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2023-05-24
|
Modeling the chemical enrichment history of the Bulge Fossil Fragment Terzan 5
|
Terzan 5 is a heavily obscured stellar system located in the inner Galaxy. It
has been postulated to be a stellar relic, a Bulge Fossil Fragment witnessing
the complex history of the assembly of the Milky Way bulge. In this paper, we
follow the chemical enrichment of a set of putative progenitors of Terzan 5 to
assess whether the chemical properties of this cluster fit within a formation
scenario in which it is the remnant of a primordial building block of the
bulge. We can explain the metallicity distribution function and the runs of
different element-to-iron abundance ratios as functions of [Fe/H] derived from
optical-infrared spectroscopy of giant stars in Terzan 5, by assuming that the
cluster experienced two major star formation bursts separated by a long
quiescent phase. We further predict that the most metal-rich stars in Terzan 5
are moderately He-enhanced and a large spread of He abundances in the cluster,
Y = 0.26-0.335. We conclude that current observations fit within a formation
scenario in which Terzan 5 originated from a pristine, or slightly
metal-enriched, gas clump about one order of magnitude more massive than its
present-day mass. Losses of gas and stars played a major role in shaping Terzan
5 the way we see it now. The iron content of the youngest stellar population is
better explained if the white dwarfs that give rise to type Ia supernovae (the
main Fe factories) sink towards the cluster center, rather than being stripped
by the strong tidal forces exerted by the Milky Way in the outer regions.
|
2305.15355v1
|
2023-06-07
|
2021 occultations and transits of Linus orbiting (22) Kalliope: I. Polygonal and `cliptracing' algorithm
|
The satellite Linus orbiting the main-belt asteroid (22) Kalliope exhibited
occultation and transit events in late 2021. A photometric campaign was
organized and observations were taken by the TRAPPIST-South, SPECULOOS-Artemis,
OWL-Net, and BOAO telescopes, with the goal to constrain models of this system.
Our dynamical model is complex, with multipoles (up to the order $\ell = 2$),
internal tides, and external tides. The model was constrained by astrometry
(spanning 2001--2021), occultations, adaptive-optics imaging, calibrated
photometry, as well as relative photometry. Our photometric model was
substantially improved. A new precise (${<}\,0.1\,{\rm mmag}$) light curve
algorithm was implemented, based on polygon intersections, which are computed
exactly -- by including partial eclipses and partial visibility of polygons.
Moreover, we implemented a `cliptracing' algorithm, based again on polygon
intersections, in which partial contributions to individual pixels are computed
exactly. Both synthetic light curves and synthetic images are then very smooth.
Based on our combined solution, we confirmed the size of Linus, $(28\pm
1)\,{\rm km}$. However, this solution exhibits some tension between the light
curves and the PISCO speckle-interferometry dataset. In most solutions, Linus
is darker than Kalliope, with the albedos $A_{\rm w} = 0.40$ vs. $0.44$. This
is confirmed on deconvolved images. A~detailed revision of astrometric data
allowed us to revise also the $J_2 \equiv -C_{20}$ value of Kalliope. Most
importantly, a~homogeneous body is excluded. For a differentiated body, two
solutions exist: low-oblateness ($C_{20} \simeq -0.12$), with a~spherical iron
core, and alternatively, high-oblateness ($C_{20} \simeq -0.22$) with an
elongated iron core. These correspond to the low- and high-energy collisions,
respectively, studied by means of SPH simulations in our previous work.
|
2306.04768v1
|
2023-07-04
|
Packed bed thermal energy storage for waste heat recovery in the iron and steel industry: An experimental study on powder hold-up and pressure drop
|
Waste heat recovery in the energy intensive industry is one of the most
important measures for the mitigation of climate change. The utilization of
just a fraction of the theoretically available waste heat potential would lead
to a significant reduction of the primary energy consumption and hence a
reduction of greenhouse gas emissions. The present study examines the
integration of a packed bed thermal energy storage for waste heat recovery in
the iron and steel industry. Along with the highly fluctuating availability of
excess heat the main difficulty of waste heat recovery in industrial processes
is the high amount of powder that is transported by the hot exhaust gases.
Therefore, investigations focus on the pressure drop and powder hold-up in a
packed bed thermal energy storage that is operated with a gas-powder two phase
exhaust gas as heat transfer fluid with the ultimate goal to assess its
suitability and robustness under such challenging operational conditions. The
results indicate, that 98 % of the powder that is introduced into the system
with the heat transfer fluid during charging accumulates in the packed bed.
Remarkably, most of the powder hold-up in the packed bed is concentrated near
the surface at which the heat transfer fluid enters the packed bed. When
reversing the flow direction of the heat transfer fluid to discharge the
storage with a clean single phase gas, this gas is not contaminated with the
powder that has been accumulated in previous charging periods. Further, the
radial distribution of the powder hold-up in the packed bed is observed to be
even which indicates that there is no risk of random flow channel formation
that could affect the thermal performance (storage capacity, thermal power
rate) of the system. The results reinforce the great potential of packed bed
thermal energy storage systems for waste heat recovery in the energy intensive
industry.
|
2307.01585v2
|
2023-07-10
|
The individual abundance distributions of disc stars across birth radii in GALAH
|
Individual abundances in the Milky Way disc record stellar birth properties
(e.g. age, birth radius ($R_{\rm birth}$)) and capture the diversity of the
star-forming environments over time. Assuming an analytical relationship
between ([Fe/H], [$\alpha$/Fe]) and $R_{\rm birth}$, we examine the
distributions of individual abundances [X/Fe] of elements C, O, Mg, Si, Ca
($\alpha$), Al (odd-z), Mn (iron-peak), Y, and Ba (neutron-capture) for stars
in the Milky Way. We want to understand how these elements might differentiate
environments across the disc. We assign tracks of $R_{\rm birth}$ in the
[$\alpha$/Fe] vs. [Fe/H] plane as informed by expectations from simulations for
$\sim 59,000$ GALAH stars in the solar neighborhood ($R\sim7-9$ kpc) which also
have inferred ages. Our formalism for $R_{\rm birth}$ shows that older stars
($\sim$10 Gyrs) have a $R_{\rm birth}$ distribution with smaller mean values
(i.e., $\bar{R}_{\mbox{birth}}$$\sim5\pm0.8$ kpc) compared to younger stars
($\sim6$ Gyrs; $\bar{R}_{\mbox{birth}}$$\sim10\pm1.5$ kpc), for a given [Fe/H],
consistent with inside-out growth. The $\alpha$-, odd-z, and iron-peak element
abundances decrease as a function of $R_{\rm birth}$, whereas the
neutron-capture abundances increase. The $R_{\rm birth}$-[Fe/H] gradient we
measure is steeper compared to the present-day gradient (-0.067 dex/kpc vs
-0.058 dex/kpc), which we also find true for $R_{\rm birth}$-[X/Fe] gradients.
These results (i) showcase the feasibility of relating the birth radius of
stars to their element abundances, (ii) the abundance gradients across $R_{\rm
birth}$ are steeper than those over current radius, and (iii) offer an
observational comparison to expectations on element abundance distributions
from hydrodynamical simulations.
|
2307.04724v1
|
2023-07-11
|
Dynamics of orbital degrees of freedom probed via isotope $^{121,123}$ Sb nuclear quadrupole moments in Sb-substituted iron-pnictide superconductors
|
Isotope $^{121,123}$Sb nuclei with large electric quadrupole moments are
applied to investigate the dynamics of orbital degrees of freedom in
Sb-substituted iron(Fe)-based compounds. In the parent compound
LaFe(As$_{0.6}$Sb$_{0.4}$)O, the nuclear spin relaxation rate
$^{121,123}(T_{1}^{-1})$ at $^{121,123}$Sb sites was enhanced at structural
transition temperature ($T_{s}\sim$ 135 K), which is higher than N\'eel
temperature ($T_{\rm N}\sim$125 K). The isotope ratio
$^{123}(T_{1}^{-1})/^{121}(T_{1}^{-1})$ indicates that the electric quadrupole
relaxation due to the dynamical electric field gradient at Sb site increases
significantly toward $T_{s}$. It is attributed to the critically enhanced
nematic fluctuations of stripe-type arrangement of Fe-$3d_{xz}$ (or $3d_{yz}$)
orbitals. In the lightly electron-doped superconducting (SC) compound
LaFe(As$_{0.7}$Sb$_{0.3}$)(O$_{0.9}$F$_{0.1}$), the nematic fluctuations are
largely suppressed in comparison with the case of the parent compound, however,
it remains a small enhancement below 80 K down to the $T_c$($\sim$ 20 K). The
results indicate that the fluctuations from both the spin and orbital degrees
of freedom on the $3d_{xz}$(or $3d_{yz}$) orbitals can be seen in lightly
electron-doped SC state of LaFeAsO-based compounds. We emphasize that isotope
$^{121,123}$Sb quadrupole moments are sensitive local probe to identify the
dynamics of orbital degrees of freedom in Fe-pnictides, which provides with a
new opportunity to discuss the microscopic correlation between the
superconductivity and both nematic and spin fluctuations simultaneously even in
the polycrystalline samples.
|
2307.05112v1
|
2023-07-11
|
The Dependence of Iron-rich Metal-poor Star Occurrence on Galactic Environment Supports an Origin in Thermonuclear Supernova Nucleosynthesis
|
It has been suggested that a class of chemically peculiar metal-poor stars
called iron-rich metal-poor (IRMP) stars formed from molecular cores with metal
contents dominated by thermonuclear supernova nucleosynthesis. If this
interpretation is accurate, then IRMP stars should be more common in
environments where thermonuclear supernovae were important contributors to
chemical evolution. Conversely, IRMP stars should be less common in
environments where thermonuclear supernovae were not important contributors to
chemical evolution. At constant $[\text{Fe/H}] \lesssim -1$, the Milky Way's
satellite classical dwarf spheroidal (dSph) galaxies and the Magellanic Clouds
have lower $[\text{$\alpha$/Fe}]$ than the Milky Way field and globular cluster
populations. This difference is thought to demonstrate the importance of
thermonuclear supernova nucleosynthesis for the chemical evolution of the Milky
Way's satellite classical dSph galaxies and the Magellanic Clouds. We use data
from the Sloan Digital Sky Survey (SDSS) Apache Point Observatory Galactic
Evolution Experiment (APOGEE) and Gaia to infer the occurrence of IRMP stars in
the Milky Way's satellite classical dSph galaxies $\eta_{\text{dSph}}$ and the
Magellanic Clouds $\eta_{\text{Mag}}$ as well as in the Milky Way field
$\eta_{\text{MWF}}$ and globular cluster populations $\eta_{\text{MWGC}}$. In
order of decreasing occurrence, we find
$\eta_{\text{dSph}}=0.07_{-0.02}^{+0.02}$,
$\eta_{\text{Mag}}=0.037_{-0.006}^{+0.007}$,
$\eta_{\text{MWF}}=0.0013_{-0.0005}^{+0.0006}$, and a 1-$\sigma$ upper limit
$\eta_{\text{MWGC}}<0.00057$. These occurrences support the inference that IRMP
stars formed in environments dominated by thermonuclear supernova
nucleosynthesis and that the time lag between the formation of the first and
second stellar generations in globular clusters was longer than the
thermonuclear supernova delay time.
|
2307.05669v2
|
2023-07-31
|
Effects of Grain Magnetic Properties and Grain Growth on Synthetic Dust Polarization of MHD Simulations in Protostellar Environments
|
Thermal dust polarization is a powerful tool to probe magnetic fields
($\textbf{B}$) and grain properties. However, a systematic study of the
dependence of dust polarization on grain properties in protostellar
environments is not yet available. In this paper, we post-process a non-ideal
MHD simulation of a collapsing protostellar core with our updated POLARIS code
to study in detail the effects of iron inclusions and grain growth on thermal
dust polarization. We found that superparamagnetic (SPM) grains can produce
high polarization degree of $p \sim 10-40\%$ beyond $\sim 500$ au from the
protostar because of their efficient alignment by magnetically enhanced
Radiative Torque mechanism. The magnetic field tangling by turbulence in the
envelope causes the decrease in $p$ with increasing emission intensity $I$ as
$p\propto I^{\alpha}$ with the slope $\alpha \sim -0.3$. But within 500 au, SPM
grains tend to have inefficient internal alignment (IA) and be aligned with
$\textbf{B}$ by RATs only, producing lower $p \sim 1\%$ and a steeper slope of
$\alpha \sim -0.6$. For paramagnetic (PM) grains, the alignment loss of grains
above $1\mu m$ in the inner $\sim 200$ au produces $p << 1\%$ and the
polarization hole with $\alpha \sim -0.9$. Grain growth can increase $p$ in the
envelope for SPM grains, but cause stronger depolarization for SPM grains in
the inner $\sim 500$ au and for PM grains in the entire protostellar core.
