directly addressing the two difficulties above. In particular, the first
difficulty is that when pointwise maximizing virtual surplus under
multi-dimensional feasibility (i.e., the Border feasibility), (1) neither the
optimal interim allocation is trivially monotone in the virtual value, (2) nor
the virtual value is monotone in the bidder's type. As a result, the optimal
interim allocations resulting from virtual surplus maximization no longer
guarantees BIC. To address (1), we prove a generalization of Border's theorem
and show that optimal interim allocation is indeed monotone in the virtual
value. To address (2), we adapt Myerson's ironing procedure to this setting by
redefining the (ironed) virtual value as a function of the lowest utility
point. The second difficulty, perhaps a more challenging one, is that the
lowest utility type in general is no longer at the endpoints of the type
interval. To address this difficulty, we show by construction that there exist
an allocation rule and an induced lowest utility type such that they form a
solution of the virtual surplus maximization and in the meanwhile guarantees
IIR. In the single bidder case, the optimal auction consists of a randomized
bundle menu and a deterministic bundle menu; while in the multiple bidder case,
the optimal auction is a randomization between two extreme mechanisms. The
optimal solutions of our setting can be implemented by a Bayesian IC and IR
auction, however, perhaps surprisingly, the revenue of this auction cannot be
achieved by any (dominant-strategy) IC and IR auction.
|
1603.00562v3
|
2016-03-04
|
The thermal instability of the warm absorber in NGC 3783
|
We model the observed X-ray spectral continuum shape, ionic column densities,
and absorption measure distribution (AMD) of the warm absorber in the Seyfert
galaxy NGC 3783. We assume a photo-ionized medium with a uniform total
(gas+radiation) pressure. The irradiation causes the wind to be radiation
pressure compressed (RPC). We compare the observational characteristics derived
from the 900 ksec Chandra observation to radiative transfer computations in
pressure equilibrium using the radiative transfer code TITAN. We explore
different values of the ionization parameter xi of the incident flux and adjust
the hydrogen-equivalent column density, N_H0 of the warm absorber to match the
observed soft X-ray continuum. We derive theoretical column densities for a
broad range of ionic species of iron and neon and an AMD that we compare to the
observations. We find an extension of the degeneracy between xi and N_H0 for
the constant pressure models previously discussed for NGC 3783. Including the
ionic column densities of iron and neon in the comparison between observations
and data we conclude that a range of ionization parameters between 4000 and
8000 ergs cm/s is preferred. For the first time, we present theoretical AMD for
a constant pressure wind in NGC 3783 that correctly reproduces the observed
level and is in approximate agreement with the observational appearance of an
instability region. Using a variety of observational indicators, we confirm
that the X-ray outflow of NGC 3783 can be described as an RPC medium in
pressure equilibrium. The observed AMD agrees with a uniformly hot or a
uniformly cold thermal state. The measured ionic column densities suggest that
the wind tends to the uniformly cold thermal state. The occurrence of thermal
instability in the warm absorber model may depend on the computational method
and the spatial scale on which the radiative transfer is solved.
|
1603.01448v1
|
2016-03-11
|
Nonlinear and time-resolved optical study of the 112-type iron-based superconductor parent Ca$_{1-x}$La$_{x}$FeAs$_{2}$ across its structural phase transition
|
The newly discovered 112-type ferropnictide superconductors contain chains of
As atoms that break the tetragonal symmetry between the $a$ and $b$ axes. This
feature eliminates the need for uniaxial strain that is usually required to
stabilize large single domains in the electronic nematic state that exists in
the vicinity of magnetic order in the iron-based superconductors. We report
detailed structural symmetry measurements of 112-type
Ca$_{0.73}$La$_{0.27}$FeAs$_{2}$ using rotational anisotropy optical second
harmonic generation. This technique is complementary to diffraction experiments
and enables a precise determination of the point group symmetry of a crystal.
By combining our measurements with density functional theory calculations, we
uncover a strong optical second harmonic response of bulk electric dipole
origin from the Fe and Ca $3d$-derived states that enables us to assign $C_2$
as the crystallographic point group. This makes the 112-type materials
high-temperature superconductors without a center of inversion, allowing for
the possible mixing of singlet and triplet Cooper pairs in the superconducting
state. We also perform pump-probe transient reflectivity experiments that
reveal a 4.6 THz phonon mode associated with the out-of-plane motion of As
atoms in the FeAs layers. We do not observe any suppression of the optical
second harmonic response or shift in the phonon frequency upon cooling through
the reported monoclinic-to-triclinic transition at 58 K. This allows us to
identify $C_1$ as the low-temperature crystallographic point group but suggests
that structural changes induced by long-range magnetic order are subtle and do
not significantly affect electronic states near the Fermi level.
|
1603.03785v1
|
2016-04-09
|
Reduced radiative conductivity of high and low spin FeO6 octahedra in the Earth's lower mantle
|
The ability of Earths mantle to conduct heat by radiation is determined by
optical properties of mantle phases. Optical properties of mantle minerals at
high pressure are accessible through diamond anvil cell experiments, but
because of the extensive thermal radiation at T above 1000 K such studies are
limited to lower temperatures. Particularly uncertain is the
temperature-dependence of optical properties of lower mantle minerals across
the spin transition as the spin state itself is a strong function of
temperature. Here we use laser-heated DACs combined with a pulsed bright
supercontinuum laser probe and a synchronized time-gated detector to examine
optical properties of high and low spin ferrous iron at 45-73 GPa and to 1600 K
in FeO6, one of the most abundant building blocks in the mantle. Siderite
(FeCO3) is used as a model for FeO6-octahedra as it contains no ferric iron and
exhibits a sharp optically apparent spin transition at 44 GPa, simplifying data
interpretation. We find that the optical absorbance of low spin FeO6 is
substantially increased at 1000-1200 K due to the partially lifted Laporte
selection rule. The temperature-induced low to high spin transition, however,
results in a dramatic drop in absorbance of the FeO6-unit. The absorption edge
(Fe-O charge transfer) red-shifts (~ 1 cm-1/K) with increasing temperature and
at T above 1600 K becomes the dominant absorption mechanism in the visible
range, suppressing radiative conductivity. This implies that the radiative
conductivity of analogous FeO6-bearing minerals such as ferropericlase, the
second most abundant mineral in the Earths lower mantle, is substantially
reduced approaching the core-mantle boundary conditions. Finally, our results
emphasize that optical properties of mantle minerals probed at room temperature
are insufficient to model radiative thermal conductivity of planetary
interiors.
|
1604.02613v1
|
2016-04-11
|
Chemical abundances and kinematics of barium stars
|
In this paper we present an homogeneous analysis of photospheric abundances
based on high-resolution spectroscopy of a sample of 182 barium stars and
candidates. We determined atmospheric parameters, spectroscopic distances,
stellar masses, ages, luminosities and scale height, radial velocities,
abundances of the Na, Al, $alpha$-elements, iron-peak elements, and s-process
elements Y, Zr, La, Ce, and Nd. We employed the local-thermodynamic-equilibrium
model atmospheres of Kurucz and the spectral analysis code {\sc moog}. We found
that the metallicities, the temperatures and the surface gravities for barium
stars can not be represented by a single gaussian distribution. The abundances
of $alpha$-elements and iron peak elements are similar to those of field giants
with the same metallicity. Sodium presents some degree of enrichment in more
evolved stars that could be attributed to the NeNa cycle. As expected, the
barium stars show overabundance of the elements created by the s-process. By
measuring the mean heavy-element abundance pattern as given by the ratio
[s/Fe], we found that the barium stars present several degrees of enrichment.
We also obtained the [hs/ls] ratio by measuring the photospheric abundances of
the Ba-peak and the Zr-peak elements. Our results indicated that the [s/Fe] and
the [hs/ls] ratios are strongly anti-correlated with the metallicity. Our
kinematical analysis showed that 90% of the barium stars belong to the thin
disk population. Based on their luminosities, none of the barium stars are
luminous enough to be an AGB star, nor to become self-enriched in the s-process
elements. Finally, we determined that the barium stars also follow an
age-metallicity relation.
|
1604.03031v1
|
2016-04-14
|
Discovery of Broad Soft X-ray Absorption Lines from the Quasar Wind in PDS 456
|
High resolution soft X-ray spectroscopy of the prototype accretion disk wind
quasar, PDS 456, is presented. Here, the XMM-Newton RGS spectra are analyzed
from the large 2013-2014 XMM-Newton campaign, consisting of 5 observations of
approximately 100 ks in length. During the last observation (hereafter OBS. E),
the quasar is at a minimum flux level and broad absorption line profiles are
revealed in the soft X-ray band, with typical velocity widths of $\sigma_{\rm
v}\sim 10,000$ km s$^{-1}$. During a period of higher flux in the 3rd and 4th
observations (OBS. C and D, respectively), a very broad absorption trough is
also present above 1 keV. From fitting the absorption lines with models of
photoionized absorption spectra, the inferred outflow velocities lie in the
range $\sim 0.1-0.2c$. The absorption lines likely originate from He and H-like
neon and L-shell iron at these energies. Comparison with earlier archival data
of PDS 456 also reveals similar absorption structure near 1 keV in a 40 ks
observation in 2001, and generally the absorption lines appear most apparent
when the spectrum is more absorbed overall. The presence of the soft X-ray
broad absorption lines is also independently confirmed from an analysis of the
XMM-Newton EPIC spectra below 2 keV. We suggest that the soft X-ray absorption
profiles could be associated with a lower ionization and possibly clumpy phase
of the accretion disk wind, where the latter is known to be present in this
quasar from its well studied iron K absorption profile and where the wind
velocity reaches a typical value of 0.3$c$.
|
1604.04196v1
|
2016-04-22
|
Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering and band structure calculations
|
We report Raman light scattering in the phase separated superconducting
single crystal Rb0.77Fe1.61Se2 with Tc = 32 K. The spectra have been measured
in a wide temperature range 3K -500K. The observed phonon lines from the
majority vacancy ordered Rb2Fe4Se5 (245) antiferromagnetic phase with TN= 525 K
demonstrate modest anomalies in frequency, intensity and halfwidth at the
superconductive phase transition. We identify phonon lines from the minority
compressed Rb{\delta}Fe2Se2 (122) conductive phase. The superconducting gap
with dx2-y2 symmetry is also detected in our spectra. In the range 0-600 cm-1
we observed the low intensive but highly polarized B1g-type background which
becomes well structured under cooling. The possible magnetic or multiorbital
origin of this background has been discussed. We argue that phase separation in
M0.8+xFe1.6+ySe2 has pure magnetic origin. It occurs below Neel temperature
when iron magnetic moment achieves some critical magnitude. We state that there
is a spacer between the majority 245 and minority 122 phases. Using ab-initio
spin polarized band structure calculations we demonstrate that compressed
vacancy ordered Rb2Fe4Se5 phase can be conductive and therefore may serve as a
protective interface spacer between the pure metallic Rb{\delta}Fe2Se2 phase
and the insulating Rb2Fe4Se5 phase providing the percolative Josephson-junction
like superconductivity in the whole sample of Rb0.8+xFe1.6+ySe2 Our lattice
dynamics calculations show significant difference in the phonon spectra of the
conductive and insulating Rb2Fe4.Se5 phases.
|
1604.06825v1
|
2016-05-10
|
Cosmic-ray energy spectrum and composition up to the ankle - the case for a second Galactic component
|
We have carried out a detailed study to understand the observed energy
spectrum and composition of cosmic rays with energies up to ~10^18 eV. Our
study shows that a single Galactic component with subsequent energy cut-offs in
the individual spectra of different elements, optimised to explain the observed
spectra below ~10^14 eV and the knee in the all-particle spectrum, cannot
explain the observed all-particle spectrum above ~2x10^16 eV. We discuss two
approaches for a second component of Galactic cosmic rays -- re-acceleration at
a Galactic wind termination shock, and supernova explosions of Wolf-Rayet
stars, and show that the latter scenario can explain almost all observed
features in the all-particle spectrum and the composition up to ~10^18 eV, when
combined with a canonical extra-galactic spectrum expected from strong radio
galaxies or a source population with similar cosmological evolution. In this
two-component Galactic model, the knee at ~ 3x10^15 eV and the second knee at
~10^17 eV in the all-particle spectrum are due to the cut-offs in the first and
second components, respectively. We also discuss several variations of the
extra-galactic component, from a minimal contribution to scenarios with a
significant component below the ankle (at ~4x10^18 eV), and find that
extra-galactic contributions in excess of regular source evolution are neither
indicated nor in conflict with the existing data. Our main result is that the
second Galactic component predicts a composition of Galactic cosmic rays at and
above the second knee that largely consists of helium or a mixture of helium
and CNO nuclei, with a weak or essentially vanishing iron fraction, in contrast
to most common assumptions. This prediction is in agreement with new
measurements from LOFAR and the Pierre Auger Observatory which indicate a
strong light component and a rather low iron fraction between ~10^17 and 10^18
eV.
|
1605.03111v2
|
2016-05-11
|
Broadband observations of the X-ray burster 4U 1705-44 with BeppoSAX
|
4U 1705-44 is one of the most-studied type I X-ray burster and Atoll sources.
