2,640 research outputs found
A Model for Abundances in Metal-Poor Stars
It is argued that the abundances of r-process related elements in stars with
-3<[Fe/H]<-1 can be explained by the contributions of three sources. The
sources are: the first generations of very massive (>100 solar masses) stars
that are formed from Big Bang debris and are distinct from SNII, and two types
of SNII, the H and L events, which can occur only at [Fe/H]>-3. The H events
are of high frequency and produce dominantly heavy (A>130) r-elements but no Fe
(presumably leaving behind black holes). The L events are of low frequency and
produce Fe and dominantly light (A<130) r-elements (essentially none above Ba).
By using the observed abundances in two ultra-metal-poor stars and the solar
r-abundances, the initial or prompt inventory of elements produced by the first
generations of very massive stars and the yields of H and L events can be
determined. The abundances of a large number of elements in a star can then be
calculated from the model by using only the observed Eu and Fe abundances. To
match the model results and the observational data for stars with -3<[Fe/H]<-1
requires that the solar r-abundances for Sr, Y, Zr, and Ba must be
significantly increased from the standard values. Whether the solar
r-components of these elements used here to obtain a fit to the stellar data
can be reconciled with those obtained from solar abundances by subtracting the
s-components calculated from models is not clear.Comment: 47 pages, 19 figures, to appear in Ap
Abundances and Kinematics of Field Halo and Disk Stars I: Observational Data and Abundance Analysis
We describe observations and abundance analysis of a high-resolution,
high-S/N survey of 168 stars, most of which are metal-poor dwarfs. We follow a
self-consistent LTE analysis technique to determine the stellar parameters and
abundances, and estimate the effects of random and systematic uncertainties on
the resulting abundances. Element-to-iron ratios are derived for key alpha,
odd, Fe-peak, r- and s-process elements. Effects of Non-LTE on the analysis of
Fe I lines are shown to be very small on the average. Spectroscopically
determined surface gravities are derived that are generally close to those
obtained from Hipparcos parallaxes.Comment: 41 pages, 7 Postscript figures. Accepted for publication in the A
Constraining the equation of state of supra-nuclear dense matter from XMM-Newton observations of neutron stars in globular clusters
We report on the detailed modelling of the X-ray spectra of three likely
neutron stars. The neutron stars, observed with XMM-Newton are found in three
quiescent X-ray binaries in the globular clusters: omega Cen, M 13 and NGC
2808. Whether they are accreting at very low rates or radiating energy from an
accretion heated core, their X-ray spectra are expected to be those of a
hydrogen atmosphere. We use and compare publicly available hydrogen atmosphere
models, with constant and varying surface gravities to constrain the masses and
radii of the neutron stars. Thanks to the high XMM-Newton throughput, and the
accurate distances available for these clusters, using the latest science
analysis software release and calibration of the XMM-Newton EPIC cameras, we
derive the most stringent constraints on the masses and radii of the neutron
stars obtained to date from these systems. A comparison of the models indicate
that previously used hydrogen atmosphere models (assuming constant surface
gravity) tend to underestimate the mass and overestimate the radius of neutron
stars. Our data constrain the allowed equations of state to those which concern
normal nucleonic matter and one possible strange quark matter model, thus
constraining radii to be from 8 km and masses up to 2.4 M.Comment: 10 pages, 8 figures, accepted to be published in The Astrophysical
Journa
The r-Process Enriched Low Metallicity Giant HD 115444
New high resolution, very high signal-to-noise spectra of ultra-metal-poor
(UMP) giant stars HD 115444 and HD 122563 have been gathered with the
High-Resolution Echelle Spectrometer of the McDonald Observatory 2.7m
Telescope. With these spectra, line identification and model atmosphere
analyses have been conducted, emphasizing the neutron-capture elements. Twenty
elements with Z > 30 have been identified in the spectrum of HD 115444. This
star is known to have overabundances of the neutron-capture elements, but it
has lacked a detailed analysis necessary to compare with nucleosynthesis
predictions. The new study features a line-by-line differential abundance
comparison of HD 115444 with the bright, well-studied halo giant HD 122563. For
HD 115444, the overall metallicity is [Fe/H]~ -3.0. The abundances of the light
and iron-peak elements generally show the same pattern as other UMP stars (e.g.