Finally, we found the increase of polarization angle dispersion function $S$
with iron inclusions and grain growth, implying the dependence of B-field
strength measured using the DCF technique on grain alignment and grain
properties.
|
2307.16829v2
|
2023-08-02
|
Numerical modeling of thermal dust polarization from aligned grains in the envelope of evolved stars with updated POLARIS
|
Magnetic fields are thought to influence the formation and evolution of
evolved star envelopes. Thermal dust polarization from magnetically aligned
grains is potentially a powerful tool for probing magnetic fields and dust
properties in these circumstellar environments. In this paper, we present
numerical modeling of thermal dust polarization from the envelope of IK Tau
using the magnetically enhanced radiative torque (MRAT) alignment theory
implemented in our updated POLARIS code. Due to the strong stellar radiation
field, the minimum size required for RAT alignment of silicate grains is $\sim
0.005 - 0.05\,\rm\mu m$. Additionally, ordinary paramagnetic grains can achieve
perfect alignment by MRAT in the inner regions of $r < 500\,\rm au$ due to
stronger magnetic fields of $B\sim 10$ mG - 1G, producing thermal dust
polarization degree of $\sim 10\,\%$. The polarization degree can be enhanced
to $\sim 20-40\%$ for grains with embedded iron inclusions. We also find that
the magnetic field geometry affects the alignment size and the resulting
polarization degree due to the projection effect in the plane-of-sky. We also
study the spectrum of polarized thermal dust emission and find the increased
polarization degree toward $\lambda > 50\,\rm\mu m$ due to the alignment of
small grains by MRAT. Furthermore, we investigate the impact of rotational
disruption by RATs (RAT-D) and find the RAT-D effect cause a decrease in the
dust polarization fraction. Finally, we compare our numerical results with
available polarization data observed by SOFIA/HAWC+ for constraining dust
properties, suggesting grains are unlikely to have embedded iron clusters and
might have slightly elongated shapes. Our modeling results suggest further
observational studies at far-infrared/sub-millimeter wavelengths to understand
the properties of magnetic fields and dust in AGB envelopes.
|
2308.01215v1
|
2023-08-02
|
Oxygen, sulfur, and iron radial abundance gradients of classical Cepheids across the Galactic thin disk
|
Classical Cepheids (CCs) are solid distance indicators and tracers of young
stellar populations. Our aim is to provide iron, oxygen, and sulfur abundances
for the largest and most homogeneous sample of Galactic CCs ever analyzed. The
current sample covers a wide range in Galactocentric distances (RG), pulsation
modes and periods. High-resolution and high S/N spectra collected with
different spectrographs were adopted to estimate the atmospheric parameters.
Individual distances are based on Gaia trigonometric parallaxes or on
near-infrared Period-Luminosity relations. We found that Fe and alpha-element
radial gradients based on CCs display a well-defined change in the slope for RG
larger than 12 kpc. Radial gradients based on open clusters, covering a wide
range in age, display similar trends, meaning that the flattening in the outer
disk is an intrinsic feature of the radial gradients since it is independent of
age. Empirical evidence indicates that the radial gradient for S is steeper
than for Fe. The difference in the slope is a factor of two in the linear fit.
We also found that S is, on average, under-abundant compared with O. We
performed a detailed comparison with Galactic chemical evolution models and we
found that a constant Star Formation Efficiency for RG larger than 12 kpc takes
account for the flattening in both Fe and alpha-elements. To further constrain
the impact that predicted S yields for massive stars have on radial gradients,
we adopted a "toy model" and we found that the flattening in the outermost
regions requires a decrease of a factor of four in the current S predictions.
Sulfur photospheric abundances, compared with other alpha-elements, have the
key advantage of being a volatile element. Therefore, stellar S abundances can
be directly compared with nebular S abundances in external galaxies.
|
2308.01928v1
|
2023-08-29
|
Planet formation throughout the Milky Way: Planet populations in the context of Galactic chemical evolution
|
As stellar compositions evolve over time in the Milky Way, so will the
resulting planet populations. In order to place planet formation in the context
of Galactic chemical evolution, we make use of a large ($N = 5\,325$) stellar
sample representing the thin and thick discs, defined chemically, and the halo,
and we simulate planet formation by pebble accretion around these stars. We
build a chemical model of their protoplanetary discs, taking into account the
relevant chemical transitions between vapour and refractory minerals, in order
to track the resulting compositions of formed planets. We find that the masses
of our synthetic planets increase on average with increasing stellar
metallicity [Fe/H] and that giant planets and super-Earths are most common
around thin-disc ($\alpha$-poor) stars since these stars have an overall higher
budget of solid particles. Giant planets are found to be very rare
($\lesssim$1\%) around thick-disc ($\alpha$-rich) stars and nearly non-existent
around halo stars. This indicates that the planet population is more diverse
for more metal-rich stars in the thin disc. Water-rich planets are less common
around low-metallicity stars since their low metallicity prohibits efficient
growth beyond the water ice line. If we allow water to oxidise iron in the
protoplanetary disc, this results in decreasing core mass fractions with
increasing [Fe/H]. Excluding iron oxidation from our condensation model instead
results in higher core mass fractions, in better agreement with the core-mass
fraction of Earth, that increase with increasing [Fe/H]. Our work demonstrates
how the Galactic chemical evolution and stellar parameters, such as stellar
mass and chemical composition, can shape the resulting planet population.
|
2308.15504v1
|
2023-09-11
|
The Scale of Stellar Yields: Implications of the Measured Mean Iron Yield of Core Collapse Supernovae
|
The scale of alpha-element yields is difficult to predict from theory because
of uncertainties in massive star evolution, supernova physics, and black hole
formation, and it is difficult to constrain empirically because the impact of
higher yields can be compensated by greater metal loss in galactic winds. We
use a recent measurement of the mean iron yield of core collapse supernovae
(CCSN) by Rodriguez et al. (RMN23), $\bar{y}_{\rm Fe}^{\rm cc} =0.058 \pm 0.007
M_\odot$, to infer the scale of alpha-element yields by assuming that the
plateau of [alpha/Fe] abundance ratios observed in low metallicity stars
represents the yield ratio of CCSN. For a Kroupa IMF and a plateau at
[alpha/Fe]=0.45, we find that the population-averaged yields of O and Mg per
unit mass of star formation are about equal to the mass fractions of these
elements in the sun. The inferred O and Fe yields agree with predictions of the
Sukhbold et al. (2016) CCSN models assuming their Z9.6+N20 neutrino-driven
engine, a scenario in which many progenitors with $M<40M_\odot$ implode to
black holes rather than exploding. The yields are lower than assumed in some
models of galactic chemical evolution (GCE) and the galaxy mass-metallicity
relation, reducing the level of outflows needed to match observed abundances.
For straightforward assumptions, we find that one-zone GCE models with
mass-loading factor $\eta\approx 0.6$ evolve to solar metallicity at late
times. By requiring that models reach [alpha/Fe]=0 at late times, and assuming
a mean Fe yield of $0.7M_\odot$ per Type Ia supernova, we infer a Hubble-time
integrated SNIa rate of $1.1\times 10^{-3} M_\odot^{-1}$, compatible with
estimates from supernova surveys. The RMN23 measurement provides one of the few
empirical anchors for the absolute scale of nucleosynthetic yields, with
wide-ranging implications for stellar and galactic astrophysics.
|
2309.05719v1
|
2023-10-11
|
Absence of topological Hall effect in Fe$_x$Rh$_{100-x}$ epitaxial films: revisiting their phase diagram
|
A series of Fe$_x$Rh$_{100-x}$ ($30 \leq x \leq 57$) films were epitaxially
grown using magnetron sputtering, and were systematically studied by
magnetization-, electrical resistivity-, and Hall resistivity measurements.
After optimizing the growth conditions, phase-pure Fe$_{x}$Rh$_{100-x}$ films
were obtained, and their magnetic phase diagram was revisited. The
ferromagnetic (FM) to antiferromagnetic (AFM) transition is limited at narrow
Fe-contents with $48 \leq x \leq 54$ in the bulk Fe$_x$Rh$_{100-x}$ alloys. By
contrast, the FM-AFM transition in the Fe$_x$Rh$_{100-x}$ films is extended to
cover a much wider $x$ range between 33 % and 53 %, whose critical temperature
slightly decreases as increasing the Fe-content. The resistivity jump and
magnetization drop at the FM-AFM transition are much more significant in the
Fe$_x$Rh$_{100-x}$ films with $\sim$50 % Fe-content than in the Fe-deficient
films, the latter have a large amount of paramagnetic phase. The
magnetoresistivity (MR) is rather weak and positive in the AFM state, while it
becomes negative when the FM phase shows up, and a giant MR appears in the
mixed FM- and AFM states. The Hall resistivity is dominated by the ordinary
Hall effect in the AFM state, while in the mixed state or high-temperature FM
state, the anomalous Hall effect takes over. The absence of topological Hall
resistivity in Fe$_{x}$Rh$_{100-x}$ films with various Fe-contents implies that
the previously observed topological Hall effect is most likely extrinsic. We
propose that the anomalous Hall effect caused by the FM iron moments at the
interfaces nicely explains the hump-like anomaly in the Hall resistivity. Our
systematic investigations may offer valuable insights into the spintronics
based on iron-rhodium alloys.
|
2310.07140v1
|
2023-10-19
|
Potential Lifshitz transition at optimal substitution in nematic pnictide Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$
|
BaNi$_2$As$_2$ is a structural analog of the pnictide superconductor
BaFe$_2$As$_2$, which, like the iron-based superconductors, hosts a variety of
ordered phases including charge density waves (CDWs), electronic nematicity,
and superconductivity. Upon isovalent Sr substitution on the Ba site, the
charge and nematic orders are suppressed, followed by a sixfold enhancement of
the superconducting transition temperature ($T_c$). To understand the
mechanisms responsible for enhancement of $T_c$, we present high-resolution
angle-resolved photoemission spectroscopy (ARPES) measurements of the
Ba$_{1-x}$Sr$_{x}$Ni$_2$As$_2$ series, which agree well with our density
functional theory (DFT) calculations throughout the substitution range.
Analysis of our ARPES-validated DFT results indicates a Lifshitz transition and
reasonably nested electron and hole Fermi pockets near optimal substitution
where $T_c$ is maximum. These nested pockets host Ni $d_{xz}$/$d_{yz}$ orbital
compositions, which we associate with the enhancement of nematic fluctuations,
revealing unexpected connections to the iron-pnictide superconductors. This
gives credence to a scenario in which nematic fluctuations drive an enhanced
$T_c$.
|
2310.13163v1
|
2023-10-31
|
First spectroscopic investigation of Anomalous Cepheid variables
|
Anomalous Cepheids (ACEPs) are intermediate mass metal-poor pulsators mostly
discovered in dwarf galaxies of the Local Group. However, recent Galactic
surveys, including the Gaia DR3, found a few hundreds of ACEPs in the Milky
Way. Their origin is not well understood. We aim to investigate the origin and
evolution of Galactic ACEPs by studying for the first time the chemical
composition of their atmospheres. We used UVES@VLT to obtain high-resolution
spectra for a sample of 9 ACEPs belonging to the Galactic halo. We derived the
abundances of 12 elements, including C, Na, Mg, Si, Ca, Sc, Ti, Cr, Fe, Ni, Y,
and Ba. We complemented these data with literature abundances for an additional
three ACEPs that were previously incorrectly classified as type II Cepheids,
thus increasing the sample to a total of 12 stars. All the investigated ACEPs
have an iron abundance [Fe/H]$<-1.5$ dex as expected from theoretical
predictions for these pulsators. The abundance ratios of the different elements
to iron show that the ACEP's chemical composition is generally consistent with
that of the Galactic halo field stars, except the Sodium, which is found
overabundant in 9 out of the 11 ACEPs where it was measured, in close
similarity with second-generation stars in the Galactic Globular Clusters. The
same comparison with dwarf and ultra-faint satellites of the Milky Way reveals
more differences than similarities so it is unlikely that the bulk of Galactic
ACEPs originated in such a kind of galaxies which subsequently dissolved in the
Galactic halo. The principal finding of this work is the unexpected
overabundance of Sodium in ACEPs. We explored several hypotheses to explain
this feature, finding that the most promising scenario is the evolution of
low-mass stars in a binary system with either mass transfer or merging.
Detailed modelling is needed to confirm this hypothesis.
|
2310.20503v1
|
2023-11-08
|
A self-synthesized origin of heavy metals in hot subdwarf stars?
|
Some He-rich hot subdwarf stars (He-sdOBs) present high abundances of
trans-iron elements, such as Sr, Y, Zr and Pb. Diffusion processes are
important in hot subdwarf stars, and it is thought that the high abundances of
heavy elements in these stars are due to the action of radiative levitation.
However, during the formation of He-sdOBs, hydrogen can be ingested into the
convective zone driven by the He-core flash. It is known that episodes in which
protons are being ingested into He-burning convective zones can lead to
neutron-capture processes and the formation of heavy elements. In this work we
aim to explore for the first time if neutron-capture processes can occur in
late He-core flashes happening in the cores of the progenitors of He-sdOBs. We
compute a detailed evolutionary model of a stripped red-giant star with a
stellar evolution code with a nuclear network comprising 32 isotopes. Then we
post-process the stellar models in the phase of He and H burning with a
post-processing nucleosynthesis code with a nuclear network of 1190 species
that allows us to follow the neutron-capture processes in detail. We find the
occurrence of neutron-capture processes in our model, with neutron densities
reaching a value of $\sim5\times10^{12}\,{\rm cm}^{-3}$. We find that the
trans-iron elements are enhanced in the surface by 1 to 2 dex as compared to
initial compositions. Moreover, the relative abundance pattern $[{\rm
X}_i/\rm{Fe}]$ produced by neutron-capture processes closely resembles those
observed in some He-sdOBs, hinting at a possible self-synthesized origin of the
heavy elements in these stars. We conclude that intermediate neutron-capture
processes can occur during a proton ingestion event in the He-core flash of
stripped red-giant stars. This mechanism offers a natural channel to produce
the heavy elements observed in some of the He-sdOBs.
|
2311.04700v1
|
2023-12-07
|
Interface-Induced Superconductivity in Magnetic Topological Insulator-Iron Chalcogenide Heterostructures
|
When two different electronic materials are brought together, the resultant
interface often shows unexpected quantum phenomena, including interfacial
superconductivity and Fu-Kane topological superconductivity (TSC). Here, we use
molecular beam epitaxy (MBE) to synthesize heterostructures formed by stacking
together two magnetic materials, a ferromagnetic topological insulator (TI) and
an antiferromagnetic iron chalcogenide (FeTe). We discover emergent
interface-induced superconductivity in these heterostructures and demonstrate
the trifecta occurrence of superconductivity, ferromagnetism, and topological
band structure in the magnetic TI layer, the three essential ingredients of
chiral TSC. The unusual coexistence of ferromagnetism and superconductivity can
be attributed to the high upper critical magnetic field that exceeds the Pauli
paramagnetic limit for conventional superconductors at low temperatures. The
magnetic TI/FeTe heterostructures with robust superconductivity and atomically
sharp interfaces provide an ideal wafer-scale platform for the exploration of
chiral TSC and Majorana physics, constituting an important step toward scalable
topological quantum computation.
|
2312.04353v1
|
2023-12-08
|
The Heavy Metal Survey: The Evolution of Stellar Metallicities, Abundance Ratios, and Ages of Massive Quiescent Galaxies Since z~2
|
We present the elemental abundances and ages of 19 massive quiescent galaxies
at $z\sim1.4$ and $z\sim2.1$ from the Keck Heavy Metal Survey. The ultra-deep
LRIS and MOSFIRE spectra were modeled using a full-spectrum stellar population
fitting code with variable abundance patterns. The galaxies have iron
abundances between [Fe/H] = -0.5 and -0.1 dex, with typical values of $-0.2$
[$-0.3$] at $z\sim1.4$ [$z\sim2.1$]. We also find a tentative
$\log\sigma_v$-[Fe/H] relation at $z\sim1.4$. The magnesium-to-iron ratios span
[Mg/Fe] = 0.1--0.6 dex, with typical values of $0.3$ [$0.5$] dex at $z\sim1.4$
[$z\sim2.1$]. The ages imply formation redshifts of $z_{\rm form}=2-8$.