This source represents a perfect candidate to test different models proposed to
self-consistently track the physical changes occurring between different
spectral states because it shows clear spectral state transitions. The
broadband coverage, the sensitivity and energy resolution of the BeppoSAX
satellite offers the opportunity to disentangle the components that form the
total X-ray spectrum and to study their changes according to the spectral
state. Using two BeppoSAX observations carried out in August and October 2000,
respectively, for a total effective exposure time of about 100 ks, we study the
spectral evolution of the source from a soft to hard state. Energy spectra are
selected according to the source position in the color-color diagram (CCD)
Results. We succeeded in modeling the spectra of the source using a physical
self-consistent scenario for both the island and banana branches (the double
Comptonization scenario). The components observed are the soft Comptonization
and hard Comptonization, the blackbody, and a reflection component with a broad
iron line. When the source moves from the banana state to the island state, the
parameters of the two Comptonization components change significantly and the
blackbody component becomes too weak to be detected. We interpret the soft
Comptonization component as emission from the hot plasma surrounding the
neutron star, hard Comptonization as emission from the disk region, and the
blackbody component as emission from the inner accretion disk. The broad
feature in the iron line region is compatible with reflection from the inner
accretion disk.
|
1605.03366v1
|
2016-05-12
|
The Soft State of Cygnus X-1 Observed with NuSTAR: A Variable Corona and a Stable Inner Disk
|
We present a multi-epoch hard X-ray analysis of Cygnus X-1 in its soft state
based on four observations with NuSTAR. Despite the basic similarity of the
observed spectra, there is clear spectral variability between epochs. To
investigate this variability, we construct a model incorporating both the
standard disk-corona continuum and relativistic reflection from the accretion
disk, based on prior NuSTAR work on Cygnus X-1, and apply this model to each
epoch independently. We find excellent consistency for the black hole spin, and
the iron abundance of the accretion disk, which are expected to remain constant
on observational timescales. In particular, we confirm that Cygnus X-1 hosts a
rapidly rotating black hole, 0.93<a*<0.96, in broad agreement with the majority
of prior studies of the relativistic disk reflection and constraints on the
spin obtained through studies of the thermal accretion disk continuum. Our work
also confirms the apparent misalignment between the inner disk and the orbital
plane of the binary system reported previously, finding the magnitude of this
warp to be ~10-15deg. This level of misalignment does not significantly change
(and may even improve) the agreement between our reflection results and the
thermal continuum results regarding the black hole spin. The spectral
variability observed by NuSTAR is dominated by the primary continuum, implying
variability in the temperature of the scattering electron plasma. Finally, we
consistently observe absorption from ionized iron at ~6.7 keV, which varies in
strength as a function of orbital phase in a manner consistent with the
absorbing material being an ionized phase of the focused stellar wind from the
supergiant companion star.
|
1605.03966v1
|
2016-06-02
|
A search for X-ray reprocessing echoes in the power spectral density functions of AGN
|
We present the results of a detailed study of the X-ray power spectra density
(PSD) functions of twelve X-ray bright AGN, using almost all the archival
XMM-Newton data. The total net exposure of the EPIC-pn light curves is larger
than 350 ks in all cases (and exceeds 1 Ms in the case of 1H 0707-497). In a
physical scenario in which X-ray reflection occurs in the inner part of the
accretion disc of AGN, the X-ray reflection component should be a filtered echo
of the X-ray continuum signal and should be equal to the convolution of the
primary emission with the response function of the disc. Our primary objective
is to search for these reflection features in the 5-7 keV (iron line) and 0.5-1
keV (soft) bands, where the X-ray reflection fraction is expected to be
dominant. We fit to the observed periodograms two models: a simple bending
power law model (BPL) and a BPL model convolved with the transfer function of
the accretion disc assuming the lamp-post geometry and X-ray reflection from a
homogeneous disc. We do not find any significant features in the best-fitting
BPL model residuals either in individual PSDs in the iron band, soft and full
band (0.3-10 keV) or in the average PSD residuals of the brightest and more
variable sources (with similar black hole mass estimates). The typical
amplitude of the soft and full-band residuals is around 3-5 per cent. It is
possible that the expected general relativistic effects are not detected
because they are intrinsically lower than the uncertainty of the current PSDs,
even in the strong relativistic case in which X-ray reflection occurs on a disc
around a fast rotating black hole having an X-ray source very close above it.
However, we could place strong constrains to the X-ray reflection geometry with
the current data sets if we knew in advance the intrinsic shape of the X-ray
PSDs, particularly its high frequency slope.
|
1606.00844v1
|
2016-06-09
|
Ternary borides Nb$_7$Fe$_3$B$_8$ and Ta$_7$Fe$_3$B$_8$ with Kagome-type iron framework
|
Two new ternary borides $TM$$_7$Fe$_3$B$_8$ ($TM$ = Nb, Ta) were synthesized
by high-temperature thermal treatment of samples obtained by arc-melting. This
new type of structure with space group $P$6/$mmm$, comprises $TM$ slabs
containing isolated planar hexagonal [B$_6$] rings and iron centered $TM$
columns in a Kagome type of arrangement. Chemical bonding analysis in
Nb$_7$Fe$_3$B$_8$ by means of the electron localizability approach reveals
two-center interactions forming the Kagome net of Fe and embedded B, while
weaker multicenter bonding present between this net and Nb atoms. Magnetic
susceptibility measurements reveal antiferromagnetic order below $T_N$ = 240 K
for Nb$_7$Fe$_3$B$_8$ and $T_N$ =265 K for Ta$_7$Fe$_3$B$_8$. Small remnant
magnetization below 0.01 $\mu_B$/f.u. is observed in the antiferromagnetic
state. The bulk nature of the magnetic transitions was confirmed by the
hyperfine splitting of the M\"o{\ss}bauer spectra, the sizable anomalies in the
specific heat capacity, and the kinks in the resistivity curves. The high-field
paramagnetic susceptibilities fitted by the Curie-Weiss law show effective
paramagnetic moments $\mu_{eff}$ about 3.1 $\mu_B$/Fe in both compounds. The
temperature dependence of the electrical resistivity also reveals metallic
character of both compounds. Density functional calculations corroborate the
metallic behaviour of both compounds and demonstrate the formation of a sizable
local magnetic moment on the Fe-sites. They indicate the presence of both
antiferro- and ferromagnetic interactions.
|
1606.03123v1
|
2016-08-02
|
Atomic-scale observation and manipulation of plaquette antiferromagnetic order in iron-based superconductor
|
The symmetry requirement and the origin of magnetic orders coexisting with
superconductivity have been strongly debated issues of iron-based
superconductors (FeSCs). Observation of C$_4$-symmetric antiferromagnetism in
violation of the inter-band nesting condition of spin-density waves in
superconducting ground state will require significant change in our
understanding of the mechanism of FeSC. The superconducting material
Sr$_2$VO$_3$FeAs, a bulk version of monolayer FeSC in contact with a perovskite
layer with its magnetism (T$_N$ ~ 50 K) and superconductivity (T$_c$ ~ 37 K)
coexisting at parent state, has no reported structural orthorhombic distortion
and thus makes a perfect system to look for theoretically expected C$_4$
magnetisms. Based on variable temperature spin-polarized scanning tunneling
microscopy (SPSTM) with newly discovered imaging mechanism that removes the
static surface reconstruction (SR) pattern by fluctuating it rapidly with
spin-polarized tunneling current, we could visualize underlying C$_4$ symmetric
(2$\times$2) magnetic domains and its phase domain walls. We find that this
magnetic order is perfectly consistent with the plaquette antiferromagnetic
order in tetragonal Fe spin lattice expected from theories based on the
Heisenberg exchange interaction of local Fe moments and the quantum order by
disorder. The inconsistency of its modulation Q vectors from the nesting
condition also implies that the nesting-based C$_2$ symmetric magnetism is not
a unique prerequisite of high-T$_c$ FeSC. Furthermore, the plaquette
antiferromagnetic domain wall dynamics under the influence of small spin torque
effect of spin-polarized tunneling current are shown to be consistent with
theoretical simulation based on the extended Landau-Lifshitz-Gilbert equation.
|
1608.00884v3
|
2016-08-19
|
Optical properties of $A$Fe$_\mathbf{2}$As$_\mathbf{2}$ ($A=\,$Ca, Sr, and Ba) single crystals
|
The detailed optical properties have been determined for the iron-based
materials $A$Fe$_2$As$_2$, where $A=\,$Ca, Sr, and Ba, for light polarized in
the iron-arsenic ($a-b$) planes over a wide frequency range, above and below
the magnetic and structural transitions at $T_N =$ 172, 195, and 138 K,
respectively. The real and imaginary parts of the complex conductivity are fit
simultaneously using two Drude terms in combination with a series of
oscillators. Above $T_N$, the free-carrier response consists of a weak, narrow
Drude term, and a strong, broad Drude term, both of which show only a weak
temperature dependence. Below $T_N$ there is a slight decrease of the plasma
frequency but a dramatic drop in the scattering rate for the narrow Drude term,
and for the broad Drude term there is a significant decrease in the plasma
frequency, while the decrease in the scattering rate, albeit significant, is
not as severe. The small values observed for the scattering rates for the
narrow Drude term for $T\ll{T_N}$ may be related to the Dirac cone-like
dispersion of the electronic bands. Below $T_N$ new features emerge in the
optical conductivity that are associated with the reconstruction Fermi surface
and the gapping of bands at $\Delta_1 \simeq$ 45 $-$ 80 meV, and $\Delta_2
\simeq$ 110 $-$ 210 meV. The reduction in the spectral weight associated with
the free carriers is captured by the gap structure, specifically, the spectral
weight from the narrow Drude term appears to be transferred into the low-energy
gap feature, while the missing weight from the broad term shifts to the
high-energy gap.
|
1608.05709v3
|
2016-09-16
|
A deep X-ray view of the bare AGN Ark 120. II. Evidence for Fe K emission transients
|
We report on the results from a large observational campaign on the bare
Seyfert galaxy Ark 120, jointly carried out in 2014 with XMM-Newton, Chandra,
and NuSTAR. The fortunate line of sight to this source, devoid of any
significant absorbing material, provides an incomparably clean view to the
nuclear regions of an active galaxy. Here we focus on the analysis of the iron
fluorescence features, which form a composite emission pattern in the 6$-$7 keV
band. The prominent K$\alpha$ line from neutral iron at 6.4 keV is resolved in
the Chandra High-Energy Transmission Grating spectrum to a full-width at half
maximum of 4700$^{+2700}_{-1500}$ km s$^{-1}$, consistent with an origin from
the optical broad-line region. Excess components are detected on both sides of
the narrow K$\alpha$ line: the red one (6.0$-$6.3 keV) clearly varies in
strength in about one year, and hints at the presence of a broad, mildly
asymmetric line from the accretion disk; the blue one (6.5$-$7.0 keV), instead,
is likely a blend of different contributions, and appears to be constant when
integrated over long enough exposures. However, the Fe K excess emission map
computed over the 7.5 days of the XMM-Newton monitoring shows that both the red
and the blue features are actually highly variable on timescales of
$\sim$10$-$15 hours, suggesting that they might arise from short-lived hotspots
on the disk surface, located within a few tens of gravitational radii from the
central supermassive black hole and possibly illuminated by magnetic
reconnection events. Any alternative explanation would still require a highly
dynamic, inhomogeneous disk/coronal system, involving clumpiness and/or
instability.
|
1609.05210v1
|
2016-10-14
|
Elemental partitioning and isotopic fractionation of Zn between metal and silicate and geochemical estimation of the S content of the Earth's core
|
Zinc metal-silicate fractionation provides experimental access to the
conditions of core formation and Zn has been used to estimate the S contents of
the Earth's core and of the bulk Earth, assuming that they share similar
volatility and that Zn was not partitioned into the Earth's core. We have
conducted a suite of partitioning experiments to characterize Zn metal-silicate
elemental and isotopic fractionation as a function of time, temperature, and
composition. Experiments were conducted at temperatures from 1473-2273K, with
run durations from 5-240 minutes for four starting materials. Chemical and
isotopic equilibrium is achieved within 10 minutes. Zinc metal-silicate
isotopic fractionation displays no resolvable dependence on temperature,
composition, or oxygen fugacity. Thus, the Zn isotopic composition of silicate
phases can be used as a proxy for bulk telluric bodies. Results from this study
and literature data were used to parameterize Zn metal-silicate partitioning as
a function of temperature, pressure, and redox state. Using this
parameterization and viable formation conditions, we have estimated a range of
Zn contents in the cores of iron meteorite parent bodies (i.e. iron meteorites)
of ~0.1-150 ppm, in good agreement with natural observations. We have
calculated the first geochemical estimates for the Zn contents of the Earth's
core and of the bulk Earth, at 242 +/-107 ppm and 114 +/-34 ppm (respectively),
that consider the slightly siderophile behavior of Zn and are therefore
significantly higher than previous estimates. Assuming similar volatility for S
and Zn, a chondritic S/Zn ratio, and considering our new estimates, we have
calculated a geochemical upper bound for the S content of the Earth's core of
6.3 +/-1.9 wt%. This indicates that S may be a major contributor to the density
deficit of the Earth's core or that the S/Zn ratio for the Earth is
non-chondritic.
|
1610.04463v1
|
2016-10-20
|
The Broadband Spectral Variability of Holmberg IX X-1
|
We present results from four new broadband X-ray observations of the extreme
ultraluminous X-ray source Holmberg IX X-1 ($L_{\rm{X}} > 10^{40}$ erg/s),
performed by $Suzaku$ and $NuSTAR$ in coordination. Combined with the archival
data, we now have broadband observations of this remarkable source from six
separate epochs. Two of these new observations probe lower fluxes than seen
previously, allowing us to extend our knowledge of the broadband spectral
variability exhibited. The spectra are well fit by two thermal blackbody
components, which dominate the emission below 10 keV, as well as a steep
($\Gamma \sim 3.5$) powerlaw tail which dominates above $\sim$15 keV.