Compared to quiescent galaxies at lower redshifts, we find [Fe/H] was $\sim0.2$
dex lower at $z=1.4-2.1$. We find no evolution in [Mg/Fe] out to $z\sim1.4$,
though the $z\sim2.1$ galaxies are $0.2$ dex enhanced compared to $z=0-0.7$. A
comparison of these results to a chemical evolution model indicates that
galaxies at higher redshift form at progressively earlier epochs and over
shorter star-formation timescales, with the $z\sim2.1$ galaxies forming the
bulk of their stars over 150 Myr at $z_{\rm form}\sim4$. This evolution cannot
be solely attributed to an increased number of quiescent galaxies at later
times; several Heavy Metal galaxies have extreme chemical properties not found
in massive galaxies at $z\sim0.0-0.7$. Thus, the chemical properties of
individual galaxies must evolve over time. Minor mergers also cannot fully
account for this evolution as they cannot increase [Fe/H], particularly in
galaxy centers. Consequently, the build-up of massive quiescent galaxies since
$z\sim2.1$ may require further mechanisms such as major mergers and/or central
star formation.
|
2312.05307v1
|
2023-12-12
|
Abundances of iron-peak elements in accreted and in situ born Galactic halo stars
|
Previous work on the abundances of C, O, Na, Mg, Si, Ca, Ti, Cr, Mn, Fe, Ni,
Cu, and Zn in low-alpha (accreted) and high-alpha (in situ born) halo stars is
extended to include the abundances of Sc, V, and Co, enabling us to study the
nucleosynthesis of all iron-peak elements along with the lighter elements. The
Sc, V, and Co abundances were determined from a 1D MARCS model-atmosphere
analysis of equivalent widths of atomic lines in high signal-to-noise, high
resolution spectra assuming local thermodynamic equilibrium (LTE). In addition,
new 3D and/or non-LTE calculations were used to correct the 1D LTE abundances
for several elements including consistent 3D non-LTE calculations for Mg. The
two populations of accreted and in situ born stars are well separated in
diagrams showing [Sc/Fe], [V/Fe], and [Co/Fe] as a function of [Fe/H]. The
[X/Mg] versus [Mg/H] trends for high-alpha and low-alpha stars were used to
determine the yields of core-collapse and Type Ia supernovae. The largest Type
Ia contribution occurs for Cr, Mn, and Fe, whereas Cu is a pure core-collapse
element. Sc, Ti, V, Co, Ni, and Zn represent intermediate cases. A comparison
with yields calculated for supernova models shows poor agreement for the
core-collapse yields. The Ia yields suggest that sub-Chandrasekhar-mass Type Ia
supernovae provide a dominant contribution to the chemical evolution of the
host galaxies of the low-alpha stars. A substructure in the abundances and
kinematics of the low-alpha stars suggests that they arise from at least two
different satellite accretion events, Gaia-Sausage-Enceladus and Thamnos.
|
2312.07768v1
|
2023-12-20
|
On the Origin of the X-ray Emission in Heavily Obscured Compact Radio Sources
|
X-ray continuum emission of active galactic nuclei (AGNs) may be reflected by
circumnuclear dusty tori, producing prominent fluorescence iron lines at X-ray
frequencies. Here we discuss the broad-band emission of three radio-loud AGN
belonging to the class of compact symmetric objects (CSOs), with detected
narrow Fe\,K$\alpha$ lines. CSOs have newly-born radio jets, forming compact
radio lobes with projected linear sizes of the order of a few to hundreds of
parsecs. We model the radio--to--$\gamma$-ray spectra of compact lobes in
{J1407+2827}, J1511+0518, and {J2022+6137}, which are among the nearest and the
youngest CSOs known to date, and are characterized by an intrinsic X-ray
absorbing column density of $N_{\rm H} > 10^{23}$\,cm$^{-2}$. In addition to
the archival data, we analyze the newly acquired \chandra\ X-ray Observatory
and Sub-Millimeter Array (SMA) observations, and also refine the $\gamma$-ray
upper limits from the \fermi\ Large Area Telescope (LAT) monitoring. The new
\chandra\ data exclude the presence of the extended X-ray emission components
on scales larger than $1.5^{\prime \prime}$. The SMA data unveil a correlation
of the spectral index of the electron distribution in the lobes and $N_{\rm
H}$, which can explain the $\gamma$-ray quietness of heavily obscured CSOs.
Based on our modeling, we argue that the inverse-Compton emission of compact
radio lobes may account for the intrinsic X-ray continuum in all these sources.
Furthermore, we propose that the observed iron lines may be produced by a
reflection of the lobes' continuum from the surrounding cold dust.
|
2312.13418v2
|
2024-01-10
|
Nematic quantum disordered state in FeSe
|
The unusual quantum-disordered magnetic ground state intertwined with
superconductivity and electronic nematicity in FeSe has been a research focus
in iron-based superconductors. However, the intrinsic spin excitations across
the entire Brillouin zone in detwinned FeSe, which forms the basis for a
microscopic understanding of the magnetic state and superconductivity, remain
to be determined. Here, we use inelastic neutron scattering to map out the spin
excitations of FeSe dewtinned with a uniaxial-strain device. We find that the
stripe spin excitations (Q=(1, 0)/(0, 1)) exhibit the $C_2$ symmetry up to
$E\approx120$ meV, while the N{\'e}el spin excitations (Q=(1, 1)) retain their
$C_4$ symmetry in the nematic state. The temperature dependence of the
difference in the spin excitations at Q=(1, 0) and (0, 1) for temperatures
above the structural phase transition unambiguously shows the establishment of
the nematic quantum disordered state. The similarity of the N\'eel excitations
in FeSe and NaFeAs suggests that the N\'eel excitations are driven by the
enhanced electron correlations in the $3d_{xy}$ orbital. By determining the key
features of the stripe excitations and fitting their dispersions using a
Heisenberg Hamiltonian with biquadratic interaction ($J_1$-$K$-$J_2$), we
establish a spin-interaction phase diagram and conclude that FeSe is close to a
crossover region between the antiferroquadrupolar, N\'eel, and stripe ordering
regimes. The results provide an experimental basis for establishing a
microscopic theoretical model to describe the origin and intertwining of the
emergent orders in iron-based superconductors.
|
2401.05092v1
|
2024-01-17
|
Aperture and Resolution Effects on Ultraviolet Star-Forming Properties: Insights from Local Galaxies and Implications for High-Redshift Observations
|
We present an analysis of the effects of spectral resolution and aperture
scales on derived galaxy properties using far-ultraviolet (FUV) spectra of
local star-forming galaxies from the International Ultraviolet Explorer (R~250,
FOV~10"x20") and Cosmic Origins Spectrograph on the Hubble Space Telescope
(R~15,000, FOV~2.5"). Using these spectra, we measured FUV luminosities,
spectral slopes, dust attenuation, and equivalent widths. We find that galaxies
with one dominant stellar cluster have FUV properties that are independent of
aperture size, while galaxies with multiple bright clusters are sensitive to
the total light fraction captured by the aperture. Additionally, we find
significant correlations between the strength of stellar and interstellar
absorption-lines and metallicity, indicating metallicity-dependent line-driven
stellar winds and interstellar macroscopic gas flows shape the stellar and
interstellar spectral lines, respectively. The observed line-strength versus
metallicity relation of stellar-wind lines agrees with the prediction of
population synthesis models for young starbursts. In particular, measurements
of the strong stellar CIV 1548,1550 line provide an opportunity to determine
stellar abundances as a complement to gas-phase abundances. We provide a
relation between the equivalent width of the CIV line and the oxygen abundance
of the galaxy. We discuss this relation in terms of the stellar-wind properties
of massive stars. As the driving lines in stellar winds are mostly ionized iron
species, the CIV line may eventually offer a method to probe
alpha-element-to-iron ratios in star-forming galaxies once consistent models
with non-solar abundance ratios are available. These results have important
implications for the galaxy-scale, low-resolution observations of high-redshift
galaxies from JWST (R~100-3,500).
|
2401.08971v1
|
2024-02-09
|
High-Resolution X-Ray Spectroscopy of Interstellar Iron Toward Cygnus X-1 and GX 339-4
|
We present a high-resolution spectral study of Fe L-shell extinction by the
diffuse interstellar medium (ISM) in the direction of the X-ray binaries Cygnus
X-1 and GX 339-4, using the XMM-Newton reflection grating spectrometer. The
majority of interstellar Fe is suspected to condense into dust grains in the
diffuse ISM, but the compounds formed from this process are unknown. Here, we
use the laboratory cross sections from Kortright & Kim (2000) and Lee et al.
(2009) to model the absorption and scattering profiles of metallic Fe, and the
crystalline compounds fayalite (Fe$_2$SiO$_4$), ferrous sulfate (FeSO$_4$),
hematite ($\alpha$-Fe$_2$O$_3$), and lepidocrocite ($\gamma$-FeOOH), which have
oxidation states ranging from Fe$^{0}$ to Fe$^{3+}$. We find that the observed
Fe L-shell features are systematically offset in energy from the laboratory
measurements. An examination of over two dozen published measurements of Fe
L-shell absorption finds a 1-2 eV scatter in energy positions of the L-shell
features. Motivated by this, we fit for the best energy-scale shift
simultaneously with the fine structure of the Fe L-shell extinction cross
sections. Hematite and lepidocrocite provide the best fits ($\approx +1.1$ eV
shift), followed by fayalite ($\approx +1.8$ eV shift). However, fayalite is
disfavored, based on the implied abundances and knowledge of ISM silicates
gained by infrared astronomical observations and meteoritic studies. We
conclude that iron oxides in the Fe$^{3+}$ oxidation state are good candidates
for Fe-bearing dust. To verify this, new absolute photoabsorption measurements
are needed on an energy scale accurate to better than 0.2 eV.
|
2402.06726v1
|
2024-02-28
|
Elastocaloric evidence for a multicomponent superconductor stabilized within the nematic state in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$
|
The iron-based high-$T_c$ superconductors exhibit rich phase diagrams with
intertwined phases, including magnetism, nematicity and superconductivity. The
superconducting $T_c$ in many of these materials is maximized in the regime of
strong nematic fluctuations, making the role of nematicity in influencing the
superconductivity a topic of intense research. Here, we use the AC
elastocaloric effect (ECE) to map out the phase diagram of
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ near optimal doping. The ECE signature at $T_c$
on the overdoped side, where superconductivity condenses without any nematic
order, is quantitatively consistent with other thermodynamic probes that
indicate a single-component superconducting state. In contrast, on the slightly
underdoped side, where superconductivity condenses within the nematic phase,
ECE reveals a second thermodynamic transition proximate to and below $T_c$. We
rule out magnetism and re-entrant tetragonality as the origin of this
transition, and find that our observations strongly suggest a phase transition
into a multicomponent superconducting state. This implies the existence of a
sub-dominant pairing instability that competes strongly with the dominant
$s^\pm$ instability. Our results thus motivate a re-examination of the pairing
state and its interplay with nematicity in this extensively studied iron-based
superconductor, while also demonstrating the power of ECE in uncovering
strain-tuned phase diagrams of quantum materials.
|
2402.17945v1
|
2024-02-28
|
METAL-Z: Measuring dust depletion in low metalicity dwarf galaxies
|
The cycling of metals between interstellar gas and dust is a critical aspect
of the baryon cycle of galaxies, yet our understanding of this process is
limited. This study focuses on understanding dust depletion effects in the low
metallicity regime (< 20% Zo) typical of cosmic noon. Using medium-resolution
UV spectroscopy from the COS onboard the Hubble Space Telescope, gas-phase
abundances and depletions of iron and sulfur were derived toward 18 sightlines
in local dwarf galaxies IC 1613 and Sextans A. The results show that the
depletion of Fe and S is consistent with that found in the Milky Way, LMC and
SMC. The depletion level of Fe increases with gas column density, indicating
dust growth in the interstellar medium (ISM). The level of Fe depletion
decreases with decreasing metallicity, resulting in the fraction of iron in gas
ranging from 3% in the MW to 9% in IC 1613 and ~19% in Sextans A. The
dust-to-gas and dust-to-metal ratios (D/G, D/M) for these dwarf galaxies were
estimated based on the MW relations between the depletion of Fe and other
elements. The study finds that D/G decreases only slightly sub-linearly with
metallicity, with D/M decreasing from 0.41 +/- 0.05 in the MW to 0.11 +/- 0.11
at 0.10 Zo (at log N(H) = 21 cm-2). The trend of D/G vs. metallicity using
depletion in local systems is similar to that inferred in Damped Ly-alpha
systems from abundance ratios but lies higher than the trend inferred from FIR
measurements in nearby galaxies.
|
2402.18733v1
|
2024-03-07
|
Effects of mechanical stress, chemical potential, and coverage on hydrogen solubility during hydrogen enhanced decohesion of ferritic steel grain boundaries: A first-principles study
|
Hydrogen enhanced decohesion (HEDE) is one of the many mechanisms of hydrogen
embrittlement, a phenomenon which severely impacts structural materials such as
iron and iron alloys. Grain boundaries (GBs) play a critical role in this
mechanism, where they can provide trapping sites or act as hydrogen diffusion
pathways. The interaction of H with GBs and other crystallographic defects, and
thus the solubility and distribution of H in the microstructure, depends on the
concentration, chemical potential and local stress. Therefore, for a
quantitative assessment of HEDE, a generalized solution energy in conjunction
with the cohesive strength as a function of hydrogen coverage is needed. In
this work, we carry out density functional theory calculations to investigate
the influence of H on the decohesion of the $\Sigma$5(310)[001] and
$\Sigma$3(112)[1$\bar{1}$0] symmetrical tilt GBs in bcc Fe, as examples for
open and close-packed GB structures. A method to identify the segregation sites
at the GB plane is proposed. The results indicate that at higher local
concentrations, H leads to a significant reduction of the cohesive strength of
the GB planes, significantly more pronounced at the $\Sigma$5 than at the
$\Sigma$3 GB. Interestingly, at finite stress the $\Sigma$3 GB becomes more
favorable for H solution, as opposed to the case of zero stress, where the
$\Sigma$5 GB is more attractive. This suggests that under certain conditions
stresses in the microstructure can lead to a re-distribution of H to the
stronger grain boundary, which opens a new path to designing H-resistant
microstructures. To round up our study, we investigate the effects of typical
alloying elements in ferritic steel, C, V, Cr and Mn, on the solubility of H
and the strength of the GBs.
|
2403.04741v1
|
2024-03-20
|
A Gap in the Densities of Small Planets Orbiting M Dwarfs: Rigorous Statistical Confirmation Using the Open-source Code RhoPop
|
Using mass-radius-composition models, small planets ($\mathrm{R}\lesssim 2
\mathrm{R_\oplus}$) are typically classified into three types: iron-rich,
nominally Earth-like, and those with solid/liquid water and/or atmosphere.
These classes are generally expected to be variations within a compositional
continuum. Recently, however, Luque & Pall\'e observed that potentially
Earth-like planets around M dwarfs are separated from a lower-density
population by a density gap. Meanwhile, the results of Adibekyan et al. hint
that iron-rich planets around FGK stars are also a distinct population. It
therefore remains unclear whether small planets represent a continuum or
multiple distinct populations. Differentiating the nature of these populations
will help constrain potential formation mechanisms. We present the RhoPop
software for identifying small-planet populations. RhoPop employs mixture
models in a hierarchical framework and a nested sampler for parameter and
evidence estimates. Using RhoPop, we confirm the two populations of Luque &
Pall\'e with $>4\sigma$ significance. The intrinsic scatter in the Earth-like
subpopulation is roughly half that expected based on stellar abundance
variations in local FGK stars, perhaps implying M dwarfs have a smaller spread
in the major rock-building elements (Fe, Mg, Si) than FGK stars. We apply
RhoPop to the Adibekyan et al. sample and find no evidence of more than one
population. We estimate the sample size required to resolve a population of
planets with Mercury-like compositions from those with Earth-like compositions
for various mass-radius precisions. Only 16 planets are needed when
$\sigma_{M_p} = 5\%$ and $\sigma_{R_p} = 1\%$. At $\sigma_{M_p} = 10\%$ and
$\sigma_{R_p} = 2.5\%$, however, over 154 planets are needed, an order of
magnitude increase.
|
2403.13961v1
|
1993-04-06
|
Element Diffusion in the Solar Interior
|
We study the diffusion of helium and other heavy elements in the solar
interior by solving exactly the set of flow equations developed by Burgers for
a multi-component fluid, including the residual heat-flow terms. No
approximation is made concerning the relative concentrations and no restriction
is placed on the number of elements considered. We give improved diffusion
velocities for hydrogen, helium, oxygen and iron, in the analytic form derived
previously by Bahcall and Loeb. These expressions for the diffusion velocities
are simple to program in stellar evolution codes and are expected to be
accurate to $\sim 15\%$. Our complete treatment of element diffusion can be
directly incorporated in a standard stellar evolution code by means of an
exportable subroutine, but, for convenience, we also give simple analytical
fits to our numerical results.
|
9304005v1
|
1994-04-20
|
Analysis of the 3d6 4s(6D)4f-5g Supermultiplet of Fe I in Laboratory and Solar Infrared Spectra
|
The combined laboratory and solar analysis of the highly-excited
subconfigurations 4f and 5g of Fe I has allowed us to classify 87 lines of the
4f-5g supermultiplet in the spectral region 2545-2585 cm-1. The level structure
of these JK-coupled configurations is predicted by semiempirical calculations
and the quadrupolic approximation. Semiempirical gf-values have been calculated
and are compared to gf values derived from the solar spectrum. The solar
analysis has shown that these lines, which should be much less sensitive than
lower excitation lines to departures from LTE and to temperature uncertainties,
lead to a solar abundance of iron which is consistent with the meteoritic value
(A_Fe = 7.51).
|
9404050v1
|
1995-03-03
|
The First Second of a Type-II Supernova: Convection, Accretion, and Shock Propagation
|
One- and two-dimensional hydrodynamical simulations of neutrino-driven
supernova explosions are discussed. The simulations cover the phase between the
stagnation of the prompt shock and about one second after core bounce.
Systematic variation of the neutrino fluxes from the neutrino sphere shows that
the explosion energy, explosion time scale, initial mass of the protoneutron
star, and explosive nucleosynthesis of iron-group elements depend sensitively
on the strength of the neutrino heating during the first few 100 ms after shock
formation. Convective overturn in the neutrino-heated region behind the shock
is a crucial help for the explosion only in a narrow window of neutrino
luminosities. Here powerful explosions can be obtained only in the
multi-dimensional case. For higher core-neutrino fluxes also spherically
symmetrical models yield energetic explosions, while for lower luminosities
even with convection no strong explosions occur.
|
9503015v1
|
1995-04-03
|
COOLING FLOW MODELS OF THE X--RAY EMISSION AND TEMPERATURE PROFILES FOR A SAMPLE OF ELLIPTICAL GALAXIES
|
A simple spherically-symmetric, steady-state, cooling-flow description with
gas loss (following Sarazin \& Ashe 1989), within galaxy models constrained by
radially extended stellar dynamical data, is shown to provide generally
reasonable fits to the existing data on X-ray emission profiles and
temperatures for a set of bright elliptical galaxies in Virgo and Fornax. Three
free parameters are needed to specify the model: the external mass flux, the
external pressure, and a dimensionless factor, which regulates the mass
deposition rate along the flow. Three different assumptions on the supernova
rate have been considered. A moderate value for the supernova rate in
elliptical galaxies is found to be preferred. Confining pressures of
$p_{ext}\sim4\div15\times10^3\kelvin$ cm$^{-3}$ and significant accretion rates
of external material, up to $4\msolar/\yr$, are suggested by our models. A
possible correlation between $L_X/L_B$ and the iron abundance in the gas inside
ellipticals is pointed out.
|
9504002v2
|
1995-05-18
|
X-RAY OUTBURST OF THE PECULIAR SEYFERT GALAXY IC 3599
|
We report optical, soft X-ray, and UV observations of the peculiar Seyfert
galaxy IC 3599 using data obtained with ROSAT and IUE. Most remarkably, we
discovered a rapid decrease of the X-ray flux by a factor of about 100 within
one year and a more gradual decrease thereafter. The X-ray spectrum of IC 3599
was soft at flux maximum and became even softer as the flux decreased.
Simultaneously with the late decrease of the X-ray flux, we observed a decrease
in the strength of highly ionized optical iron lines. We discuss several
explanations for this behaviour including an accretion disk instability and
tidal disruption of a star orbiting a central massive black hole.
|
9505085v1
|
1995-10-19
|
Model Atmospheres for Low Field Neutron Stars
|
We compute model atmospheres and emergent spectra for low field (B<10^10 G)
neutron stars, using new opacity and equation of state data from the OPAL
project. These computations, incorporating improved treatments of flux
transport and convective stability, provide spectra for hydrogen, solar
abundance and iron atmospheres. We compare our results to high field magnetic
atmospheres, available only for hydrogen. An application to apparently thermal
flux from the low field millisecond pulsar PSR J0437--4715 shows that H
atmospheres fit substantially better than Fe models. We comment on extension to
high fields and the implication of these results for neutron star luminosities
and radii.
|
9510094v2
|
1995-11-22
|
Metal Enhancements in the X-ray Gas around Central Cluster Galaxies
|
The X-ray emission by hot gas around the central galaxies of galaxy clusters
is commonly modeled assuming the existence of steady-state, multiphase cooling
flows. The inflowing gas will be chemically enriched by type Ia supernovae and
stellar mass loss occurring in the outer parts of the central galaxy. This may
give rise to a substantial metallicity enhancement towards the center, whose
amplitude is proportional to the ratio of the central galaxy luminosity to the
mass inflow rate. The metallicity of the hotter phases is expected to be higher
than that of the colder, denser phases. The metallicity profile expected for
the Centaurus cluster is in good agreement with the iron abundance gradient
recently inferred from ASCA measurements (Fukazawa et al. 1994). However,
current data do not rule out alternative models where cooling is balanced by
some heat source. In either case, the enhancement expected from injection by
type Ia supernovae is roughly as observed. Most of this work is described in
more detail in Reisenegger, Miralda-Escud\'e, \& Waxman (1996;
astro-ph/9511044).
|
9511105v1
|
1995-12-12
|
The first simultaneous X-ray/gamma-ray observations of Cyg X-1 by Ginga and OSSE
|
We present the results of 4 simultaneous observations of Cygnus X-1 by Ginga
and OSSE. The X-ray/gamma-ray spectra can be described by an intrinsic
continuum and a component due to Compton reflection including an iron K-alpha
line. The intrinsic spectrum at X-ray energies is a power-law with a photon
spectral index of Gamma=1.6. The intrinsic gamma-ray spectrum can be
phenomenologically described by either a power-law without cutoff up to 150
keV, and an exponential cutoff above that energy, or by an exponentially cutoff
power law and a second hard component.
|
9512073v1
|
1996-05-10
|
The Compton Mirror in NGC 4151
|
We show that the sharp cutoff in the hard X-ray spectrum of NGC 4151, unusual
for Seyfert 1 galaxies, can be reconciled with the average Seyfert 1 spectrum
if we assume that the central source is completely hidden from our line of
sight by the thick part of the accretion disk or by the broad emission line
clouds. The observed X-ray radiation is produced by scattering of the Seyfert
1-type spectrum in the higher, cooler parts of the accretion disk corona, or in
a wind. A sharp cutoff appears as a result of the Compton recoil effect. This
model naturally explains a discrepancy regarding the inclination of the central
source, inferred to be low (face-on) from observations of the iron $K\alpha$
emission line, but inferred to be high on the basis of optical and UV
observations.
|
9605052v1
|
1996-07-08
|
Physical Conditions in Low Ionization Regions of the Orion Nebula
|
We reexamine the spectroscopic underpinnings of recent suggestions that [O I]
and [Fe II] lines from the Orion H II region are produced in gas where the
iron-carrying grains have been destroyed and the electron density is
surprisingly high. Our new observations show that previous detections of [O I]
5577 were dominated by telluric emission. Our limits are consistent with a
moderate density (10^4 cm^{-3}) photoionized gas. We show that a previously
proposed model of the Orion H II region reproduces the observed [O I] and [Fe
II] spectrum. These lines are fully consistent with formation in a dusty region
of moderate density.
|
9607045v1
|
1996-08-19
|
Scattering of X-ray emission lines by the neutral and molecular hydrogen in the Sun's atmosphere and in the vicinity of active galactic nuclei and compact sources
|
In many astrophysical objects an account for scattering of the X-ray
fluorescent and resonance lines of iron ions by the electrons, bound in atomic
and molecular hydrogen is important. Specific distortions of the scattered line
profile spectrum appear in comparison with scattering by free electrons.
Analysis of scattered line spectra may provide information on the ionization
state, helium abundance and geometry of scattering media in the vicinity of
AGNs, compact X-ray sources and on the surface of X-ray flaring stars and the
Sun.
|
9608114v1
|
1996-10-09
|
Galaxy evolution: the effect of dark matter on the chemical evolution of ellipticals and galaxy clusters
|
In this paper we discuss the chemical evolution of elliptical galaxies and
its consequences on the evolution of the intracluster medium (ICM). We use
chemical evolution models taking into account dark matter halos and compare the
results with previous models where dark matter was not considered. In
particular, we examine the evolution of the abundances of some relevant heavy
elements such as oxygen, magnesium and iron and conclude that models including
dark matter halos and an initial mass function (IMF) containing more massive
stars than the Salpeter (1955) IMF, better reproduce the observed abundances of
Mg and Fe both in the stellar populations and in the ICM (ASCA results). We
also discuss the origin of gas in galaxy clusters and conclude that most of it
should have a primordial origin.
|
9610059v1
|
1996-10-11
|
Spallation of Iron in Black Hole Accretion Flows
|
In the local Galactic interstellar medium there is approximate energy
equipartition between cosmic rays, magnetic fields and radiation. If this holds
in the central regions of AGN, in particular Seyfert galaxies, then consideral
nuclear spallation of Fe occurs, resulting in enhanced abundances of the sub-Fe
elements Ti, V, Cr and Mn. These elements produce a cluster of X-ray
flourescence lines at energies just below the 6.4 keV Fe-K$\alpha$ line. It is
suggested that the red wings on the Fe lines observed with ASCA from various
Seyfert AGN are due to the unresolved line emission from these elements. Future
observations with more sensitive X-ray instruments should resolve these lines.