Remarkably, while the 0.3-10.0 keV flux varies by a factor of $\sim$3 between
all these epochs, the 15-40 keV flux varies by only $\sim$20%. Although the
spectral variability is strongest in the $\sim$1-10 keV band, both of the
thermal components are required to vary when all epochs are considered. We also
re-visit the search for iron absorption features, leveraging the high-energy
$NuSTAR$ data to improve our sensitivity to extreme velocity outflows in light
of the ultra-fast outflow recently detected in NGC 1313 X-1. Iron absorption
from a similar outflow along our line of sight can be ruled out in this case.
We discuss these results in the context of super-Eddington accretion models
that invoke a funnel-like geometry for the inner flow, and propose a scenario
in which we have an almost face-on view of a funnel that expands to larger
radii with increasing flux, resulting in an increasing degree of geometrical
collimation for the emission from intermediate temperature regions.
|
1610.06611v2
|
2016-10-21
|
Two Groups of Red Giants with Distinct Chemical Abundances in the Bulge Globular Cluster NGC 6553 Through the Eyes of APOGEE
|
Multiple populations revealed in globular clusters (GCs) are important
windows to the formation and evolution of these stellar systems. The metal-rich
GCs in the Galactic bulge are an indispensable part of this picture, but the
high optical extinction in this region has prevented extensive research. In
this work, we use the high resolution near-infrared (NIR) spectroscopic data
from APOGEE to study the chemical abundances of NGC 6553, which is one of the
most metal-rich bulge GCs. We identify ten red giants as cluster members using
their positions, radial velocities, iron abundances, and NIR photometry. Our
sample stars show a mean radial velocity of $-0.14\pm5.47$ km s$^{-1}$, and a
mean [Fe/H] of $-0.15\pm 0.05$. We clearly separate two populations of stars in
C and N in this GC for the first time. NGC 6553 is the most metal-rich GC where
the multiple stellar population phenomenon is found until now. Substantial
chemical variations are also found in Na, O, and Al. However, the two
populations show similar Si, Ca, and iron-peak element abundances. Therefore,
we infer that the CNO, NeNa, and MgAl cycles have been activated, but the MgAl
cycle is too weak to show its effect on Mg. Type Ia and Type II supernovae do
not seem to have significantly polluted the second generation stars. Comparing
with other GC studies, NGC 6553 shows similar chemical variations as other
relatively metal-rich GCs. We also confront current GC formation theories with
our results, and suggest possible avenues for improvement in the models.
|
1610.06763v1
|
2016-10-24
|
Observations of the X-ray pulsar LMC X-4 with NuSTAR: limit on the magnetic field and tomography of the system in the fluorescent iron line
|
We present results of the spectral and timing analysis of the X-ray pulsar
LMC X-4 with the NuSTAR observatory in the broad energy range 3-79 keV. Along
with the detailed analysis of the averaged source spectrum, the high-precision
pulse phased-resolved spectra were obtained for the first time. It has been
shown that the comptonization model gives the best approximation of the
obtained spectra. The evolution of its parameters was traced depending on the
pulse phase as well. The search for the possible cyclotron absorption line was
performed for all energy spectra in the 5-55 keV energy range. The obtained
upper limit for the depth of the cyclotron absorption line $\tau\simeq0.15$
($3\sigma$) indicates no cyclotron absorption line in this energy range, which
provides an estimate of the magnitude of the magnetic field on the surface of
the neutron star: $B < 3 \times 10^{11}$ G or $B > 6.5 \times 10^{12}$ G. The
latter one is agree with the estimate of the magnetic field obtained from the
analysis of the power spectrum of the pulsar: $B \sim 3 \times 10^{13}$ G.
Based on results of the pulse phase-resolved spectroscopy we revealed a delay
between maxima of the source emission and the equivalent width of the
fluorescent iron line. This delay can be apparently associated with the travel
time of photons between the emitting regions in the vicinity of the neutron
star and the relatively cold regions where this emission is reflected
(presumably, at the inflowing stream or at the place of an interaction of the
stream and the outer edge of the accretion disk).
|
1610.08092v1
|
2016-10-27
|
IC 3639 - A new bona fide Compton thick AGN unveiled by NuSTAR
|
We analyse high-quality NuSTAR observations of the local (z = 0.011) Seyfert
2 active galactic nucleus (AGN) IC 3639, in conjunction with archival Suzaku
and Chandra data. This provides the first broadband X-ray spectral analysis of
the source, spanning nearly two decades in energy (0.5-30 keV). Previous X-ray
observations of the source below 10 keV indicated strong reflection/obscuration
on the basis of a pronounced iron fluorescence line at 6.4 keV. The hard X-ray
energy coverage of NuSTAR, together with self-consistent toroidal reprocessing
models, enables direct broadband constraints on the obscuring column density of
the source. We find the source to be heavily Compton-thick (CTK) with an
obscuring column in excess of $3.6\times10^{24}$ cm$^{-2}$, unconstrained at
the upper end. We further find an intrinsic 2-10 keV luminosity of
$\textrm{log}_{10}(L_{\textrm{2-10 keV}} \textrm{[erg s}^{-1}]) =
43.4^{+0.6}_{-1.1}$ to 90% confidence, almost 400 times the observed flux, and
consistent with various multi-wavelength diagnostics. Such a high intrinsic to
observed flux ratio in addition to an Fe-K$\alpha$ fluorescence line equivalent
width exceeding 2 keV is extreme amongst known bona fide CTK AGN, which we
suggest are both due to the high level of obscuration present around IC 3639.
Our study demonstrates that broadband spectroscopic modelling with NuSTAR
enables large corrections for obscuration to be carried out robustly, and
emphasises the need for improved modelling of AGN tori showing intense iron
fluorescence.
|
1610.08997v1
|
2016-11-04
|
Chemical trends in the Galactic Halo with APOGEE data
|
The galaxy formation process in the $\Lambda$-Cold Dark Matter scenario can
be constrained from the analysis of stars in the Milky Way's halo system. We
examine the variation of chemical abundances in distant halo stars observed by
the Apache Point Galactic Evolution Experiment (APOGEE), as a function of
distance from the Galactic center ($r$) and iron abundance ([M/H]), in the
range 5 $\lesssim r \lesssim$ 30 kpc and $-2.5 <$ [M/H] $<$ 0.0. We perform a
statistical analysis of the abundance ratios derived by the APOGEE pipeline
(ASPCAP) and distances calculated by several approaches. Our analysis reveals
signatures of a different chemical enrichment between the inner and outer
regions of the halo, with a transition at about 15 kpc. The derived metallicity
distribution function exhibits two peaks, at [M/H] $\sim -1.5$ and $\sim -2.1$,
consistent with previously reported halo metallicity distributions. We obtain a
difference of $\sim 0.1$ dex for $\alpha$-element-to-iron ratios for stars at
$r > 15$ kpc and [M/H] $> -1.1$ (larger in the case of O, Mg and S) with
respect to the nearest halo stars. This result confirms previous claims for
low-$\alpha$ stars found at larger distances. Chemical differences in elements
with other nucleosynthetic origins (Ni, K, Na, and Al) are also detected. C and
N do not provide reliable information about the interstellar medium from which
stars formed because our sample comprises RGB and AGB stars and can experience
mixing of material to their surfaces.
|
1611.01249v1
|
2016-11-11
|
Competing instabilities, orbital ordering and splitting of band degeneracies from a parquet renormalization group analysis of a 4-pocket model for iron-based superconductors: application to FeSe
|
We report the results of a parquet renormalization group (RG) study of
competing instabilities in the full 2D four pocket, three orbital low-energy
model for iron-based superconductors. We derive and analyze the RG flow of the
couplings, which describe all symmetry-allowed interactions between low-energy
fermions. Despite that the number of the couplings is large, we argue that
there are only two stable fixed trajectories of the RG flow and one weakly
unstable fixed trajectory with a single unstable direction. Each fixed
trajectory has a finite basin of attraction in the space of initial system
parameters. On the stable trajectories, either interactions involving only
$d_{xz}$ and $d_{yz}$ or only $d_{xy}$ orbital components on the electron
pockets dominate, while on the weakly unstable trajectory interactions
involving $d_{xz}$ ($d_{yz}$) and $d_{xy}$ orbital states on the electron
pockets remain comparable. The behavior along the two stable fixed trajectories
has been analyzed earlier [A.V. Chubukov, M. Khodas, and R.M. Fernandes,
arXiv:1602.05503]. Here we focus on the system behavior along the weakly
unstable trajectory and apply the results to FeSe. We find, based on the
analysis of susceptibilities along this trajectory, that the leading
instability upon lowering the temperature is towards a {\it three-component}
d-wave orbital nematic order. Two components are the differences between
fermionic densities on $d_{xz}$ and $d_{yz}$ orbitals on hole pockets and on
electron pockets, and the third one is the difference between the densities of
$d_{xy}$ orbitals on the two electron pockets. We argue that this order is
consistent with the splitting of band degeneracies, observed in recent
photoemission data on FeSe by A. Fedorov et al [arXiv:1606.03022].
|
1611.03912v1
|
2016-11-17
|
A Chemical Composition Survey of the Iron-Complex Globular Cluster NGC 6273 (M 19)
|
Recent observations have shown that a growing number of the most massive
Galactic globular clusters contain multiple populations of stars with different
[Fe/H] and neutron-capture element abundances. NGC 6273 has only recently been
recognized as a member of this "iron-complex" cluster class, and we provide
here a chemical and kinematic analysis of > 300 red giant branch (RGB) and
asymptotic giant branch (AGB) member stars using high resolution spectra
obtained with the Magellan-M2FS and VLT-FLAMES instruments. Multiple lines of
evidence indicate that NGC 6273 possesses an intrinsic metallicity spread that
ranges from about [Fe/H] = -2 to -1 dex, and may include at least three
populations with different [Fe/H] values. The three populations identified here
contain separate first (Na/Al-poor) and second (Na/Al-rich) generation stars,
but a Mg-Al anti-correlation may only be present in stars with [Fe/H] > -1.65.
The strong correlation between [La/Eu] and [Fe/H] suggests that the s-process
must have dominated the heavy element enrichment at higher metallicities. A
small group of stars with low [alpha/Fe] is identified and may have been
accreted from a former surrounding field star population. The cluster's large
abundance variations are coupled with a complex, extended, and multimodal blue
horizontal branch (HB). The HB morphology and chemical abundances suggest that
NGC 6273 may have an origin that is similar to omega Cen and M 54.
|
1611.05830v1
|
2017-01-04
|
A re-analysis of the NuSTAR and XMM-Newton broad-band spectrum of Ser~X-1
|
Context: Ser X-1 is a well studied LMXB which clearly shows a broad iron
line. Recently, Miller et al. (2103) have presented broad-band, high quality
NuSTAR data of SerX-1.Using relativistically smeared self-consistent reflection
models, they find a value of R_in close to 1.0 R_ISCO (corresponding to 6 R_g),
and a low inclination angle, less than 10 deg. Aims: The aim of this paper is
to probe to what extent the choice of reflection and continuum models (and
uncertainties therein) can affect the conclusions about the disk parameters
inferred from the reflection component. To this aim we re-analyze all the
available public NuSTAR and XMM-Newton. Ser X-1 is a well studied source, its
spectrum has been observed by several instruments, and is therefore one of the
best sources for this study. Methods: We use slightly different continuum and
reflection models with respect to those adopted in literature for this source.