The estimated gamma ray emission from nuclear deexcitation and neutral pion
production is calculated and found to be below the sensitivities of any current
instruments. However, very luminous nearby Seyferts displaying Fe lines with
red wings would have $>100$ MeV continuum emission detectable by future
instruments such as GLAST.
|
9610081v1
|
1996-10-29
|
Comment on a paper by M. A. Bautista ``Atomic data from the Iron Project. XVI. Photoionization cross sections and oscillator strengths for Fe V''
|
Recently, Bautista (1996, A&AS, 119, 105) reported new calculations of
photoionization cross sections for Fe V, and compared them with the earlier
results of Reilman & Manson (1979, ApJS, 40, 815) and Verner et al. (1993,
Atomic Data Nucl. Data Tables, 55, 233). Bautista claimed that beyond 10 Ry the
new cross sections ``converge well toward the results by Reilman and Manson'',
whereas ``the calculations by Verner et al. still underestimate the cross
section by almost factor of two''. We show that Bautista erroneously compared
the total (summed over shells) cross sections from Reilman & Manson with the
partial 3d-shell cross sections from Verner et al. Actually, the total cross
sections from Verner et al. and Reilman & Manson agree within 3% at all
energies.
|
9610244v1
|
1996-11-13
|
Galactic Drips and How to Stop Them!
|
The temperature of hot interstellar gas at large radii in elliptical galaxies
can be lower than the mean galactic virial temperature. If so, a nonlinear
cooling wave can form in the hot interstellar gas and propagate slowly toward
the galactic core. If the cooling wave survives hydrodynamic instabilities, it
can intermittently deposit cold gas within about 15 effective radii. For a
bright elliptical the total mass deposited in this manner can approach 10^10
solar masses. The cold gas that drips out at large galactic radii may account
for the young stellar populations and extended gas at $\sim 10^4$ K observed in
many ellipticals, features that are often attributed to galactic mergers.
Galactic drips are expected in relatively isolated (field) ellipticals provided
(i) the galactic stellar velocity ellipsoids are radially oriented at large
galactic radii and (ii) the current Type Ia supernova rate is sufficiently
small to be consistent with interstellar iron abundances found in recent X-ray
studies. Galactic drips are surpressed in ellipticals located within clusters
of galaxies; when the pressure in the ambient cluster gas exceeds that in the
outer parts of the galactic interstellar medium, some cluster gas flows into
the galaxy which surpresses the drips.
|
9611096v1
|
1996-11-29
|
Ultraviolet Light from Old Stellar Populations
|
We consider the general theoretical problem of ultraviolet light from old
stellar populations (t > 2 Gyr), and the interpretation of galaxy spectra at
short wavelengths (lamda < 3200A) The sources believed to be responsible for
the observed `ultraviolet upturn phenomenon' (UVX) are Post-AGB (P-AGB),
extreme horizontal branch (EHB) and AGB-Manque (AGBM) stars. The production of
EHB stars depends on mass loss on the red giant branch, which is poorly
understood, making the far-UV flux problematical as an indicator of the gross
properties, such as the age, of a given galaxy. We discuss the current state of
the comparison between theory and observation for the UVX phenomenon, and
revisit the interpretation of the well-known UVX-Mg2 correlation. In
particular, we draw attention to the fact that the UVX appears not to be
correlated with indices that measure the iron abundance, which has implications
for models that explain the UV-Mg2 correlation as an abundance-driven effect.
Finally, we note the potential utility of ultraviolet observations in
distinguishing age and metallicity from galaxy spectral energy distributions.
|
9612008v1
|
1996-12-19
|
The Great Annihilator 1E1740.7-2942: Molecular cloud connection and coronal structure
|
Using 12CO and 13CO observations we present column density maps of the
molecular cloud (V = -135 km/s) in the direction of 1E1740.7-2942. Hydrogen
column densities of the cloud scatter between (3.5 - 11) 10**22 cm-2, depending
on the method used. From this we conclude, deriving first a simple analytic
formula, that despite of the weakness of the iron fluorescent 6.4 keV line
(Churazov et al. 1996), the source may lie inside the cloud, or at least close
to its edge. The combined ASCA/BATSE spectrum from September 1993 and 1994 can
be modelled with a two-phase accretion disc corona model, where the hot region
is detached from the cold disc. Geometrically, the hot phase can be interpreted
e.g. as a number of active regions (magnetic loops) above the disc, or as a
spherical hot cloud around the central object.
|
9612194v1
|
1997-01-08
|
The ASCA X-ray spectrum of the powerful radio galaxy 3C109
|
We report the results from an ASCA X-ray observation of the powerful Broad
Line Radio Galaxy, 3C109. The ASCA spectra confirm our earlier ROSAT detection
of intrinsic X-ray absorption associated with the source. The absorbing
material obscures a central engine of quasar-like luminosity. The luminosity is
variable, having dropped by a factor of two since the ROSAT observations 4
years before. The ASCA data also provide evidence for a broad iron emission
line from the source, with an intrinsic FWHM of ~ 120,000 km/s. Interpreting
the line as fluorescent emission from the inner parts of an accretion disk, we
can constrain the inclination of the disk to be $> 35$ degree, and the inner
radius of the disk to be $< 70$ Schwarzschild radii. Our results support
unified schemes for active galaxies, and demonstrate a remarkable similarity
between the X-ray properties of this powerful radio source, and those of lower
luminosity, Seyfert 1 galaxies.
|
9701033v1
|
1997-01-22
|
General relativistic effects in the neutrino-driven wind and r-process nucleosynthesis
|
We discuss general relativistic effects in the steady-state neutrino-driven
``wind'' which may arise from nascent neutron stars. In particular, we
generalize previous analytic estimates of the entropy per baryon $S$, the mass
outflow rate $\dot M$, and the dynamical expansion time scale $\tau_{dyn}$. We
show that $S$ increases and $\tau_{dyn}$ decreases with increasing values of
the mass-to-radius ratio describing the supernova core. Both of these trends
indicate that a more compact core will lead to a higher number of neutrons per
iron peak seed nucleus. Such an enhancement in the neutron-to-seed ratio may be
required for successful r-process nucleosynthesis in neutrino-heated supernova
ejecta.
|
9701178v1
|
1997-04-18
|
Gamma rays and neutrinos from the Crab Nebula produced by pulsar accelerated nuclei
|
We investigate the consequences of the acceleration of heavy nuclei (e.g.
iron nuclei) by the Crab pulsar. Accelerated nuclei can photodisintegrate in
collisions with soft photons produced in the pulsar's outer gap, injecting
energetic neutrons which decay either inside or outside the Crab Nebula. The
protons from neutron decay inside the nebula are trapped by the Crab Nebula
magnetic field, and accumulate inside the nebula producing gamma-rays and
neutrinos in collisions with the matter in the nebula. Neutrons decaying
outside the Crab Nebula contribute to the Galactic cosmic rays. We compute the
expected fluxes of gamma-rays and neutrinos, and find that our model could
account for the observed emission at high energies and may be tested by
searching for high energy neutrinos with future neutrino telescopes currently
in the design stage.
|
9704186v1
|
1997-05-08
|
Spectropolarimetry of FIRST 0840+3633
|
We present Keck spectropolarimetry of a rare ``Iron Lo-BALQSO,'' FIRST
0840+3633. The continuum is 4% polarized near 2000 angstroms rest-frame, but
falls to 2% at longer wavelengths, and maintains a relatively constant position
angle of 50 degrees. The emission lines are unpolarized. The polarization
increases up to 8% in the low-ionization absorption troughs of Mg II 2800 and
Al III 1860. The polarization and its position angle vary in a complicated
manner across the metastable Fe II absorption lines, suggesting that more than
one mechanism is at work or that the system geometry is complex.
|
9705053v1
|
1997-05-12
|
Physical Conditions of the Coronal Line Region in Seyfert Galaxies
|
The launch of the Infrared Space Observatory and new atomic data have opened
a window to the study of high ionization gas in active galactic nuclei (AGN).
We present the results of a large number of photoionization simulations of the
``coronal line'' region in AGN, employing new atomic data from the Opacity and
Iron Projects. Our grid of line emission spans 8 orders of magnitude in gas
density and 14 orders of magnitude in ionizing flux in an effort to identify
the optimal conditions in which these lines form. We show that coronal lines
form at distances from just outside the broad line region to ~400L_{43.5}^{1/2}
pc, in gas with ionization parameter -2.0 < log U(H) < 0.75, corresponding to
gas densities of 10$^2$ to 10$^{8.5}$ cm$^{-3}$, with electron temperatures
~12,000K -- 150,000K. A large range of distances from the central source
implies significant line width variation among the coronal lines. We identify
several line ratios which could be used to measure relative abundances, and we
use these to show that the coronal line gas is likely to be dust free.
|
9705082v1
|
1997-06-02
|
Decoupled Nuclei and Nuclear Polar Rings in Regular Spiral Galaxies. NGC 2841
|
The Sb galaxy NGC 2841 was observed at the 6 m telescope of SAO RAS with the
Multi-Pupil Field Spectrograph and at the 1 m telescope of SAO RAS with the
long-slit spectrograph. An unresolved nucleus of NGC 2841 is shown to be
chemically decoupled both in magnesium and in iron with abundance break
estimates of 0.36 dex for Mg and 0.6 dex for Fe; an abundance gradient in the
bulge is seen only in the magnesium index and is typical for early-type disk
galaxies. The rotation axis of the nuclear ionized gas in NGC 2841 is
orthogonal to that of the central stellar population; an existence of a bulge
stellar component with decoupled rotation momentum in the radius range 5" -12"
is suspected. A possible scenario for the origin of the unusual central
structure in NGC 2841 is proposed.
|
9706003v1
|
1997-06-07
|
X-ray detections of weak Seyfert2's with BeppoSAX
|
We report the detection in the X rays of two weak Seyfert2's (NGC3393 and
NGC4941) with the Italian-Dutch satellite BeppoSAX. These are among the first
sources observed in a sample of 12 Seyfert2's which are being studied within
the BeppoSAX core program, in an effort to probe the putative torus at high
X-ray energies, calibrate an isotropic luminosity indicator for absorbed
nuclei, and determine the distribution of torus thicknesses, Nh. Both a Compton
thick spectrum, with a reflected power law and a large equivalent width iron
line, and a Compton thin spectrum, with the intrinsic power law transmitted
through a large column density absorber, provide acceptable fits to both
sources, with some preference for the latter model in the case of NGC4941. The
high initial detection rate in our program points to a large final X-ray
sample.
|
9706070v1
|
1997-07-08
|
Detailed NLTE Model Atmospheres for Novae during Outburst: I. New Theoretical Results
|
We present new detailed NLTE calculations for model atmospheres of novae
during outburst. This fully self-consistent NLTE treatment for a number of
model atoms includes 3922 NLTE levels and 47061 NLTE primary transitions. We
discuss the implication of departures from LTE for the strengths of the lines
in nova spectra. The new results show that our large set of NLTE lines
constitute the majority of the total line blanketing opacity in nova
atmospheres. Although we include LTE background lines, their effect are small
on the model structures and on the synthetic spectra. We demonstrate that the
assumption of LTE leads to incorrect synthetic spectra and that NLTE
calculations are required for reliably modeling nova spectra. In addition, we
show that detailed NLTE treatment for a number of ionization stages of iron
changes the results of previous calculations and improve the fit to observed
nova spectra.
|
9707104v1
|
1997-07-22
|
Spectropolarimetry of FIRST BAL QSOs
|
We present Keck spectropolarimetry of two rare low-ionization broad
absorption line (BAL) QSOs, FIRST J084044.5+363328 and FIRST J155633.8+351758,
that also exhibit narrow absorption lines from metastable excited levels of Fe
II (``Iron Lo-BALs''). These QSOs were discovered in optical follow-ups to a
deep radio survey; FIRST J155633.8+351758 is radio-loud, the first BAL QSO so
identified. FIRST J084044.5+363328 is highly polarized and exhibits many
features found in other BAL QSOs, and the polarization and its position angle
vary in a complicated manner across the metastable Fe II absorption lines,
suggesting that more than one mechanism is at work, or that the system geometry
is complex. FIRST J155633.8+351758 may be the most highly polarized BAL QSO
known, and exhibits other unusual polarization properties compared to other
highly polarized BAL QSOs. Currently available data cannot yet discriminate
among the possible lines of sight to BAL QSOs (edge-on, pole-on, or random).
|
9707238v1
|
1997-07-22
|
A Second UV "Light Bulb" behind the SN 1006 Remnant
|
A point X-ray source located 9 arcmin northeast of the center of SN~1006 has
been spectroscopically identified as a background QSO, with a redshift of
0.335. The object is moderately bright, with magnitude V=18.3. If its
ultraviolet spectrum is typical of low-z quasars, this object will be a second
(after the Schweizer-Middleditch star) source to use for absorption
spectroscopy of material within SN 1006. Absorption spectra provide a unique
probe for unshocked ejecta within this supernova remnant, and can possibly
solve the long-standing problem of "missing" iron in the remnants of Type Ia
supernovae.
|
9707247v1
|
1997-08-25
|
ASCA observations of type-2 Seyfert Galaxies. III. Orientation and X-ray Absorption
|
We discuss the spectral properties of a sample of type-2 Seyfert galaxies
based upon the analysis of \asca data. In this paper we consider the sources
for which the X-ray spectra appear to be dominated by the nuclear continuum,
transmitted through a large column of absorbing material. We find that both
Seyfert-2 galaxies and NELGs show iron K$\alpha$ line profiles indicative of
reprocessing of nuclear X-rays in a face-on accretion disk. Such line profiles
are also observed in Seyfert-1 galaxies. This result is contrary to unification
models, which would predict the inner regions of Seyfert-2 galaxies to be
observed edge-on. This raises some questions as to the orientation of the
circumnuclear absorber. If the observed differences between Seyfert type-1 and
type-2 galaxies, and NELGs are not due to differences in the orientation of the
absorbing material, then we suggest that differences in dust composition and
grain size, and in the density of the circumnuclear gas could be of primary
importance.
|
9708231v1
|
1997-09-02
|
New Measurement of Metal Abundance in the Elliptical Galaxy NGC 4636 with Asca
|
High quality X-ray spectra of NGC 4636 are obtained with ASCA. Theoretical
models are found unable to reproduce the data in the Fe-L line region. Spectral
data above 1.4 keV indicate that Mg to Si abundance ratio is $\sim 1$ solar.
Assuming that the abundance ratios among $\alpha$-elements are the same with
the solar ratios, spectral fit with increased systematic error in the 0.4--1.6
keV range gives abundances of $\alpha$-elements and Fe to be both $\sim$ 1
solar by taking one solar of Fe to be 3.24$\times 10^{-5}$ by number. These new
abundance results solve discrepancy between stellar and hot-gas metallicity,
but still a low supernova rate is implied. We also detect strong abundance
gradients for both $\alpha$-elements and iron in a similar fashion. The
abundance is $\sim 1$ solar within $4'$, and decreases outerwards down to
$0.2\sim 0.3$ solar at $10'$ from the galaxy center. Dilution due to an
extended hot gas is suggested.
|
9709009v1
|
1997-09-22
|
Discovery of a Large-Scale Abundance Gradient in the Cluster of Galaxies AWM7 with ASCA
|
A large-scale gradient in the metal abundance has been detected with ASCA
from an X-ray bright cluster of galaxies AWM7. The metal abundance shows a peak
of 0.5 solar at the center and smoothly declines to <~ 0.2 solar at a radius of
500 kpc. The gas temperature is found to be constant at 3.8 keV. The radial
distribution of iron can be fit with a beta-model with beta ~ 0.8 assuming the
same core radius (115 kpc) as that of the intracluster medium. The metal
distribution in AWM7 suggests that the gas injected from galaxies is not
efficiently mixed in the cluster space and traces the distribution of galaxies.
|
9709208v1
|
1997-10-17
|
The IntraCluster Medium: An Invariant Stellar IMF
|
Evidence supporting the hypothesis of an invariant stellar Initial Mass
Function is strong and varied. The intra-cluster medium in rich clusters of
galaxies is one of the few contrary locations where recent interpretations of
the chemical abundances have favoured an IMF that is biased towards massive
stars, compared to the `normal' IMF. This interpretation hinges upon the
neglect of Type Ia supernovae to the ICM enrichment, and a particular choice of
the nucleosynthesis yields of Type II supernovae. We demonstrate here that when
one adopts yields determined empirically from observations of Galactic stars,
rather than the uncertain model yields, a self-consistent picture may be
obtained with an invariant stellar IMF, and about half of the iron in the ICM
being produced by Type Ia supernovae.
|
9710195v1
|
1997-11-14
|
Cygnus X-1: X-ray Emission Mechanism and Geometry
|
We summarize recent observations of Cyg X-1 during its 1996 state transition.
The results favor an emission geometry with a central optically thin corona
surrounded by an optically thick, geometrically thin accretion disk. As the
soft state is approached, the inner edge of the disk moves closer to the black
hole, and the corona shrinks, perhaps due to more efficient cooling. Soft
X-rays are from the innermost disk, and inverse-Comptionization by the corona
appears to be responsible for the hard X-ray emission. Both Compton reflection
and an iron emission line have been observed in both states. They are likely to
originate in the inner region of the disk. Also briefly discussed is the
theoretical effort to simultaneously model the observed spectral and temporal
X-ray properties of Cyg X-1.
|
9711166v1
|
1997-11-19
|
Line emission from an accretion disk around black hole: effects of the disk structure
|
The observed iron K-alpha fluorescence lines in Seyfert galaxies provide
strong evidence for an accretion disk near a supermassive black hole as a
source of the line emission. Previous studies of line emission have considered
only geometrically thin disks, where the gas moves along geodesics in the
equatorial plane of a black hole. Here we extend this work to include effects
on line profiles from finite disk thickness, radial accretion flow and
turbulence. We adopt the Novikov-Thorne solution, and find that within this
framework, turbulent broadening is the most significant effect. The most
prominent changes in the skewed, double-horned line profiles is a substantial
reduction in the maximum flux at both red and blue peaks. We show that at the
present level of signal-to-noise in X-ray spectra, proper treatment of the
actual structure of the accretion disk can change estimates of the inclination
angle of the disk. Thus these effects will be important for future detailed
modeling of high quality observational data.
|
9711214v1
|
1997-11-20
|
High Energy Break and Reflection Features in the Seyfert Galaxy MCG+8-11-11
|
We present the results from ASCA and OSSE simultaneous observations of the
Seyfert 1.5 galaxy MCG+8-11-11 performed in August-September 1995. The ASCA
observations indicate a modest flux increase (20%) in 3 days, possibly
correlated to a softening of the 0.6-9 keV spectrum. The spectrum is well
described by a hard power law (Gamma=1.64) absorbed by a column density
slightly larger than the Galactic value, with an iron line at 6.4 keV of EW=400
eV. The simultaneous OSSE data are characterized by a much softer power law
with photon index Gamma=3.0, strongly suggesting the presence of a spectral
break in the hard X/soft gamma-ray band. A joint fit to OSSE and ASCA data
clearly shows an exponential cut-off at about 300 keV, and strong reflection
component. MCG+8-11-11 features a spectral break in the underlying continuum
unambiguously. This, together with the inferred low compactness of this source,
favours thermal or quasi-thermal electron Comptonization in a structured Corona
as the leading process of high energy radiation production.
|
9711242v1
|
1997-11-29
|
Wide-Band X-Ray Spectra and Images of the Starburst Galaxy M82
|
The ASCA results of the starburst galaxy M82 are presented. The X-rays in the
0.5--10 keV band exhibit a thin thermal spectrum with emission lines from
highly ionized magnesium, silicon, and sulfur, as well as a hard tail extending
to higher than 10keV energy. The soft X-rays are spatially extended, while the
hard X-rays show an unresolved point-like structure with possible a long-term
flux variability. The flux ratio of the emission lines and the spatially
extended structure in the low-energy band indicate that at least
two-temperature thin thermal plasmas are present. The abundances of the oxygen,
neon, magnesium, silicon, sulfur, and iron in the thin thermal plasmas are
found to be significantly lower than the cosmic value. Neither type-Ia nor
type-II supernova explosions can reproduce the observed abundance ratio. The
origin of the unresolved hard X-rays is uncertain, but is probably an obscured
low-luminosity AGN.
|
9712002v1
|
1997-12-17
|
BeppoSAX observations of 3C 273
|
We present preliminary results of BeppoSAX AO1 observations of 3C 273
performed in January 1997. We also present a close comparison with data
obtained during the satellite SVP, in July 1996. On average, the AO1 flux is
about a factor 2 higher than the flux detected during the SVP, and roughly on
the middle of the historical X-ray flux range. Power law fits with galactic
absorption to all observations yield spectral indices in the range 1.53-1.6,
with the spectrum extending from 0.2 to at least up to 200 keV without any
significant slope change. The broad band spectrum appears basically
featureless, marking a clear difference from the SVP data, where an absorption
feature at low energy and a fluorescence iron emission line are present. The
lack of cold/warm matter signatures in our data may indicate that, at this
"high" level of luminosity, the featureless continuum produced in a
relativistic jet overwhelms any thermal and/or reprocessing component, while
the two components were at least comparable during the "low" state of July
1996.
|
9712234v1
|
1997-12-18
|
1H0419-577: a two-state Seyfert Galaxy?
|
The preliminary results of the BeppoSAX observation of the radio-quiet AGN
1H0419-577 are presented. Despite its broad line optical spectrum, the
intermediate X-ray spectrum (i.e. 2--10 keV) is flatter than typically observed
in Seyfert 1s and no iron line is significantly detected. Even more
intriguingly, a 1992 ROSAT pointed observation suggests a dramatic (~ 1) change
in the spectral shape for E < 2 keV. Such behavior is briefly discussed in the
framework of our current understanding of Comptonization scenarios in the
nuclear regions of radio-quiet AGN.
|
9712248v1
|
1998-01-27
|
A Comparison of Cosmic Ray Composition Measurements at the Highest Energies
|
In recent years the Fly's Eye and Akeno groups have presented analyses of the
cosmic ray mass composition at energies above 10^17 eV. While the analysis of
the Fly's Eye group points to a likely change in mass composition from heavy to
light at energies above 10^18 eV, the Akeno analysis favours an unchanging
composition. However, the two groups base their conclusions on simulations
using quite different hadronic models. Here we present a comparison of the
experiments using the same hadronic model and find that the agreement between
the experiments is much improved. Under this model, both experiments measure a
composition rich in iron around 10^17 eV which becomes lighter at higher
energies. However, the agreement is not complete, which indicates scope for
improvement of the interaction model, or perhaps the need for a re-examination
of the experimental results.
|
9801260v2
|
1998-02-08
|
Can Stellar Yields Accurately Constrain the Upper Limit to the Initial Mass Function?
|
Recent determinations of the upper mass limit to the local initial mass
function (IMF) claim a value of m(upper)=50+/-10 solar masses, based upon
direct comparisons of the observed oxygen and iron abundances in metal-poor
stars with the predicted stellar yields from Type II supernovae (SNe). An
unappreciated uncertainty in these analyses is the input physics intrinsic to
each SNe grid, and its effect upon stellar nucleosynthesis. We demonstrate how
such uncertainties, coupled with the uncertain metal-poor halo star
normalization, while allowing us to set a lower bound to m(upper) of
approximately 40 solar masses, nullifies any attempt at constraining the upper
bound.
|
9802091v1
|
1998-02-11
|
ASCA Measurements of Silicon and Iron Abundances in the Intracluster Medium
|
We analyzed the ASCA X-ray data of 40 nearby clusters of galaxies, whose
intracluster-medium temperature distributes in the range of 0.9--10 keV. We
measured the Si and Fe abundances of the intracluster medium, spatially
averaging over each cluster, but excluding the central $\sim 0.15 h_{50}^{-1}$
Mpc region in order to avoid any possible abundance gradients and complex
temperature structures. The Fe abundances of these clusters are 0.2--0.3 solar,
with only weak dependence on the temperature of the intracluster medium, hence
on the cluster richness. In contrast, the Si abundance is observed to increase
from 0.3 to 0.6--0.7 solar from the poorer to richer clusters. These results
suggest that the supernovae of both type-Ia and type-II significantly
contribute to the metal enrichment of the intracluster medium, with the
relative contribution of type-II supernovae increasing towards richer clusters.
We suggest a possibility that a considerable fraction of type-II supernova
products escaped from poorer systems.
|
9802126v1
|
1998-02-13
|
Intracluster Medium Abundances Revisited
|
We examine the origin of heavy elements in the intracluster medium (ICM) of
galaxy clusters, concentrating upon the roles played by supernovae (SNe) Types
Ia and II. The most accurately determined elemental abundances, Si and Fe,
imply a mild predominance of Type II SNe as the source of ICM Fe, contributing
approximately 60-80% of its total (and approximately 100% of the
alpha-elements). (Currently) intractable uncertainties in measuring X-ray
alpha-element ICM abundances, the initial mass function (IMF), and stellar
evolution ingredients, make a more precise determination of the Ia:II ICM iron
``ratio'' impossible.
|
9802172v1
|
1998-02-26
|
ASCA spectroscopy of the luminous infrared galaxy NGC6240: X-ray emission from a starburst and a buried active nucleus
|
We present an X-ray spectral study of the prototype far-infrared galaxy
NGC6240 from ASCA. The soft X-ray spectrum (below 2 keV) shows clear signatures
of thermal emission well described with a multi-temperature optically-thin
plasma, which probably originates in a powerful starburst. Strong hard X-ray
emission is also detected with ASCA and its spectrum above 3 keV is extremely
flat with a prominent iron K line complex, very similar to that seen in the
Seyfert 2 galaxy NGC1068 but about an order of magnitude more luminous
[L(3-10keV)=1.4E42 erg/s]. The hard X-ray spectrum indicates that only
reflected X-rays of an active galactic nucleus (AGN) buried in a heavy
obscuration [N(H)>2E24 cm-2] are visible. This is evidence for an AGN in
NGC6240 emitting possibly at a quasar luminosity and suggests its significant
contribution to the far-infrared luminosity.
|
9802336v1
|
1998-03-31
|
IRAS 06562-0337, The Iron Clad Nebula: A New Young Star Cluster
|
IRAS 06562-0337 has been the recent subject of a classic debate:
proto-planetary nebula or young stellar object? We present the first 2 micron
image of IRAS 06562-0337, which reveals an extended diffuse nebula containing
approximately 70 stars inside a 30 arcsec radius around a bright, possibly
resolved, central object. The derived stellar luminosity function is consistent
with that expected from a single coeval population, and the brightness of the
nebulosity is consistent with the predicted flux of unresolved low-mass stars.
The stars and nebulosity are spatially coincident with strong CO line emission.
We therefore identify IRAS 06562-0337 as a new young star cluster embedded in
its placental molecular cloud. The central object is likely a Herbig Be star, M
approx 20 M_sun, which may be seen in reflection. We present medium resolution,
high S/N, 1997 epoch optical spectra of the central object. Comparison with
previously published spectra shows new evidence for time variable permitted and
forbidden line emission, including SiII, FeII, [FeII], and [OI]. We suggest the
origin is a dynamic stellar wind in the extended, stratified atmosphere of the
massive central star in IRAS 06562-0337.
|
9803345v1
|
1998-05-06
|
Doppler Imaging of Ap Stars
|
Doppler imaging, a technique which inverts spectral line profile variations
of an Ap star into a two-dimensional abundance maps, provides new observational
constraints on diffusion mechanism in the presence of a global magnetic field.