In particular we fit the iron line and other reflection features with
self-consistent reflection models as reflionx (with a power-law illuminating
continuum modified with a high energy cutoff to mimic the shape of the incident
Comptonization spectrum) and rfxconv. With these models we fit NuSTAR and
XMM-Newton spectra yielding consistent spectral results. Results: Our results
are in line with those already found by Miller et al. (2013) but less extreme.
In particular, we find the inner disk radius at about 13 R_g and an inclination
angle with respect to the line of sight of about 27 deg. We conclude that,
while the choice of the reflection model has little impact on the disk
parameters, as soon as a self-consistent model is used, the choice of the
continuum model can be important in the precise determination of the disk
parameters from the reflection component. Hence broad-band X-ray spectra are
highly preferable to constrain the continuum and disk parameters.
|
1701.01069v1
|
2017-01-22
|
Determination of robust metallicities for metal-rich red giant branch stars
|
The study of the Milky Way relies on our ability to interpret the light from
stars correctly. This calls for a reinvestigation of how reliably we can
determine, e.g., iron abundances in such stars and how well they reproduce
those of dwarf stars. Here we explore robust ways to determine the iron content
of metal-rich giant stars. We aim to understand what biases and shortcomings
widely applied methods suffer from. In this study we are mainly concerned with
standard methods to analyse stellar spectra. This includes the analysis of
individual lines to determine stellar parameters, analysis of the broad wings
of certain lines (e.g., H$\alpha$ and calcium lines) to determine effective
temperature and surface gravity for the stars. For NGC 6528 we find that [Fe/H]
= $+0.04$ dex with a scatter of $\sigma=0.07$ dex, which gives an error in the
derived mean abundance of 0.02 dex. Our work has two important conclusions for
analysis of metal-rich red giant branch stars. 1) For spectra with S/N below
about 35 per reduced pixel [Fe/H] become too high, 2) Determination of $T_{\rm
eff}$ using the wings of the H$\alpha$ line results in [Fe/H] values about 0.1
dex higher than if excitational equilibrium is used. The last conclusion is
perhaps not surprising as we expect NLTE effect to become more prominent in
cooler stars and we can not use the the wings of the H$\alpha$ line to
determine $T_{\rm eff}$ for the cool stars in our sample. We therefore
recommend that in studies of metal-rich red giant stars care needs to be taken
to obtain sufficient calibration data in order to be able to also use the
cooler stars.
|
1701.06132v1
|
2017-03-05
|
Reliable estimation of prediction uncertainty for physico-chemical property models
|
The predictions of parameteric property models and their uncertainties are
sensitive to systematic errors such as inconsistent reference data, parametric
model assumptions, or inadequate computational methods. Here, we discuss the
calibration of property models in the light of bootstrapping, a sampling method
akin to Bayesian inference that can be employed for identifying systematic
errors and for reliable estimation of the prediction uncertainty. We apply
bootstrapping to assess a linear property model linking the 57Fe Moessbauer
isomer shift to the contact electron density at the iron nucleus for a diverse
set of 44 molecular iron compounds. The contact electron density is calculated
with twelve density functionals across Jacob's ladder (PWLDA, BP86, BLYP, PW91,
PBE, M06-L, TPSS, B3LYP, B3PW91, PBE0, M06, TPSSh). We provide systematic-error
diagnostics and reliable, locally resolved uncertainties for isomer-shift
predictions. Pure and hybrid density functionals yield average prediction
uncertainties of 0.06-0.08 mm/s and 0.04-0.05 mm/s, respectively, the latter
being close to the average experimental uncertainty of 0.02 mm/s. Furthermore,
we show that both model parameters and prediction uncertainty depend
significantly on the composition and number of reference data points.
Accordingly, we suggest that rankings of density functionals based on
performance measures (e.g., the coefficient of correlation, r2, or the
root-mean-square error, RMSE) should not be inferred from a single data set.
This study presents the first statistically rigorous calibration analysis for
theoretical Moessbauer spectroscopy, which is of general applicability for
physico-chemical property models and not restricted to isomer-shift
predictions. We provide the statistically meaningful reference data set MIS39
and a new calibration of the isomer shift based on the PBE0 functional.
|
1703.01685v2
|
2017-08-03
|
Observations of the Ultraviolet-Bright Star Y453 in the Globular Cluster M4 (NGC 6121)
|
We present a spectral analysis of the UV-bright star Y453 in M4. Model fits
to the star's optical spectrum yield T_eff ~ 56,000 K. Fits to the star's FUV
spectrum, obtained with the Cosmic Origins Spectrograph (COS) on board the
Hubble Space Telescope, reveal it to be considerably hotter, with T_eff ~
72,000 K. We adopt T_eff = 72,000 +/- 2000 K and log g = 5.7 +/- 0.2 as our
best-fit parameters. Scaling the model spectrum to match the star's optical and
near-infrared magnitudes, we derive a mass M_* = 0.53 +/- 0.24 M_sun and
luminosity log L/L_sun = 2.84 +/- 0.05, consistent with the values expected of
an evolved star in a globular cluster. Comparing the star with post-horizontal
branch evolutionary tracks, we conclude that it most likely evolved from the
blue horizontal branch, departing the AGB before third dredge-up. It should
thus exhibit the abundance pattern (O-poor and Na-rich) characteristic of the
second-generation (SG) stars in M4. We derive the star's photospheric
abundances of He, C, N, O, Si, S, Ti, Cr, Fe, and Ni. CNO abundances are
roughly 0.25 dex greater than those of the cluster's SG stars, while the Si and
S abundances agree match the cluster values. Abundances of the iron-peak
elements (except for iron itself) are enhanced by 1 to 3 dex. Rather than
revealing the star's origin and evolution, this pattern reflects the combined
effects of diffusive and mechanical processes in the stellar atmosphere.
|
1708.01315v1
|
2017-09-04
|
Iron Snow in the Martian Core?
|
The decline of Mars' global magnetic field some 3.8-4.1 billion years ago is
thought to reflect the demise of the dynamo that operated in its liquid core.
The dynamo was probably powered by planetary cooling and so its termination is
intimately tied to the thermochemical evolution and present-day physical state
of the Martian core. Bottom-up growth of a solid inner core, the
crystallization regime for Earth's core, has been found to produce a long-lived
dynamo leading to the suggestion that the Martian core remains entirely liquid
to this day. Motivated by the experimentally-determined increase in the Fe-S
liquidus temperature with decreasing pressure at Martian core conditions, we
investigate whether Mars' core could crystallize from the top down. We focus on
the "iron snow" regime, where newly-formed solid consists of pure Fe and is
therefore heavier than the liquid. We derive global energy and entropy
equations that describe the long-timescale thermal and magnetic history of the
core from a general theory for two-phase, two-component liquid mixtures,
assuming that the snow zone is in phase equilibrium and that all solid falls
out of the layer and remelts at each timestep. Formation of snow zones occurs
for a wide range of interior and thermal properties and depends critically on
the initial sulfur concentration, x0. Release of gravitational energy and
latent heat during growth of the snow zone do not generate sufficient entropy
to restart the dynamo unless the snow zone occupies at least 400 km of the
core. Snow zones can be 1.5-2 Gyrs old, though thermal stratification of the
uppermost core, not included in our model, likely delays onset. Models that
match the available magnetic and geodetic constraints have x0~10% and snow
zones that occupy approximately the top 100 km of the present-day Martian core.
|
1709.01100v1
|
2017-09-24
|
Fe I in the β Pictoris circumstellar gas disk I. Physical properties of the neutral iron gas
|
The young planetary system {\beta} Pictoris is surrounded by a circumstellar
disk of dust and gas. Because both dust and gas have a lifetime shorter than
the system age, they need to be replenished continuously. The gas composition
is partly known, but its location and its origin are still a puzzle. The gas
source could be the exocomets (or so-called falling and evaporating bodies,
FEBs), which are observed as transient features in absorption lines of
refractory elements (Mg, Ca, and Fe) when they transit in front of the star at
several tens of stellar radii.
Nearly 1700 high-resolution spectra of {\beta} Pictoris have been obtained
from 2003 to 2015 using the HARPS spectrograph. In these spectra, the
circumstellar disk is always detected as a stable component among the numerous
variable absorption signatures of transiting exocomets.
Summing all the 1700 spectra allowed us to reach a signal-to-noise ratio
higher than 1000, which is an unprecedentedly high number for a {\beta}
Pictoris spectrum. It revealed many weak Fe I absorption lines of the
circumstellar gas in more than ten excited states. These weak lines bring new
information on the physical properties of the neutral iron gas in the
circumstellar disk. The population of the first excited levels follows a
Boltzmann distribution with a slope consistent with a gas temperature of about
1300 K; this temperature corresponds to a distance to the star of ~ 38 RStar
and implies a turbulence of {\xi} ~ 0.8 km/s.
|
1709.08170v1
|
2017-09-26
|
Near L-Edge Single and Multiple Photoionization of Singly Charged Iron Ions
|
Absolute cross sections for m-fold photoionization (m=1,...,6) of Fe+ by a
single photon were measured employing the photon-ion merged-beams setup PIPE at
the PETRA III synchrotron light source, operated by DESY in Hamburg, Germany.
Photon energies were in the range 680-920 eV which covers the photoionization
resonances associated with 2p and 2s excitation to higher atomic shells as well
as the thresholds for 2p and 2s ionization. The corresponding resonance
positions were measured with an uncertainty of +- 0.2 eV. The cross section for
Fe+ photoabsorption is derived as the sum of the individually measured
cross-sections for m-fold ionization. Calculations of the Fe+ absorption cross
sections have been carried out using two different theoretical approaches,
Hartree-Fock including relativistic extensions and fully relativistic
Multi-Configuration Dirac Fock. Apart from overall energy shifts of up to about
3 eV, the theoretical cross sections are in good agreement with each other and
with the experimental results. In addition, the complex deexcitation cascades
after the creation of inner-shell holes in the Fe+ ion have been tracked on the
atomic fine-structure level. The corresponding theoretical results for the
product charge-state distributions are in much better agreement with the
experimental data than previously published configuration-average results. The
present experimental and theoretical results are valuable for opacity
calculations and are expected to pave the way to a more accurate determination
of the iron abundance in the interstellar medium.
|
1709.09092v1
|
2017-10-10
|
High-precision abundances of elements in Kepler LEGACY stars. Verification of trends with stellar age
|
HARPS-N spectra with S/N > 250 and MARCS model atmospheres were used to
derive abundances of C, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, and Y in ten
stars from the Kepler LEGACY sample (including the binary pair 16 Cyg A and B)
selected to have metallicities in the range -0.15 < [Fe/H] < +0.15 and ages
between 1 and 7 Gyr. Stellar gravities were obtained from seismic data and
effective temperatures were determined by comparing non-LTE iron abundances
derived from FeI and FeII lines. Available non-LTE corrections were also
applied when deriving abundances of the other elements. The results support the
[X/Fe]-age relations previously found for solar twins. [Mg/Fe], [Al/Fe], and
[Zn/Fe] decrease by ~0.1 dex over the lifetime of the Galactic thin disk due to
delayed contribution of iron from Type Ia supernovae relative to prompt
production of Mg, Al, and Zn in Type II supernovae. [Y/Mg] and [Y/Al], on the
other hand, increase by ~0.3 dex, which can be explained by an increasing
contribution of s-process elements from low-mass AGB stars as time goes on. The
trends of [C/Fe] and [O/Fe] are more complicated due to variations of the ratio
between refractory and volatile elements among stars of similar age. Two stars
with about the same age as the Sun show very different trends of [X/H] as a
function of elemental condensation temperature Tc and for 16 Cyg, the two
components have an abundance difference, which increases with Tc. These
anomalies may be connected to planet-star interactions.
|
1710.03544v1
|
2017-10-23
|
The velocity ellipsoid in the Galactic disc using Gaia DR1
|
The stellar velocity ellipsoid of the solar neighbour is re-examined using
intermediate-old mono-abundances stellar groups with high quality chemistry
data together with parallaxes and proper motions from Gaia DR1. We find the
average velocity dispersion values for the three space velocity components for
the thin and thick disc of (\sigma_{U},\sigma_{V},\sigma_{W})_{thin} = (33 \pm
4, 28 \pm 2, 23 \pm 2) and (\sigma_{U},\sigma_{V},\sigma_{W})_{thick} = (57 \pm
6, 38 \pm 5, 37 \pm 4) km s^{-1}, respectively. The mean values of the ratio
between the semi-axes of the velocity ellipsoid for the thin disc are found to
be, \sigma_{V}/\sigma_{U} = 0.70 \pm 0.13 and \sigma_{W}/\sigma_{U} is 0.64 \pm
0.08, while for the thick disc \sigma_{V}/\sigma_{U} = 0.67 \pm 0.11 and
\sigma_{W}/\sigma_{U} is 0.66 \pm 0.11. Inputting these dispersions into the
linear Str\"omberg relation for the thin disc groups, we find the Sun's
velocity with respect to the LSR in Galactic rotation to be V_{\sun} = 13.9 \pm
3.4 km s^{-1}. A relation is found between the vertex deviation and the
chemical abundances for the thin disc, ranging from -5 to +40^{\circ} as
iron-abundance increases. For the thick disc we find a vertex deviation of
l_{uv} \sim -15^{\circ}. The tilt angle (l_{uw}) in the U-W plane for the thin
disc groups ranges from -10 to +15^\circ, but there is no evident relation
between l_{uw} and the mean abundances. However we find a weak relation for
l_{uw} as a function of iron abundances and \alpha-elements for most of the
groups in the thick disc, where the tilt angle decreases from -5 to -20^\circ
when [Fe/H] decreases and [\alpha/Fe] increases. The velocity anisotropy
parameter is independent of the chemical group abundances and its value is
nearly constant for both discs (\beta \sim 0.5), suggesting that the combined
disc is dynamically relaxed.