A programme is presented here with the aim to obtain abundance distributions of
at least five elements on each star, in order to study how different diffusion
processes act under influence of a stellar magnetic field. The importance of
this multi-element approach is demonstrated, by presenting the abundance maps
of helium, magnesium, silicon, chromium and iron for the magnetic B9pSi star CU
Virginis.
|
9805065v1
|
1998-05-15
|
X-ray spectroscopy of the broad line radio galaxy 3C111
|
We present an ASCA observation of the broad line radio galaxy 3C111. The
X-ray spectrum is well described by a model consisting of a
photoelectrically-absorbed power-law form. The inferred absorbing column
density is significantly greater than expected on the basis of 21-cm
measurements of Galactic HI. Whilst this may be due intrinsic absorption from a
circumnuclear torus or highly warped accretion disk, inhomogeneities and
molecular gas within the foreground giant molecular cloud may also be
responsible for some of this excess absorption. We also claim a marginal
detection of a broad iron Ka line which is well explained as being a
fluorescent line originating from the central regions of a
radiatively-efficient accretion disk. This line appears weak in comparison to
those found in (radio-quiet) Seyfert nuclei. We briefly discuss the
implications of this fact.
|
9805217v1
|
1998-05-26
|
The stellar population of the decoupled nucleus in M 31
|
The results of a spectroscopic and photometric investigation of the central
region of M 31 are presented. An analysis of absorption-index radial profiles
involving magnesium, calcium, and iron lines has shown that the unresolved
nucleus of M 31 is distinct by its increased metallicity; unexpectedly, among
two nuclei of M 31, it is the faintest one located exactly in the dynamical
center of the galaxy (and dynamically decoupled) which is chemically distinct.
The Balmer absorption line H-beta has been included into the analysis to
disentangle metallicity and age effects; an age difference by a factor 3 is
detected between stellar populations of the nucleus and of the bulge, the
nucleus being younger. The morphological analysis of CCD images has revealed
the presence of a nuclear stellar-gaseous disk with a radius of some 100 pc,
the gas component of which looks non-stationary, well inside the bulge of M 31.
|
9805318v1
|
1998-05-29
|
Quasi-periodic oscillations discovered in the new X-ray pulsar XTE J1858+034
|
We report the discovery of low frequency quasi-periodic oscillations centered
at 0.11 Hz in the newly discovered 221 s X-ray pulsar XTE J1858+034. Among
about 30 known transient X-ray pulsars this is the sixth source in which QPOs
have been observed. If the QPOs are produced because of inhomogeneities in the
accretion disk at the magnetospheric boundary, the low frequency of the QPOs
indicate a large magnetosphere for this pulsar. Both the Keplerian frequency
model and the beat frequency model are applicable for production of QPOs in
this source. The QPOs and regular pulsations are found to be stronger at higher
energy which favours the beat frequency model. The magnetic field of the pulsar
is calculated as a function of its distance. The energy spectrum is found to be
very hard, consisting of two components, a cut-off power law and an iron
fluorescence line.
|
9805366v1
|
1998-06-03
|
Heavy obscuration in X-ray weak AGNs
|
We present observations in the 0.1-100 keV spectral band of 8 Seyfert 2
galaxies, obtained by means of BeppoSAX. These sources were selected according
to their [OIII] optical emission line flux, to avoid biases against obscuration
on the pc scale. All sources were detected. All of them are weak X-ray
emitters, and most of them are characterized by prominent iron lines at 6.4-7
keV (EW > 500 eV) and by a flat continuum, indicative of heavy obscuration
along our line of sight (N_H > 10^25 cm^-2 in most cases). These results 1)
provide further evidence in favor of the unified scenario, and 2) indicate that
the average obscuration of type 2 AGNs is very likely much higher than deduced
by former X-ray surveys. These findings have important implications for the
synthesis of the X-ray background.
|
9806055v2
|
1998-06-10
|
Line Emission from an Accretion Disk around a Black hole: Effects of Disk Structure
|
The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide
strong evidence for an accretion disk near a supermassive black hole as a
source of the line emission. These lines serve as powerful probes for examining
the structure of inner regions of accretion disks. Previous studies of line
emission have considered geometrically thin disks only, where the gas moves
along geodesics in the equatorial plane of a black hole. Here we extend this
work to consider effects on line profiles from finite disk thickness, radial
accretion flow and turbulence. We adopt the Novikov and Thorne (1973) solution,
and find that within this framework, turbulent broadening is the dominant new
effect. The most prominent change in the skewed, double-horned line profiles is
a substantial reduction in the maximum flux at both red and blue peaks. The
effect is most pronounced when the inclination angle is large, and when the
accretion rate is high. Thus, the effects discussed here may be important for
future detailed modeling of high quality observational data.
|
9806134v1
|
1998-06-17
|
The hidden X-ray Seyfert nucleus in 3C 273: BeppoSAX results
|
We present the results of BeppoSAX AO1 Core Program observations of 3C~273
performed in Jan. 1997 and compare them in detail with data obtained during the
satellite Science Verification Phase (SVP), in Jul. 1996. Though a power law is
an acceptable representation of the data in the whole 0.1-200 keV range, there
is indication of a steepening of the spectrum as the energy increases. Our data
mark a difference with respect to the SVP data, where a steeper power law below
0.5 keV, an absorption feature at 0.6 keV, and a more prominent fluorescence
iron line have been found. The weakening of cold/warm matter signatures in our
data with respect to the SVP ones may indicate that, at higher luminosities,
the featureless continuum produced in a relativistic jet overwhelms any thermal
and/or reprocessed radiation, while the two components were almost comparable
during the lower state of Jul. 1996. We conclude that this radio--loud source
shows evidence not only for thermal disk--like emission, but also substantial
reprocessing of X-rays onto cold matter.
|
9806229v2
|
1998-06-22
|
A New Dual-Component Photoionization Model for the Narrow Emission-Line Regions in Active Galactic Nuclei
|
Having found that type 1 Seyfert nuclei have excess [FeVII]6087 emission with
respect to type 2s, Murayama & Taniguchi have proposed that the high-ionization
nuclear emission-line region (HINER) traced by the [FeVII]6087 emission resides
in the inner wall of dusty tori. The covering factor of the torus is usually
large (e.g., ~ 0.9). Further, electron density in the tori (e.g., ~ 10^7-10^8
cm-3) is considered to be higher significantly than that (e.g., ~ 10^3-10^4
cm^-3) in the narrow-line region (NLR). Therefore it is expected that the torus
emission contributes to the majority of the higher-ionization emission lines.
Taking this HINER component into account, we have constructed new
dual-component (i.e., a typical NLR with a HINER torus) photoionization models.
Comparison of our model with the observations show that that if the torus
emission contributes ~ 10 % of the NLR emission, our dual-component model can
explain the observed high [FeVII]6087/[OIII]5007 intensity ratios of the
Seyfert 1s without invoking any unusual assumptions (e.g., the overabundance of
iron).
|
9806287v1
|
1998-07-02
|
Contribution of Type Ia and Type II Supernovae for Intra-Cluster Medium Enrichment
|
The origin of the chemical composition of the intracluster medium (ICM) is
discussed in this paper. In particular, the contribution from Type Ia
supernovae (SNe Ia) to the ICM enrichment is shown to exist by adopting the
fitting formulas which have been used in the analysis of the solar system
abundances. Our analysis means that we can use the frequency of SNe Ia relative
to SNe II as the better measure than $M_{Fe, SN Ia}/M_{Fe, total}$ for
estimating the contribution of SNe Ia. Moreover, the chemical compositions of
ICMs are shown to be similar to that of the solar system abundances. We can
also reproduce the sulfur/iron abundance ratio within a factor of 2, which
means that the abundance problem of sulfur needs not to be emphasized too
strongly. We need more precise observations to conclude whether ICMs really
suffer the shortage problem of sulfur or not.
|
9807022v1
|
1998-07-21
|
The swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory
|
BeppoSAX observed the low-luminous Seyfert 1 Galaxy NGC4051 in a ultra-dim
X-ray state. The 2-10 keV flux (1.26 x 10^{-12} erg/cm^2/s) was about 20 times
fainter than its historical average value, and remained steady along the whole
observation (~2.3 days). The observed flat spectrum (\Gamma ~ 0.8) and intense
iron line (EW ~600 eV) are best explained assuming that the active nucleus has
switched off, leaving only a residual reflection component visible.
|
9807213v2
|
1998-08-14
|
Redshift determination in the X-ray band of gamma-ray bursts
|
If gamma-ray bursts originate in dense stellar forming regions, the
interstellar material can imprint detectable absorption features on the
observed X-ray spectrum. Such features can be detected by existing and planned
X-ray satellites, as long as the X-ray afterglow is observed after a few
minutes from the burst. If the column density of the interstellar material
exceeds ~10^{23} cm^{-2} there exists the possibility to detect the K_alpha
fluorescent iron line, which should be visible for more than one year, long
after the X-ray afterglow continuum has faded away. Detection of these X-ray
features will make possible the determination of the redshift of gamma-ray
bursts even when their optical afterglow is severely dimmed by extinction.
|
9808156v1
|
1998-09-04
|
Age Dating of a High-Redshift QSO B1422+231 at Z=3.62 and its Cosmological Implications
|
The observed Fe II(UV+optical)/Mg II lambda lambda 2796,2804 flux ratio from
a gravitationally lensed quasar B1422+231 at z=3.62 is interpreted in terms of
detailed modeling of photoionization and chemical enrichment in the broad-line
region (BLR) of the host galaxy. The delayed iron enrichment by Type Ia
supernovae is used as a cosmic clock. Our standard model, which matches the Fe
II/Mg II ratio, requires the age of 1.5 Gyr for B1422+231 with a lower bound of
1.3 Gyr, which exceeds the expansion age of the Einstein-de Sitter Omega_0=1
universe at a redshift of 3.62 for any value of the Hubble constant in the
currently accepted range, H_0=60-80 km,s^{-1},Mpc^{-1}. This problem of an age
discrepancy at z=3.62 can be unraveled in a low-density Omega_0<0.2 universe,
either with or without a cosmological constant, depending on the allowable
redshift range of galaxy formation. However, whether the cosmological constant
is a required option in modern cosmology awaits a thorough understanding of
line transfer processes in the BLRs.
|
9809047v1
|
1998-09-08
|
ASCA Observations of the Seyfert 2 Galaxy NGC 7582: An Obscured and Scattered View of the Hidden Nucleus
|
ASCA observations of the Seyfert 2 galaxy NGC 7582 revealed it was highly
variable on the timescale of $\sim2\times10^4$ s in the hard X-ray (2-10 keV)
band, while the soft X-ray (0.5-2 keV) flux remained constant during the
observations.
The spectral analysis suggests that this object is seen through an obscuring
torus with the thickness of N$_{\rm H}\sim1.0\times 10^{23}\rm cm^{-2}$. The
hard X-ray is an absorbed direct continuum from a hidden Seyfert 1 nucleus; the
soft X-ray is dominated by the scattered central continuum from an extended
spatial region. Thus we have an obscured/absorbed and a scattered view of this
source as expected from the unification model for Seyfert galaxies.
More interestingly, the inferred X-ray column was observed to increase by
$\sim4\times10^{22} \rm cm^{-2}$ from 1994 to 1996, suggesting a ``patchy''
torus structure, namely the torus might be composed of many individual clouds.
The observed iron line feature near 6.4 keV with the equivalent width of 170 eV
is also consistent with the picture of the transmission of nuclear X-ray
continuum through a non-uniform torus.
|
9809086v1
|
1998-09-14
|
Detection of a cyclotron line and its second harmonic in 4U1907+09
|
We report the detection of a cyclotron absorption line and its second
harmonic in the average spectrum of the high mass X-ray binary 4U1907+09
observed by the BeppoSAX satellite on 1997 September 27 and 28. The broad band
spectral capability of BeppoSAX allowed a good determination of the continuum
against which the two absorption features are evident at ~ 19 and ~ 39 keV.
Correcting for the gravitational redshift of a ~ 1.4 solar mass neutron star,
the inferred surface magnetic field strength is B = 2.1 10E12 G that is typical
for this kind of systems. We also detected an iron emission line at ~ 6.4 keV,
with an equivalent width of ~ 60 eV.
|
9809167v1
|
1998-09-20
|
Emission-Line Probes of Circumnuclear Dust in AGNs
|
Emission lines that trace elements subject to strong depletion onto grains
provide a means for probing the dust content of AGNs. Examples include infrared
[FeII] and optical [CaII] lines. The excitation mechanisms underlying the
[FeII] lines remain controversial, resulting in related disagreement over the
gas-phase abundance of iron in Seyfert narrow-line regions. In this
contribution we emphasize the utility of the [CaII] lines as a consistency test
for claims of grain destruction affecting the [FeII] lines. A search for [CaII]
emission in NGC 1068 at a location of strongest [FeII] emission along the radio
jet yields a strong upper limit, but no detection. This result suggests that
grains survive largely intact in a region that otherwise shows strong evidence
of shock processing.
|
9809247v1
|
1998-10-05
|
Core Collapse Supernovae --- Theory between Achievements and New Challenges
|
Multi-dimensional hydrodynamic simulations of the post-bounce evolution of
collapsed stellar iron cores have demonstrated that convective overturn between
the stalled shock and the neutrinosphere can have an important effect on the
neutrino-driven explosion mechanism. Whether a model yields a successful
explosion or not, however, still depends on the power of neutrino energy
deposition behind the stalled shock. The neutrino interaction with the stellar
gas in the ``hot bubble'' also determines the duration of the shock stagnation
phase, the explosion energy, and the composition of the neutrino-heated
supernova ejecta. More accurate models require a more precise calculation of
the neutrino luminosities and spectra and of the angular distributions of the
neutrinos in the heating region. Therefore it is necessary to improve the
numerical treatment of the neutrino transport, to develop a better
understanding of the neutrino opacities of the dense nuclear medium, and to
take into account convective processes {\it inside} the newly formed neutron
star.
|
9810058v1
|
1998-10-22
|
On the possibility that rotation causes latitudinal abundance variations in stars
|
The effect of rotation of a star on the distribution of chemical species in
radiative zones is discussed. Gravity darkening generates a large radiative
force on heavy element ions which is directed toward the equatorial plane.