|
1710.08479v1
|
2017-11-06
|
K2-141 b: A 5-M$_\oplus$ super-Earth transiting a K7 V star every 6.7 hours
|
We report on the discovery of K2-141 b (EPIC 246393474 b), an
ultra-short-period super-Earth on a 6.7-hour orbit transiting an active K7 V
star based on data from K2 campaign 12. We confirmed the planet's existence and
measured its mass with a series of follow-up observations: seeing-limited
MuSCAT imaging, NESSI high-resolution speckle observations, and FIES and HARPS
high-precision radial-velocity monitoring. K2-141 b has a mass of $5.31 \pm
0.46 $ $M_{\oplus}$ and radius of $1.54^{+0.10}_{-0.09}$ $R_{\oplus}$, yielding
a mean density of $8.00_{ - 1.45 } ^ { + 1.83 }$ $\mathrm{g\,cm^{-3}}$ and
suggesting a rocky-iron composition. Models indicate that iron cannot exceed
$\sim$70 % of the total mass. With an orbital period of only 6.7 hours, K2-141
b is the shortest-period planet known to date with a precisely determined mass.
|
1711.02097v2
|
2017-12-15
|
Interior Structures and Tidal Heating in the TRAPPIST-1 Planets
|
With seven planets, the TRAPPIST-1 system has the largest number of
exoplanets discovered in a single system so far. The system is of
astrobiological interest, because three of its planets orbit in the habitable
zone of the ultracool M dwarf. Assuming the planets are composed of
non-compressible iron, rock, and H$_2$O, we determine possible interior
structures for each planet. To determine how much tidal heat may be dissipated
within each planet, we construct a tidal heat generation model using a single
uniform viscosity and rigidity for each planet based on the planet's
composition. With the exception of TRAPPIST-1c, all seven of the planets have
densities low enough to indicate the presence of significant H$_2$O in some
form. Planets b and c experience enough heating from planetary tides to
maintain magma oceans in their rock mantles; planet c may have eruptions of
silicate magma on its surface, which may be detectable with next-generation
instrumentation. Tidal heat fluxes on planets d, e, and f are lower, but are
still twenty times higher than Earth's mean heat flow. Planets d and e are the
most likely to be habitable. Planet d avoids the runaway greenhouse state if
its albedo is $\gtrsim$ 0.3. Determining the planet's masses within $\sim0.1$
to 0.5 Earth masses would confirm or rule out the presence of H$_2$O and/or
iron in each planet, and permit detailed models of heat production and
transport in each planet. Understanding the geodynamics of ice-rich planets f,
g, and h requires more sophisticated modeling that can self-consistently
balance heat production and transport in both rock and ice layers.
|
1712.05641v2
|
2018-01-16
|
The origin of ICM enrichment in the outskirts of present-day galaxy clusters from cosmological hydrodynamical simulations
|
The uniformity of the intra-cluster medium (ICM) enrichment level in the
outskirts of nearby galaxy clusters suggests that chemical elements were
deposited and widely spread into the intergalactic medium before the cluster
formation. This observational evidence is supported by numerical findings from
cosmological hydrodynamical simulations, as presented in Biffi et al. (2017),
including the effect of thermal feedback from active galactic nuclei. Here, we
further investigate this picture, by tracing back in time the spatial origin
and metallicity evolution of the gas residing at z=0 in the outskirts of
simulated galaxy clusters. In these regions, we find a large distribution of
iron abundances, including a component of highly-enriched gas, already present
at z=2. At z>1, the gas in the present-day outskirts was distributed over tens
of virial radii from the the main cluster and had been already enriched within
high-redshift haloes. At z=2, about 40% of the most Fe-rich gas at z=0 was not
residing in any halo more massive than 1e11 Msun/h in the region and yet its
average iron abundance was already 0.4, w.r.t. the solar value by Anders &
Grevesse (1989). This confirms that the in situ enrichment of the ICM in the
outskirts of present-day clusters does not play a significant role, and its
uniform metal abundance is rather the consequence of the accretion of both
low-metallicity and pre-enriched (at z>2) gas, from the diffuse component and
through merging substructures. These findings do not depend on the mass of the
cluster nor on its core properties.
|
1801.05425v2
|
2018-03-07
|
Extracting a less model dependent cosmic ray composition from $X_\mathrm{max}$ distributions
|
At higher energies the uncertainty in the estimated cosmic ray mass
composition, extracted from the observed distributions of the depth of shower
maximum $X_\text{max}$, is dominated by uncertainties in the hadronic
interaction models. Thus, the estimated composition depends strongly on the
particular model used for its interpretation. To reduce this model dependency
in the interpretation of the mass composition, we have developed a novel
approach which allows the adjustment of the normalisation levels of the proton
$<X_\text{max}>$ and $\sigma (X_\text{max})$ guided by real observations of
$X_\text{max}$ distributions. In this paper we describe the details of this
approach and present a study of its performance and its limitations. Using this
approach we extracted cosmic ray mass composition information from the
published Pierre Auger $X_\text{max}$ distributions. We have obtained a
consistent mass composition interpretation for Epos-LHC, QGSJetII-04 and
Sibyll2.3. Our fits suggest a composition consisting of predominantly iron.
Below $10^{18.8}$ eV, the small proportions of proton, helium and nitrogen
vary. Above $10^{18.8}$ eV, there is little proton or helium, and with
increasing energy the nitrogen component gradually gives way to the growing
iron component, which dominates at the highest energies. The fits suggest that
the normalisation level for proton $<X_\mathrm{max}>$ is much deeper than the
initial predictions of the hadronic interaction models. The fitted
normalisation level for proton $\sigma (X_\text{max})$ is also greater than the
model predictions. When fixing the expected normalisation of
$\sigma(X_\mathrm{max})$ to that suggested by the QGSJetII-04 model, a slightly
larger fraction of protons is obtained. These results remain sensitive to the
other model parameters that we keep fixed, such as the elongation rate and the
$<X_\text{max}>$ separation between p and Fe.
|
1803.02520v1
|
2018-03-23
|
Neutron-induced reactions in nuclear astrophysics
|
The quest for the origin of the chemical elements, which we find in our body,
in our planet (Earth), in our star (Sun), or in our galaxy (Milky Way) could
only be resolved with a thorough understanding of the nuclear physics
properties of stable and unstable atomic nuclei. While the elements until iron
are either created during the big bang or during fusion reactions in stars,
most of the elements heavier than iron are produced via neutron-induced
reactions. Therefore, neutron capture cross sections of stable and unstable
isotopes are important. So far, time-of-flight or activation methods have been
applied very successfully, but these methods reach their limits once the
isotopes with half-lives shorter than a few months are of interest. A
combination of a radioactive beam facility, an ion storage ring and a high flux
reactor or a spallation source would allow a direct measurement of
neutron-induced reactions over a wide energy range of isotopes with half-lives
down to minutes. The idea is to measure neutron-induced reactions on
radioactive ions in inverse kinematics. This means, the radioactive ions will
pass through a neutron target. In order to efficiently use the rare nuclides as
well as to enhance the luminosity, the exotic nuclides can be stored in an ion
storage ring. The neutron target can be the core of a research reactor, where
one of the central fuel elements is replaced by the evacuated beam pipe of the
storage ring. Alternatively, a large moderator surrounding a spallation source
can be intersected by the beam pipe of an ion storage ring. Using particle
detectors and Schottky spectroscopy, most of the important neutron-induced
reactions, such as (n,$\gamma$), (n,p), (n,$\alpha$), (n,2n), or (n,f), could
be investigated.
|
1803.08678v1
|
2018-03-26
|
Distinct pressure evolution of coupled nematic and magnetic order in FeSe
|
FeSe, despite being the structurally simplest compound in the family of
iron-based superconductors, shows an astoundingly rich interplay of physical
phenomena including nematicity and pressure-induced magnetism. Here, we present
a microscopic study of these two phenomena by high-energy x-ray diffraction and
time-domain M\"ossbauer spectroscopy on FeSe single crystals over a wide
temperature and pressure range. The topology of the pressure-temperature phase
diagram is a surprisingly close parallel to the well-known doping-temperature
phase diagram of BaFe2As2 generated through partial Fe/Co and Ba/Na
substitution. In FeSe with pressure p as a control parameter, the
magneto-structural ground state can be tuned from "pure" nematic - paramagnetic
with an orthorhombic lattice distortion - through a strongly coupled
magnetically ordered and orthorhombic state to a magnetically ordered state
without an orthorhombic lattice distortion. The magnetic hyperfine field
increases monotonically over a wide pressure range. However, the orthorhombic
distortion initially decreases under increasing pressure, but is stabilized by
cooperative coupling to the pressure-induced magnetic order. Close to the
reported maximum of the superconducting critical temperature Tc (occuring at p
= 6.8 GPa), the orthorhombic distortion suddenly disappears and FeSe remains
tetragonal down to the lowest temperature measured. Analysis of the structural
and magnetic order parameters suggests an independent origin of the structural
and magnetic ordering phenomena, and their cooperative coupling leads to the
similarity with the canonical phase diagram of iron pnictides.
|
1803.09449v1
|
2018-04-01
|
The 1.5 Ms Observing Campaign on IRAS 13224-3809: X-ray Spectral Analysis I
|
We present a detailed spectral analysis of the recent 1.5\,Ms XMM-Newton
observing campaign on the narrow line Seyfert 1 galaxy IRAS~13224$-$3809, taken
simultaneously with 500\,ks of NuSTAR data. The X-ray lightcurve shows three
flux peaks, registering at about 100 times the minimum flux seen during the
campaign, and rapid variability with a time scale of kiloseconds. The spectra
are well fit with a primary powerlaw continuum, two relativistic-blurred
reflection components from the inner accretion disk with very high iron
abundance, and a simple blackbody-shaped model for the remaining soft excess.
The spectral variability is dominated by the power law continuum from a corona
region within a few gravitational radii from the black hole. Additionally,
blueshifted Ne \textsc{x}, Mg \textsc{xii}, Si \textsc{xiv} and S \textsc{xvi}
absorption lines are identified in the stacked low-flux spectrum, confirming
the presence of a highly ionized outflow with velocity up to $v= 0.267$ and
$0.225$\,c. We fit the absorption features with \texttt{xstar} models and find
a relatively constant velocity outflow through the whole observation. Finally,
we replace the \texttt{bbody} and supersolar abundance reflection models by
fitting the soft excess successfully with the extended reflection model
\texttt{relxillD}, which allows for higher densities than the standard
\texttt{relxill} model. This returns a disk electron density $n_{\rm
e}>10^{18.7}$\,cm$^{-3}$ and lowers the iron abundance from $Z_{\rm
Fe}=24^{+3}_{-4}Z_\odot$ with $n_{\rm e}\equiv10^{15}$\,cm$^{-3}$ to $Z_{\rm
Fe}=6.6^{+0.8}_{-2.1}Z_\odot$.
|
1804.00349v1
|
2018-04-08
|
A high-density relativistic reflection origin for the soft and hard X-ray excess emission from Mrk 1044
|
We present the first results from a detailed spectral-timing analysis of a
long ($\sim$130 ks) XMM-Newton observation and quasi-simultaneous NuSTAR and
Swift observations of the highly-accreting narrow-line Seyfert 1 galaxy Mrk
1044. The broadband (0.3$-$50 keV) spectrum reveals the presence of a strong
soft X-ray excess emission below $\sim$1.5 keV, iron K$_{\alpha}$ emission
complex at $\sim$6$-$7 keV and a `Compton hump' at $\sim$15$-$30 keV. We find
that the relativistic reflection from a high-density accretion disc with a
broken power-law emissivity profile can simultaneously explain the soft X-ray
excess, highly ionized broad iron line and the Compton hump. At low frequencies
($[2-6]\times10^{-5}$ Hz), the power-law continuum dominated 1.5$-$5 keV band
lags behind the reflection dominated 0.3$-$1 keV band, which is explained with
a combination of propagation fluctuation and Comptonization processes, while at
higher frequencies ($[1-2]\times10^{-4}$ Hz), we detect a soft lag which is
interpreted as a signature of X-ray reverberation from the accretion disc. The
fractional root-mean-squared (rms) variability of the source decreases with
energy and is well described by two variable components: a less variable
relativistic disc reflection and a more variable direct coronal emission. Our
combined spectral-timing analyses suggest that the observed broadband X-ray
variability of Mrk~1044 is mainly driven by variations in the location or
geometry of the optically thin, hot corona.