Taking iron as an example, it is shown that this force may produce drift
velocities similar to, and larger than, the typical velocities of bulk motion
due to meridional circulation. This potentially allows large chemical abundance
inhomogeneities to build up across a meridian over the lifetime of the star --
particularly near the equatorial plane. This enhancement may be significantly
reduced if the mass loss of the star is strongly metallicity dependent, in
which case the mass-loss rate may be enhanced in the equatorial plane.
|
9810360v1
|
1998-10-22
|
Calibrator Design for the COBE Far Infrared Absolute Spectrophotometer (FIRAS)
|
The photometric errors of the external calibrator for the FIRAS instrument on
the COBE are smaller than the measurement errors on the cosmic microwave
background (CMBR) spectrum (typically 0.02 MJy/sr, 1 sigma), and smaller than
0.01% of the peak brightness of the CMBR. The calibrator is a re-entrant cone,
shaped like a trumpet mute, made of Eccosorb iron-loaded epoxy. It fills the
entire beam of the instrument and is the source of its accuracy. Its known
errors are caused by reflections, temperature gradients, and leakage through
the material and around the edge. Estimates and limits are given for all known
error sources. Improvements in understanding the temperature measurements of
the calibrator allow an improved CMBR temperature determination of 2.725 +/-
0.002 K.
|
9810373v1
|
1998-11-03
|
Microwave Emission by Dust: Mechanisms, Properties and Prospects for ISM Studies
|
I review my work with Bruce Draine on dust emissivity at microwave
frequencies (3 cm - 3 mm). This emissivity explains the recently detected
"anomalous" component of the galactic foreground emission. Both small (a<0.001
micron) and large grains contribute to this emission. Small grains have
electric dipole moments and emit while they rotate; the microwave emission of
large grains is mostly due to magneto-dipole radiation. Most efficient
magneto-dipole emitters are strongly magnetic, e.g. ferrimagnetic or
ferromagnetic, materials. The relative role of the two mechanisms can be
established through observations of microwave emissivity from dark clouds. New
microwave window is a window of opportunity for interstellar studies. Magnetic
fields inside dark clouds may be successfully studied via microwave
polarization. Microwave emissivity constrains the abundance of strongly
magnetic materials. For instance, the available data at 90 GHz indicate that
not more than 5% of interstellar Fe is in the form of metallic iron grains or
inclusions (e.g., in ``GEMS''). Future missions, e.g. MAP and PLANCK, will
bring a wealth of microwave data that can be successfully used to study ISM.
Such a study would be appreciated by cosmologists who franticly try to remove
all foregrounds from their data.
|
9811043v1
|
1998-11-16
|
BeppoSAX confirms extreme relativistic effects in the X-ray spectrum of MCG-6-30-15
|
We report the first simultaneous measure of the X-ray broadband (0.1--200
keV) continuum and of the iron K-alpha fluorescent line profile in the Seyfert
1 galaxy MCG-6-30-15. Our data confirms the ASCA detection of a skewed and
redshifted line profile (Tanaka et al. 1995). The most straightforward
explanation is that the line photons are emitted in the innermost regions of a
X-ray illuminated relativistic disk. The line Equivalent Width (~200 eV) is
perfectly consistent with the expected value for solar abundances, given the
observed amount of Compton reflection. We report also the discovery of a
cut-off in the nuclear primary emission at the energy of ~160 keV.
|
9811246v1
|
1998-11-25
|
Enrichment of the Intracluster Medium
|
The relevance of galaxies of different luminosity and mass for the chemical
enrichment of the intracluster medium (ICM) is analysed. For this purpose, I
adopt the composite luminosity function of cluster galaxies from Trentham
(1998), which exhibits a significant rise at the very faint end. The model -
adopting a universal Salpeter IMF - is calibrated on reproducing the
M_Fe/L_tot, M_Fe/M_*, and alpha/Fe ratios observed in clusters. Although the
contribution to total luminosity and ICM metals peaks around L* galaxies (M*
approx -20), faint objects with M_B>-18 still provide at least 30 per cent of
the metals present in the ICM. In consistency with the solar alpha/Fe ratios
determined by {ASCA}, the model predicts that 60 per cent of the ICM iron comes
from Type Ia supernovae. The predicted slope of the relation between
intracluster gas mass and cluster luminosity emerges shallower than the
observed one, indicating that the fraction of primordial gas increases with
cluster richness.
|
9811409v1
|
1998-12-16
|
ASCA Observations of GRO J1744-28
|
We report the ASCA results of the bursting X-ray pulsar \gro, which was
observed in February 1996 and March 1997. The source flux in the 2--10 keV band
was $2.0\times10^{-8}$ erg/sec/cm$^2$ in 1996 and $5.0\times10^{-9}$
erg/sec/cm$^2$ in 1997. We detected 12 and 17 Type II bursts during the two
observations with mean bursting intervals of about 27 min and 37 min. Each
burst is followed by an intensity dip with the depleted flux depending on the
burst fluence. The energy spectra are approximated by an absorbed power law
with additional structure around 6--7 keV\@. Constant absorption column,
$(5-6)\times10^{22}$ cm$^{-2}$, independent of the observation dates and
emission phases (persistent, burst and dip) is interpreted as an interstellar
absorption. The source may be actually located near the Galactic center, at a
distance of 8.5 kpc. The structure in the energy spectrum at 6--7 keV is most
probably due to iron and maybe reproduced by a disk line model with additional
broadening mechanism.
|
9812298v1
|
1999-01-11
|
RXTE Observations of the X-ray Pulsar XTE J1855-026 - a Possible New Supergiant System
|
A new X-ray source, XTE J1855-026, was discovered during RXTE scans along the
galactic plane. The source shows pulsations at a period of 361 s and also
modulation at a period of 6.1 days which we interpret as the orbital period of
the system. The X-ray spectrum above ~3 keV can be fitted with an absorbed
power law model with a high-energy cut-off, and an iron emission line at
approximately 6.4 keV. We interpret these results as indicating that XTE
J1855-026 is likely to consist of a neutron star accreting from the wind of an
O or B supergiant primary. A less likely interpretation is that XTE J1855-026
is instead a Be/neutron star binary, in which case it would have the shortest
known orbital period for such a system.
|
9901125v1
|
1999-02-15
|
Role of disk galaxies in the chemical enrichment of the intracluster medium
|
Elliptical galaxies are often assumed to be the primary source of heavy
elements in the intracluster medium (ICM), with the contribution of other
morphological types being negligible. In this paper we argue that a role of
spiral galaxies in the chemical evolution of the ICM is also important. This
statement rests upon our recent calculations of the heavy element loss from a
disk galaxy through the hot steady-state galactic wind and dust grains
expulsion by stellar radiation pressure. We show that a typical disk galaxy is
nearly as effective in enriching the ICM as an elliptical galaxy of the same
mass. Having estimated the oxygen and iron loss from a single galaxy, we
integrate them over the galactic mass spectrum. We show that the "effective"
loss (per unit luminosity) from spiral galaxies is comparable to the loss from
ellipticals. The dominant role of early-type galaxies in rich clusters is
caused by that they outnumber spirals.
|
9902202v1
|
1999-02-23
|
X-ray reflection spectra from ionized slabs
|
X-ray reflection spectra are an important component in the X-ray spectra of
many active galactic nuclei and Galactic black hole candidates. It is likely
that reflection takes place from highly ionized surfaces of the accretion disc
in some cases. This can lead to strong Comptonization of the emergent iron, and
other, absorption and emission features. We present such reflection spectra
here, computed in a self-consistent manner with the method described by Ross
and Fabian. In particular we emphasise the range where the ionization parameter
(the flux to density ratio) \xi is around and above 10^4. Such spectra may be
relevant to the observed spectral features found in black hole candidates such
as Cygnus X-1 in the low/hard state.
|
9902325v1
|
1999-02-25
|
Microwave Emission from Galactic Dust Grains
|
Observations of the cosmic microwave background have revealed a component of
10-60 GHz emission from the Galaxy which correlates with 100-140um emission
from interstellar dust but has an intensity much greater than expected for the
low-frequency tail of the "electric dipole vibrational" emission peaking at
\~130um. This "anomalous emission" is more than can be accounted for by
dust-correlated free-free emission. The anomalous emission could be due in part
to magnetic dipole emission from thermal fluctuations of the magnetization
within interstellar dust grains, but only if a substantial fraction of the Fe
in interstellar dust resides in magnetic materials such as metallic iron or
magnetite. The observed anomalous emission is probably due primarily to
electric dipole radiation from spinning ultrasmall interstellar dust grains.
This rotational emission is expected to be partially polarized. From the
standpoint of minimizing confusion with non-CBR foregrounds, 60-120 GHz appears
to be the optimal frequency window.
|
9902356v1
|
1999-03-10
|
Heavy nuclei at the end of the cosmic ray spectrum?
|
We provide an account of the possible acceleration of iron nuclei up to
energies $\sim300$ EeV in the nearby, metally-rich starburst galaxy NGC 253. It
is suggested that particles can escape from the nuclear region with energies of
$\sim10^{15}$ eV and then could be reaccelerated at the terminal shock of the
galactic superwind generated by the starburst, avoiding in this way the
photodisintegration expected if the nuclei were accelerated in the central
region of high photon density. We have also made estimates of the expected
arrival spectrum, which displays a strong dependency with the energy cutoff at
the source.
|
9903145v3
|
1999-03-29
|
Striking Photospheric Abundance Anomalies in Blue Horizontal-Branch Stars in Globular Cluster M13
|
High-resolution optical spectra of thirteen blue horizontal-branch (BHB)
stars in the globular cluster M13 show enormous deviations in element
abundances from the expected cluster metallicity. In the hotter stars (T_eff >
12000 K), helium is depleted by factors of 10 to 100 below solar, while iron is
enhanced to three times the solar abundance, two orders of magnitude above the
canonical metallicity [Fe/H] ~= -1.5 dex for this globular cluster. Nitrogen,
phosphorus, and chromium exhibit even more pronounced enhancements, and other
metals are also mildly overabundant, with the exception of magnesium, which
stays very near the expected cluster metallicity. These photospheric anomalies
are most likely due to diffusion --- gravitational settling of helium, and
radiative levitation of the other elements --- in the stable radiative
atmospheres of these hot stars. The effects of these mechanisms may have some
impact on the photometric morphology of the cluster's horizontal branch and on
estimates of its age and distance.
|
9903437v1
|
1999-04-13
|
An RXTE Observation of NGC 6300: a new bright Compton reflection Dominated Seyfert 2 Galaxy
|
Scanning and pointed RXTE observations of the nearby Seyfert 2 galaxy NGC
6300 reveal that it is a source of hard X-ray continuum and large equivalent
width Fe K$\alpha$ emission. These properties are characteristic of
Compton-reflection dominated Seyfert 2 galaxies. The continuum can be modeled
as Compton-reflection; subsolar iron abundance is required and a high
inclination preferred. However, the poor energy resolution of RXTE means that
this description is not unique, and the continuum can also be modeled using a
``dual absorber'', i.e. a sum of absorbed power laws. Observations with higher
energy resolution detectors will cleanly discriminate between these two models.
Optical observations support the Compton-reflection dominated interpretation as
$L_X/L_{[OIII]}$ is low. NGC 6300 is notable because with $F_{2-10}\approx 6.4
\times 10^{-12} ergs cm^{-2} s^{-1}$, it is the second brightest such object
known.
|
9904155v1
|
1999-04-14
|
Comparison between predicted and empirical DV(Bump -HB) in Galactic Globular Clusters
|
We present observational estimates of DV(Bump-HB) in a sample of 28 Galactic
Globular Clusters (GGCs) observed by HST. The photometric accuracy and the
sizable number of stars measured in each cluster allowed us to single out the
RGB Bump both in metal-poor and in metal-rich GGCs. Empirical values are
compared with homogeneous theoretical predictions which account for both H and
He burning phases over a wide range of metal abundances (0.0001 < Z < 0.02). We
found that, within current observational uncertainties on both iron and
$\alpha$-element abundances, theory and observations are in very good
agreement, provided that the metallicity scale by Carretta & Gratton (1997) as
extended by Cohen et al. (1999) is adopted. Interesting enough, we also found
that both theoretical and observed values show a change in the slope of the
DV(Bump-HB)-[M/H] relation toward higher metal contents.
|
9904183v1
|
1999-04-26
|
A BeppoSAX observation of the massive X-ray binary 4U1700-37
|
A 0.5-200 keV BeppoSAX spectrum of the non-pulsating high-mass X-ray binary
4U1700-37 is presented. The spectrum is well characterized by the standard
accreting pulsar model of an absorbed power-law with a photon index of 1.07
+0.02 -0.03, sharply modified by an exponential cutoff above 5.9 +/- 0.2 keV.
The e-folding energy of the cutoff is 23.9 +/-0.5 keV. A soft bremsstrahlung
component with a temperature of 0.2 +/- 0.1 keV is also required, together with
a narrow iron line at 6.5 keV. Both continuum components are absorbed by a
column of (5.1 +/- 0.2) x 10E22 atom/cm2. There is some evidence for the
presence of a broad cyclotron absorption feature at ~37 keV, although we cannot
exclude the possibility that this is due to an incorrect modeling of the
continuum, or instrumental effects. The hypothesis that the compact object is a
neutron star rather than a black hole seems most likely.
|
9904349v1
|
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