|
1804.02703v2
|
2018-04-19
|
The Peculiar Atmospheric Chemistry of KELT-9b
|
The atmospheric temperatures of the ultra-hot Jupiter KELT-9b straddle the
transition between gas giants and stars, and therefore between two
traditionally distinct regimes of atmospheric chemistry. Previous theoretical
studies assume the atmosphere of KELT-9b to be in chemical equilibrium. Despite
the high ultraviolet flux from KELT-9, we show using photochemical kinetics
calculations that the observable atmosphere of KELT-9b is predicted to be close
to chemical equilibrium, which greatly simplifies any theoretical
interpretation of its spectra. It also makes the atmosphere of KELT-9b, which
is expected to be cloudfree, a tightly constrained chemical system that lends
itself to a clean set of theoretical predictions. Due to the lower pressures
probed in transmission (compared to emission) spectroscopy, we predict the
abundance of water to vary by several orders of magnitude across the
atmospheric limb depending on temperature, which makes water a sensitive
thermometer. Carbon monoxide is predicted to be the dominant molecule under a
wide range of scenarios, rendering it a robust diagnostic of the metallicity
when analyzed in tandem with water. All of the other usual suspects (acetylene,
ammonia, carbon dioxide, hydrogen cyanide, methane) are predicted to be
subdominant at solar metallicity, while atomic oxygen, iron and magnesium are
predicted to have relative abundances as high as 1 part in 10,000. Neutral
atomic iron is predicted to be seen through a forest of optical and
near-infrared lines, which makes KELT-9b suitable for high-resolution
ground-based spectroscopy with HARPS-N or CARMENES. We summarize future
observational prospects of characterizing the atmosphere of KELT-9b.
|
1804.07137v3
|
2018-04-23
|
Metal Pollution of Low-Mass Population III Stars through Accretion of Interstellar Objects like `Oumuamua
|
We calculate accretion mass of interstellar objects (ISOs) like `Oumuamua
onto low-mass population III stars (Pop.~III survivors), and estimate surface
pollution of Pop.~III survivors. An ISO number density estimated from the
discovery of `Oumuamua is so high ($\sim 0.2$~au$^{-3}$) that Pop.~III
survivors have chances at colliding with ISOs $\gtrsim 10^5$ times per $1$~Gyr.
`Oumuamua itself would be sublimated near Pop.~III survivors, since it has
small size, $\sim 100$~m. However, ISOs with size $\gtrsim 3$~km would reach
the Pop.~III survivor surfaces. Supposing an ISO cumulative number density with
size larger than $D$ is $n \propto D^{-\alpha}$, Pop.~III survivors can accrete
ISO mass $\gtrsim 10^{-16}M_\odot$, or ISO iron mass $\gtrsim 10^{-17}M_\odot$,
if $\alpha < 4$. This iron mass is larger than the accretion mass of
interstellar medium (ISM) by several orders of magnitude. Taking into account
material mixing in a convection zone of Pop.~III survivors, we obtain their
surface pollution is typically [Fe/H] $\lesssim -8$ in most cases, however the
surface pollution of Pop.~III survivors with $0.8M_\odot$ can be [Fe/H]
$\gtrsim -6$ because of the very shallow convective layer. If we apply to
Pop.III survivors located at the Galactocentric distance of 8 kpc, the
dependence of the metal pollustion is as follows. If $\alpha > 4$, Pop.~III
survivors have no chance at colliding with ISOs with $D \gtrsim 3$~km, and keep
metal-free. If $3 < \alpha < 4$, Pop.~III survivors would be most polluted by
ISOs up to [Fe/H] $\sim -7$. If $\alpha < 3$ up to $D \sim 10$~km, Pop.~III
survivors could hide in metal-poor stars so far discovered. Pop.~III survivors
would be more polluted with decreasing the Galactocentric distance. Although
the metal pollution depends on $\alpha$ and the Galactocentric distance, we
first show the importance of ISOs for the metal pollution of Pop.~III
survivors.
|
1804.08200v2
|
2018-05-02
|
Electronic phase separation in iron selenide (Li, Fe)OHFeSe superconductor system
|
The phenomenon of phase separation into antiferromagnetic (AFM) and
superconducting (SC) or normal-state regions has great implication for the
origin of high-temperature (high-Tc) superconductivity. However, the occurrence
of an intrinsic antiferromagnetism above the Tc of (Li, Fe)OHFeSe
superconductor is questioned. Here we report a systematic study on a series of
(Li, Fe)OHFeSe single crystal samples with Tc up to ~41 K. We observe an
evident drop in the static magnetization at Tafm ~125 K, in some of the SC (Tc
< ~38 K, cell parameter c < ~9.27 {\AA}) and non-SC samples. We verify that
this AFM signal is intrinsic to (Li, Fe)OHFeSe. Thus, our observations indicate
mesoscopic-to-macroscopic coexistence of an AFM state with the normal (below
Tafm) or SC (below Tc) state in (Li, Fe)OHFeSe. We explain such coexistence by
electronic phase separation, similar to that in high-Tc cuprates and iron
arsenides. However, such an AFM signal can be absent in some other samples of
(Li, Fe)OHFeSe, particularly it is never observed in the SC samples of Tc > ~38
K, owing to a spatial scale of the phase separation too small for the
macroscopic magnetic probe. For this case, we propose a microscopic electronic
phase separation. It is suggested that the microscopic static phase separation
reaches vanishing point in high-Tc (Li, Fe)OHFeSe, by the occurrence of
two-dimensional AFM spin fluctuations below nearly the same temperature as Tafm
reported previously for a (Li, Fe)OHFeSe (Tc ~42 K) single crystal. A complete
phase diagram is thus established. Our study provides key information of the
underlying physics for high-Tc superconductivity.
|
1805.00688v1
|
2018-05-02
|
On a new and homogeneous metallicity scale for Galactic classical Cepheids - I. Physical parameters
|
We gathered more than 1130 high-resolution optical spectra for more than 250
Galactic classical Cepheids. The spectra were collected with different optical
spectrographs: UVES at VLT, HARPS at 3.6m, FEROS at 2.2m MPG/ESO, and STELLA.
To improve the effective temperature estimates, we present more than 150 new
line depth ratio (LDR) calibrations that together with similar calibrations
already available in the literature allowed us to cover a broad range in
wavelength (between 5348 and 8427 angstrom) and in effective temperatures
(between 3500 and 7700 K). This means the unique opportunity to cover both the
hottest and coolest phases along the Cepheid pulsation cycle and to limit the
intrinsic error on individual measurements at the level of ~100 K. Thanks to
the high signal-to-noise ratio of individual spectra we identified and measured
hundreds of neutral and ionized lines of heavy elements, and in turn, have the
opportunity to trace the variation of both surface gravity and microturbulent
velocity along the pulsation cycle. The accuracy of the physical parameters and
the number of Fe I (more than one hundred) and Fe II (more than ten) lines
measured allowed us to estimate mean iron abundances with a precision better
than 0.1 dex. Here we focus on 14 calibrating Cepheids for which the current
spectra cover either the entire or a significant portion of the pulsation
cycle. The current estimates of the variation of the physical parameters along
the pulsation cycle and of the iron abundances agree quite well with similar
estimates available in the literature. Independent homogeneous estimates of
both physical parameters and metal abundances based on different approaches
that can constrain possible systematics are highly encouraged.
|
1805.00727v1
|
2018-05-09
|
Near-infrared spectral evolution of the Type Ia supernova 2014J in the nebular phase: implications for the progenitor system
|
As the closest Type Ia supernova in decades, SN 2014J provides a unique
opportunity for detailed investigation into observational signatures of the
progenitor system and explosion mechanism in addition to burning product
distribution. We present a late-time near-infrared spectral series from
Gemini-N at $307-466$ days after the explosion. Following the $H$-band
evolution probes the distribution of radioactive iron group elements, the
extent of mixing, and presence of magnetic fields in the expanding ejecta.
Comparing the isolated $1.6440$ $\mu$m [Fe II] emission line with synthetic
models shows consistency with a Chandrasekhar-mass white dwarf of
$\rho_c=0.7\times10^9$ g cm${}^{-3}$ undergoing a delayed detonation. The ratio
of the flux in the neighboring $1.54$ $\mu$m emission feature to the flux in
the $1.6440$ $\mu$m feature shows evidence of some limited mixing of stable and
radioactive iron group elements in the central regions. Additionally, the
evolution of the $1.6440$ $\mu$m line shows an intriguing asymmetry. When
measuring line-width of this feature, the data show an increase in line width
not seen in the evolution of the synthetic spectra, corresponding to
$\approx1{,}000$ km s${}^{-1}$, which could be caused by a localized transition
to detonation producing asymmetric ionization in the ejecta. Using the
difference in width between the different epochs, an asymmetric component in
the central regions, corresponding to approximately the inner $2\times10^{-4}$
of white dwarf mass suggests an off-center ignition of the initial explosion
and hence of the kinematic center from the chemical center. Several additional
models investigated, including a He detonation and a merger, have difficulty
reproducing the features seen these spectra.
|
1805.03556v2
|
2018-05-18
|
Consistent Iron Abundances Derived from Neutral and Singly-Ionized Iron Lines in Ultraviolet and Optical Spectra of Six Warm Metal-Poor Stars
|
Neutral Fe lines in metal-poor stars yield conflicting abundances depending
on whether and how deviations from local thermodynamic equilibrium (LTE) are
considered. We have collected new high resolution and high signal-to-noise
ultraviolet (UV) spectra of three warm dwarf stars with [Fe/H] = -2.9 with the
Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We locate
archival UV spectra for three other warm dwarfs with [Fe/H] = -3.3, -2.2, and
-1.6, supplemented with optical spectra for all six stars. We calculate stellar
parameters using methods that are largely independent of the spectra, adopting
broadband photometry, color-temperature relations, Gaia parallaxes, and assumed
masses. We use the LTE line analysis code MOOG to derive Fe abundances from
hundreds of Fe I and Fe II lines with wavelengths from 2290 to 6430 Angstroms.
The [Fe/H] ratios derived separately from Fe I and Fe II lines agree in all six
stars, with [Fe II/H] - [Fe I/H] ranging from +0.00 +/- 0.07 to -0.12 +/- 0.09
dex, when strong lines and Fe I lines with lower excitation potential < 1.2 eV
are excluded. This constrains the extent of any deviations from LTE that may
occur within this parameter range. While our result confirms non-LTE
calculations for some warm, metal-poor dwarfs, it may not be generalizable to
more metal-poor dwarfs, where deviations from LTE are predicted to be larger.
We also investigate trends of systematically lower abundances derived from Fe I
lines in the Balmer continuum region (3100-3700 Angstroms), and we conclude
that no proposed explanation for this effect can fully account for the
observations presently available.
|
1805.07390v1
|
2018-05-30
|
A Recommendation Algorithm to Predict Giant Exoplanet Host Stars Using Stellar Elemental Abundances
|
The presence of certain elements within a star, and by extension its planet,
strongly impacts the formation and evolution of the planetary system. The
positive correlation between a host star's iron-content and the presence of an
orbiting giant exoplanet has been confirmed; however, the importance of other
elements in predicting giant planet occurrence is less certain despite their
central role in shaping internal planetary structure. We designed and applied a
machine learning algorithm to the Hypatia Catalog (Hinkel et a. 2014) to
analyze the stellar abundance patterns of known host stars to determine those
elements important in identifying potential giant exoplanet host stars. We
analyzed a variety of different elements ensembles, namely volatiles,
lithophiles, siderophiles, and Fe. We show that the relative abundances of
oxygen, carbon, and sodium, in addition to iron, are influential indicators of
the presence of a giant planet. We demonstrate the predictive power of our
algorithm by analyzing stars with known giant planets and found that they had
median 75% prediction score. We present a list of ~350 stars with no currently
discovered planets that have a $\geq$90% prediction probability likelihood of
hosting a giant exoplanet. We investigated archival HARPS data and found
significant trends that HIP62345, HIP71803, and HIP10278 host long-period giant
planet companions with estimated minimum $M_p\sin(i)$ values of 3.7, 6.8, and
8.5 M$_{J}$, respectively. We anticipate that our findings will revolutionize
future target selection, the role that elements play in giant planet formation,
and the determination of giant planet interior structure models.
|
1805.12144v2
|
2018-06-12
|
Galactic Archeology with the AEGIS Survey: The Evolution of Carbon and Iron in the Galactic Halo
|
Understanding the evolution of carbon and iron in the Milky Way's halo is of
importance because these two elements play crucial roles constraining star
formation, Galactic assembly, and chemical evolution in the early Universe.
Here, we explore the spatial distributions of carbonicity, [C/Fe], and
metallicity, [Fe/H], of the halo system based on medium-resolution ($R \sim$
1,300) spectroscopy of $\sim$58,000 stars in the Southern Hemisphere from the
AAOmega Evolution of Galactic Structure (AEGIS) survey. The AEGIS carbonicity
map exhibits a positive gradient with distance, as similarly found for the
Sloan Digital Sky Survey (SDSS) carbonicity map of Lee et al. The metallicity
map confirms that [Fe/H] decreases with distance, from the inner halo to the
outer halo. We also explore the formation and chemical-evolution history of the
halo by considering the populations of carbon-enhanced metal-poor (CEMP) stars
present in the AEGIS sample. The cumulative and differential frequencies of
CEMP-no stars (as classified by their characteristically lower levels of
absolute carbon abundance, $A$(C) $\leq$ 7.1 for sub-giants and giants)
increases with decreasing metallicity, and is \textit{substantially higher than
previous determinations} for CEMP stars as a whole. In contrast, that of
CEMP-$s$ stars (with higher $A$(C)), remains almost flat, at a value
$\sim$10\%, in the range $-\,4.0 \lesssim$ [Fe/H] $\lesssim-$2.0. The
distinctly different behaviors of the CEMP-no and CEMP-$s$ stars relieve the
tension with population-synthesis models assuming a binary mass-transfer
origin, which previously struggled to account for the higher reported
frequencies of CEMP stars, taken as a whole, at low metallicity.
|
1806.04738v1
|
2018-06-26
|
Modeling dust emission in PN IC 418
|
We investigated the infrared (IR) dust emission from PN IC 418, using a
detailed model controlled by a previous determination of the stellar properties
and the characteristics of the photoionized nebula, keeping as free parameters
the dust types, amounts and distributions relative to the distance of the
central star. The model includes the ionized region and the neutral region
beyond the recombination front (Photodissociation region, or PDR), where the
[OI] and [CII] IR lines are formed. We succeeded in reproducing the observed
infrared emission from 2 to 200~\mm. The global energy budget is fitted by
summing up contributions from big grains of amorphous carbon located in the
neutral region and small graphite grains located in the ionized region (closer
to the central star).
Two emission features seen at 11.5 and 30~\mm are also reproduced by assuming
them to be due to silicon carbide (SiC) and magnesium and iron sulfides
(Mg$_x$Fe$_{1-x}$S), respectively. For this, we needed to consider ellipsoidal
shapes for the grains to reproduce the wavelength distribution of the features.
Some elements are depleted in the gaseous phase: Mg, Si, and S have sub-solar
abundances (-0.5 dex below solar by mass), while the abundance of C+N+O+Ne by
mass is close to solar. Adding the abundances of the elements present in the
dusty and gaseous forms leads to values closer to but not higher than solar,
confirming that the identification of the feature carriers is plausible. Iron
is strongly depleted (3 dex below solar) and the small amount present in dust
in our model is far from being enough to recover the solar value. A remaining
feature is found as a residue of the fitting process, between 12 and 25~\mm,
for which we do not have identification.
|
1806.10248v2
|
2018-11-02
|
NuSTAR and XMM-Newton broad-band spectrum of SAX J1808.4-3658 during its latest outburst in 2015
|
The first discovered accreting millisecond pulsar, SAX J1808.4-3658, went
into X-ray outburst in April 2015. We triggered a 100 ks XMM-Newton ToO, taken
at the peak of the outburst, and a 55 ks NuSTAR ToO, performed four days apart.
We report here the results of a detailed spectral analysis of both the
XMM-Newton and NuSTAR spectra. While the XMM-Newton spectrum appears much
softer than in previous observations, the NuSTAR spectrum confirms the results
obtained with XMM-Newton during the 2008 outburst. We find clear evidence of a
broad iron line that we interpret as produced by reflection from the inner
accretion disk. For the first time, we use a self-consistent reflection model
to fit the reflection features in the NuSTAR spectrum; in this case we find a
statistically significant improvement of the fit with respect to a simple
Gaussian or diskline model to fit the iron line, implying that the reflection
continuum is also significantly detected. Despite the differences evident
between the XMM-Newton and NuSTAR spectra, the smearing best-fit parameters
found for these spectra are consistent with each other and are compatible with
previous results. In particular, we find an upper limit to the inner disk
radius of $\sim 12~R_g$. In all the cases, a high inclination angle
($>50^\circ$) of the system is required. This inclination angle, combined with
measurements of the radial velocity of the optical companion, results in a low
value for the neutron star mass ($<0.8\,M_\odot$), a result that deserves
further investigation.
|
1811.00940v1
|
2018-11-09
|
The chemical composition of planet building blocks as predicted by stellar population synthesis
|
Future space missions will improve considerably our understanding of the
formation and history of planetary systems. Currently, observations show that
the presence of planetary companions is closely linked to the metallicity and
the chemical abundances of the host stars. We aim to build an integrated tool
to predict the planet building blocks composition as a function of the stellar
populations, for the interpretation of the ongoing and future large surveys. We
synthesize stellar populations with the Besan\c{c}on Galaxy model (BGM) which
includes stellar evolutionary tracks computed with the stellar evolution code
STAREVOL. We integrate to the BGM a simple stoichiometric model to determine
the expected composition of the planet building blocks. We determine the
expected PBB composition around FGK stars, for the four galactic populations
within the Milky Way. Our solar neighborhood simulations are in good agreement
with the recent results obtained with the HARPS survey for f_iron, f_w and the
heavy mass fraction f_Z. We present evidence of the clear dependence of f_iron
and f_w with the initial alpha abundances [\alpha/Fe] of the host star. We find
that the different initial [\alpha/Fe] distributions in the different galactic
populations lead to a bimodal distribution of PBB composition and to an
iron/water valley separating PBB with high and low iron/water mass fractions.
We linked host star abundances and expected PBB composition in an integrated
model of the Galaxy. Derived trends are an important step for statistical
analyses of expected planet properties. In particular, internal structure
models may use these results to derive statistical trends of rocky planets
properties, constrain habitability and prepare interpretation of on-going and
future large scale surveys of exoplanet search.
|
1811.04096v1
|
2018-11-22
|
Effect of PEG binder on gas sensitivity of ferric oxide films synthesized by spin coating and doctor blade methods
|
Ferric oxide (Fe2O3) thin films were fabricated on amorphous nonconductive
and FTO conductive glass substrates starting from iron acetate nanoparticles by
the doctor blade and the spin coating techniques. Thin films of Fe2O3 with and
without the PEG binder were investigated. Films were subsequently annealed at
500 0C in air for 1 hour to crystallize the phase Fe2O3. The structural and
chemical properties of Fe2O3 thin films were determined using XRD and FTIR.
According to XRD, the single phase of Fe2O3 crystallized in the thin film after
annealing. The optical band gap of synthesized Fe2O3 thin films were determined
by UV-Visible spectrums. Gas sensitivity, respond time and recovery time of
Fe2O3 thin films were measured in 1000ppm of CO2 gas using AUTO LAB. Thin films
synthesized by the doctor blade method were thicker and more uniform than the
thin films prepared by the spin coating technique. Gas sensitivity of Fe2O3
films synthesized by the doctor blade method is slightly higher than that of
the films fabricated by the spin coating technique. Fe2O3 films with the PEG
binder posses a higher gas sensitivity, lower respond time and lower recovery
time compared to films without PEG binder. After adding the PEG binder, the
optical band gap of Fe2O3 thin films reduces. The reduction of the activation
energy is the reason for the improvement of the gas sensitivity after adding
the PEG. However, the peak positions of FTIR do not indicate any significant
change due to the adding of the PEG. Adding the binder increased the gas
sensitivity of iron oxide by 15.44%. Both FTIR and UV-Visible data confirm the
formation of Fe2O3 in the thin film.
|
1811.09614v1
|
2019-02-04
|
Quantum-critical scale invariance in a transition metal alloy
|
Quantum-mechanical fluctuations between competing phases at $T=0$ induce
exotic finite-temperature collective excitations that are not described by the
standard Landau Fermi liquid framework. These excitations exhibit anomalous
temperature dependences, or non-Fermi liquid behavior, in the transport and
thermodynamic properties in the vicinity of a quantum critical point, and are
often intimately linked to the appearance of unconventional Cooper pairing as
observed in strongly correlated systems including the high-$T_c$ cuprate and
iron pnictide superconductors. The presence of superconductivity, however,
precludes direct access to the quantum critical point, and makes it difficult
to assess the role of quantum-critical fluctuations in shaping anomalous
finite-temperature physical properties. Here we report temperature-field scale
invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in
a non-superconducting iron-pnictide,
Ba(Fe$_{1/3}$Co$_{1/3}$Ni$_{1/3}$)$_{2}$As$_{2}$, indicative of quantum
criticality at zero temperature and zero applied magnetic field. Beyond a
linear in temperature resistivity, the hallmark signature of strong
quasiparticle scattering, we find the scattering rate that obeys a universal
scaling relation between temperature and applied magnetic fields down to the
lowest energy scales. Together with the dominance of hole-like carriers close
to the zero-temperature and zero-field limits, the scale invariance, isotropic
field response, and lack of applied pressure sensitivity suggests a unique
quantum critical system that does not drive a pairing instability.
|
1902.01034v3
|
2019-02-13
|
Spectral analysis of the barium central star of the planetary nebula Hen 2-39
|
Barium stars are peculiar red giants characterized by an overabundance of
s-process elements along with an enrichment in carbon. These stars are
discovered in binaries with white dwarf companions. The more recently formed of
these stars are still surrounded by a planetary nebula. Precise abundance
determinations of the various s-process elements, especially, of the lightest,
short-lived radionuclide technetium will establish constraints for the
formation of s-process elements in asymptotic giant branch stars as well as
mass transfer through, for example, stellar wind, Roche-lobe overflow, and
common-envelope evolution. We performed a detailed spectral analysis of the
K-type subgiant central star of the planetary nebula Hen 2-39 based on
high-resolution optical spectra obtained with the Ultraviolet and Visual
Echelle Spectrograph at the Very Large Telescope using LTE model atmospheres.
We confirm the effective temperature of $T_\mathrm{eff} = 4350 \pm 150$ K for
the central star of the planetary nebula Hen 2-39. It has a photospheric carbon
enrichment of $[\mathrm{C/H}]= 0.36 \pm 0.08$ and a barium overabundance of
$[\mathrm{Ba/Fe}]= 1.8 \pm 0.5$. We find a deficiency for most of the
iron-group elements (calcium to iron) and establish an upper abundance limit
for technetium ($\log \epsilon_\mathrm{Tc} < 2.5$). The quality of the
available optical spectra is not sufficient to measure abundances of all
s-process elements accurately. Despite large uncertainties on the abundances as
well as on the model yields, the derived abundances are most consistent with a
progenitor mass in the range 1.75-3.00 $M_\odot$ and a metallicity of
$[\mathrm{Fe/H}]= -0.3 \pm 1.0$. This result leads to the conclusion that the
formation of such systems requires a relatively large mass transfer that is
most easily obtained via wind-Roche lobe overflow.
|
1902.04897v1
|
2019-03-01
|
Signature of Dispersing 1D Majorana Channels in an Iron-based Superconductor
|
The possible realization of Majorana fermions as quasiparticle excitations in
condensed matter physics has created much excitement. Most recent studies have
focused on Majorana bound states which can serve as topological qubits. More
generally, akin to elementary particles, Majorana fermions can propagate and
display linear dispersion. These excitations have not yet been directly
observed, and can also be used for quantum information processing. One route to
realizing this is in a line junction between two phase-shifted superconductors
coupled to topological surface states. Recent theory indicates that in
iron-based superconductors, a particular type of crystalline defect, i.e., a
domain wall (DW) between two regions with a half-unit cell shift between them,
should create a $\pi$-phase shift in the superconducting order parameter.
Combined with recent data showing topological surface states in
FeSe$_x$Te$_{1-x}$ we find that this is the ideal system to realize helical
1D-dispersing Majorana modes. Here we report scanning tunneling spectroscopic
(STS) measurements of crystalline DWs in FeSe$_{0.45}$Te$_{0.55}$. By analyzing
large-area superconducting gap maps, we identify the gap in the topological
surface state, demonstrating that our sample is an effective Fu-Kane
proximitized topological system. We further locate DWs across which the atoms
shift by half a unit cell. STS data on these DWs reveal a flat density of
states inside the superconducting gap, a hallmark of linearly dispersing modes
in 1D. This unique signature is absent in DWs in the related superconductor,
FeSe which is not in the topological phase. Our combined data are consistent
with the observation of dispersing Majorana states at a $\pi$-phase shift DW in
a proximitized topological material.
|
1903.00515v4
|
2019-08-05
|
Spectral and orbital survey of medium-sized meteoroids
|
We investigate the spectra, material properties, and orbital distribution of
millimeter- to decimeter-sized meteoroids. Our study aims to distinguish the
characteristics of populations of differently sized meteoroids and reveal the
heterogeneity of identified meteoroid streams. We verify the surprisingly large
ratio of pure iron meteoroids on asteroidal orbits detected among mm-sized
bodies. Emission spectra and multi-station meteor trajectories were collected
within the AMOS network observations. The sample is based on 202 meteors of -1
to -14 magnitude, corresponding to meteoroids of mm to dm sizes. Meteoroid
composition is studied by relative intensity ratios of Na, Mg, and Fe.
Heliocentric orbits, trajectory parameters, and material strengths inferred
from empirical $K_B$ and $P_E$ parameters were determined for 146 meteoroids.
An overall increase of Na content compared to the population of mm-sized
meteoroids was detected, reflecting weaker effects of space weathering
processes on larger meteoroids. We report a very low ratio of pure iron
meteoroids and the discovery of a new spectral group of Fe-rich meteors. The
majority of meteoroids on asteroidal orbits were found to be chondritic.
Thermal processes causing Na depletion and physical processes resulting in
Na-rich spectra are described and linked to characteristically increased
material strengths. Numerous major and minor shower meteors were identified in
our sample, revealing various degrees of heterogeneity within Halley-type,
ecliptical, and sungrazing meteoroid streams. Our results imply a scattered
composition of the fragments of comet 2P/Encke and 109P/Swift-Tuttle. The
largest disparities were detected within the $\alpha$-Capricornids and
$\delta$-Aquarids. We also find a spectral similarity between $\kappa$-Cygnids
and Taurids, which could imply a similar composition of the parent objects of
the two streams.
|
1908.01565v2
|
2019-08-08
|
High-$T_c$ Iron-phosphide Superconductivity Enhanced by Reemergent Antiferromagnetic Spin Fluctuations in (Sr$_4$Sc$_2$O$_6$)Fe$_2$(As$_{1-x}$P$_{x}$)$_2$ probed by NMR
|
We report a systematic NMR study on
[Sr$_4$Sc$_2$O$_6$]Fe$_2$(As$_{1-x}$P$_x$)$_2$, for which the local lattice
parameters of the iron-pnictogen (Fe$Pn$) layer are similar to those of the
series LaFe(As$_{1-x'}$P$_{x'}$)O, which exhibit two segregated
antiferromagnetic (AFM) order phases, AFM1 at $x'$=0-0.2 and AFM2 at
$x'$=0.4-0.7. Our results revealed that the parent AFM1 phase at $x$=0
disappears at $x$=0.3-0.4, corresponding to a pnictogen height ($h_{pn}$) from
the Fe-plane of 1.3-1.32 \AA, which is similar to that of
LaFe(As$_{1-x'}$P$_{x'}$)O and various parent Fe-pnictides. By contrast, the
AFM2 order reported for LaFe(As$_{0.4}$P$_{0.6}$)O does not appear at
$x\sim$0.8, although the local lattice parameters of the Fe$Pn$ layer and the
microscopic electronic states are quite similar. Despite the absence of the
{\it static} AFM2 order, reemergent {\it dynamical} AFM spin fluctuations were
observed at approximately $x\sim$0.8, which can be attributed to the
instability of the AFM2 phase. We suggest this re-enhancement of AFM spin
fluctuations to play a significant role in enhancing the $T_c$ to 17 K for
$x$=0.8-1. Finally, we discuss the universality and diversity of the
complicated magnetic ground states from a microscopic point of view, including
the difference in the origins of the AFM1 and AFM2 phases, and their relations
with the high superconducting transitions in Fe-pnictides.
|
1908.02980v1
|
2019-08-09
|
Abundances of alpha-Process Elements in Thin-Disk, Thick-Disk, and Halo Stars of the Galaxy: Non-LTE Analysis
|
The atmospheric parameters and abundances of Mg, Si, Ca, and Ti have been
determined for 20 stars using the Gaia DR2 parallaxes, high-resolution spectra,
and the non-local thermodynamic equilibrium (non-LTE) line formation modeling.
A sample of stars with homogeneous data on the abundances of alpha-process
elements has thus been increased to 94. It is shown that applying a non-LTE
approach and classical 1D atmospheric models with spectroscopically determined
surface gravities based on Fe~I and Fe~II lines yields reliable results.
Analysis of the full sample confirms the conclusions of earlier studies
indicating enhancements of Mg, Si, Ca, and Ti relative to Fe for halo and
thick-disk stars, and larger enhancements for the thick-disk stars compared to
the thin-disk stars of similar metallicities. The following new results are
obtained. The ratios [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe] in the thick disk
remain constant and similar to each other at the level 0.3 when [Fe/H] < -0.4,
and fall off when the metallicity becomes higher, suggesting the onset of the
production of iron in Type Ia supernovae. Halo stars have the same [alpha/Fe]
values independent of their distance (within ~ 8 kpc of the Sun), providing
evidence for a universal evolution of the abundances of alpha-process elements
in different parts of the Galaxy. The enhancements relative to iron for halo
stars are, on average, similar, at the level 0.3 dex, for Mg, Si, Ca, and Ti.
These data are important for constraining the nucleosynthesis models. The
star-to-star scatter of [alpha/Fe] increases for [Fe/H] < -2.6, while the
scatter of the ratios between the different alpha-process elements remains
small, possibly indicating incomplete mixing of nucleosynthesis products at the
epoch of the formation of these stars.
|
1908.03370v1
|
2019-09-16
|
An evolving broad iron line from the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124
|
We present a spectral study of the ultraluminous Be/X-ray transient pulsar
Swift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER)
observations during the system's 2017--2018 giant outburst. The 1.2--10~keV
energy spectrum of the source can be approximated with an absorbed cut-off
power law model. We detect strong, luminosity-dependent emission lines in the
6--7 keV energy range. A narrow 6.42 keV line, observed in the sub-Eddington
regime, is seen to evolve into a broad Fe-line profile in the super-Eddington
regime. Other features are found at 6.67 and 6.97 keV in the Fe-line complex.
An asymmetric broad line profile, peaking at 6.67 keV, is possibly due to
Doppler effects and gravitational redshift. The 1.2--79 keV broadband spectrum
from NuSTAR and NICER observations at the outburst peak is well described by an
absorbed cut-off power law plus multiple Gaussian lines and a blackbody
component. Physical reflection models are also tested to probe the broad iron
line feature. Depending on the mass accretion rate, we found emission sites
that are evolving from ~5000 km to a range closer to the surface of the neutron
star. Our findings are discussed in the framework of the accretion disk and its
implication on the magnetic field, the presence of optically thick accretion
curtain in the magnetosphere, jet emission, and the massive, ultra-fast outflow
expected at super-Eddington accretion rates. We do not detect any signatures of
a cyclotron absorption line in the NICER or NuSTAR data.
|
1909.07338v1
|
2019-11-06
|
The Impact of Starbursts on Element Abundance Ratios
|
We investigate the impact of bursts in star formation on the predictions of
one-zone chemical evolution models, adopting oxygen (O), iron (Fe), and
strontium (Sr), as representative $\alpha$, iron-peak, and s-process elements,
respectively. To this end, we develop the Versatile Integrator for Chemical
Evolution (VICE), a python package. Starbursts driven by a temporary boost of
gas accretion rate create loops in [O/Fe]-[Fe/H] evolutionary tracks and a peak
in the stellar [O/Fe] distribution at intermediate values. Bursts driven by a
temporary boost of star formation efficiency have a similar effect, and they
also produce a population of $\alpha$-deficient stars during the depressed star
formation phase that follows the burst. This $\alpha$-deficient population is
more prominent if the outflow rate is tied to a time-averaged star formation
rate (SFR) instead of the instantaneous SFR. Theoretical models of Sr
production predict a strong metallicity dependence of supernova and asymptotic
giant branch (AGB) star yields, though comparison to data suggests an
additional source that is nearly metallicity-independent. Evolution of [Sr/Fe]
and [Sr/O] during a starburst is complex because of the yield metallicity
dependence and the multiple timescales in play. Moderate amplitude (10-20\%)
sinusoidal oscillations in SFR produce loops in [O/Fe]-[Fe/H] tracks and
multiple peaks in [O/Fe] distributions, which could be one source of intrinsic
scatter in observed sequences. We investigate models that have a factor of ~2
enhancement of SFR at t = 12 Gyr, as suggest by some recent Milky Way
observations. A late episode of enhanced star formation could help explain the
existence of young stars with moderate $\alpha$-enhancements and the
surprisingly young median age found for solar metallicity stars in the solar
neighborhood, while also raising the possibility that this starburst has not
fully decayed.
|
1911.02598v1
|
2019-11-14
|
Stellar population astrophysics (SPA) with the TNG. Revisiting the metallicity of Praesepe (M44)
|
Open clusters exquisitely track the Galactic disc chemical properties and its
time evolution; a substantial number of studies and large spectroscopic surveys
focus mostly on the chemical content of relatively old clusters (age $\gtrsim$
1 Gyr). Interestingly, the less studied young counterpart populating the solar
surrounding has been found to be solar (at most), with a notable surprising
lack of young metal-rich objects. While there is wide consensus about the
moderately above-solar composition of the Hyades cluster, the metallicity of
Praesepe is still controversial. Recent studies suggest that these two clusters
share identical chemical composition and age, but this conclusion is disputed.
With the aim of reassessing the metallicity of Praesepe, and its difference (if
any) with the Hyades cluster, we present in this paper a spectroscopic
investigation of ten solar-type dwarf members. We exploited $GIARPS$ at the TNG
to acquire high-resolution, high-quality optical and near-IR spectra and
derived stellar parameters, metallicity ([Fe/H]), light elements, $\alpha$- and
iron-peak elements, by using a strictly differential (line-by-line) approach.
We also analysed in the very same way the solar spectrum and the Hyades solar
analogue HD 28099. Our findings suggest that Praesepe is more metal-rich than
the Hyades, at the level of $\Delta$[Fe/H]=+0.05$\pm$0.01 dex, with a mean
value of [Fe/H]=+0.21$\pm0.01$ dex. All the other elements scale with iron, as
expected. This result seems to reject the hypothesis of a common origin for
these two open clusters. Most importantly, Praesepe is currently the most
metal-rich, young open cluster living in the solar neighbourhood.
|
1911.06337v2
|
2020-01-15
|
Utilizing Essential Symmetry Breaking in Auxiliary-Field Quantum Monte Carlo: Application to the Spin Gaps of the C$_{36}$ Fullerene and an Iron Porphyrin Model Complex
|
We present three distinct examples where phaseless auxiliary-field Quantum
Monte Carlo (ph-AFQMC) can be reliably performed with a single-determinant
trial wavefunction with essential symmetry breaking. We first utilized
essential time-reversal symmetry breaking with ph-AFQMC to compute the
triplet-singlet energy gap in the TS12 set. We found statistically better
performance of ph-AFQMC with complex-restricted orbitals than with
spin-unrestricted orbitals. We then showed the utilization of essential spin
symmetry breaking when computing the single-triplet gap of a known
biradicaloid, C$_{36}$. ph-AFQMC with spin-unrestricted Hartree-Fock
(ph-AFQMC+UHF) fails catastrophically even with spin-projection and predicts no
biradicaloid character. With approximate Brueckner orbitals obtained from
regularized orbital-optimized second-order M{\o}ller-Plesset perturbation
theory ($\kappa$-OOMP2), ph-AFQMC quantitatively captures strong biradicaloid
character of C$_{36}$. Lastly, we applied ph-AFQMC to the computation of the
quintet-triplet gap in a model iron porphyrin complex where brute-force methods
with a small active space fail to capture the triplet ground state. We show
unambiguously that neither triplet nor quintet is strongly correlated using
UHF, $\kappa$-OOMP2, and coupled-cluster with singles and doubles (CCSD)
performed on UHF and $\kappa$-OOMP2 orbitals. There is no essential symmetry
breaking in this problem. By virtue of this, we were able to perform
UHF+ph-AFQMC reliably with a cc-pVTZ basis set and predicted a triplet ground
state for this model geometry. The largest ph-AFQMC in this work correlated 186
electrons in 956 orbitals. Our work highlights the utility, scalability, and
accuracy of ph-AFQMC with a single determinant trial wavefunction with
essential symmetry breaking for systems mainly dominated by dynamical
correlation with little static correlation.
|
2001.05109v2
|
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