overdeficiencies of manganese and chromium, overabundances of cobalt), but the
differential analysis indicates several nucleosynthesis signatures that are
unique to each star.Comment: To Appear in the Astrophysical Journa
Langevin Analysis of Eternal Inflation
It has been widely claimed that inflation is generically eternal to the
future, even in models where the inflaton potential monotonically increases
away from its minimum. The idea is that quantum fluctuations allow the field to
jump uphill, thereby continually revitalizing the inflationary process in some
regions. In this paper we investigate a simple model of this process,
pertaining to inflation with a quartic potential, in which analytic progress
may be made. We calculate several quantities of interest, such as the expected
number of inflationary efolds, first without and then with various selection
effects. With no additional weighting, the stochastic noise has little impact
on the total number of inflationary efoldings even if the inflaton starts with
a Planckian energy density. A "rolling" volume factor, i.e. weighting in
proportion to the volume at that time, also leads to a monotonically decreasing
Hubble constant and hence no eternal inflation. We show how stronger selection
effects including a constraint on the initial and final states and weighting
with the final volume factor can lead to a picture similar to that usually
associated with eternal inflation.Comment: 22 pages, 2 figure
Abundance Patterns of Heavy Elements in Damped Lyman-Alpha Galaxies
We present a quantitative analysis of the abundances of heavy elements in
damped Ly-alpha galaxies in the sample of Lu et al. (1996). In particular, we
compare the observed gas-phase abundances with those expected when the
intrinsic (i.e., nucleosynthetic) pattern is the same as that in either the Sun
or in Galactic halo stars and when the depletion pattern is the same as that in
the warm Galactic interstellar medium, but with various values of the
dust-to-metals ratio. We find that the observations are equally consistent with
the solar and halo-star intrinsic patterns and that they favor some depletion,
the typical dust-to-metals ratio being 40%-90% of that in the Milky Way today.
However, neither intrinsic pattern matches the observations perfectly. For the
solar pattern, the discrepancy is mainly with [Mn/Fe], while for the halo-star
pattern, the discrepancy is with [Zn/Fe], [Ni/Fe], and possibly [Al/Fe]. Our
analysis does not support the claim by Lu et al. that the damped Ly-alpha
galaxies have halo-star abundance patterns and no dust depletion.Comment: latex, AAS Macros, 15 pages, 2 figures, moderately revised version,
to be published in ApJ Letters, 1997 July 2
Keck-Nirspec Infrared OH Lines: Oxygen Abundances in Metal-Poor Stars Down to [Fe/H] = -2.9
Infrared OH lines at 1.5 - 1.7 um in the H band were obtained with the
NIRSPEC high-resolution spectrograph at the 10m Keck Telescope for a sample of
seven metal-poor stars. Detailed analyses have been carried out, based on
optical high-resolution data obtained with the FEROS spectrograph at ESO.
Stellar parameters were derived by adopting infrared flux method effective
temperatures, trigonometric and/or evolutionary gravities and metallicities
from FeII lines. We obtain that the sample stars with metallicities [Fe/H] <
-2.2 show a mean oxygen abundance [O/Fe] ~ 0.54, for a solar oxygen abundance
of epsilon(O) = 8.87, or [O/Fe] ~ 0.64 if epsilon(O) = 8.77 is assumed.Comment: To be published in ApJ 575 (August 10
The Earliest Phases of Galaxy Evolution
In this paper we study the very early phases of the evolution of our Galaxy
by means of a chemical evolution model which reproduces most of the
observational constraints in the solar vicinity and in the disk. We have
restricted our analysis to the solar neighborhood and present the predicted
abundances of several elements (C, N, O, Mg, Si, S, Ca, Fe) over an extended
range of metallicities to compared to previous
models. We adopted the most recent yield calculations for massive stars taken
from different authors (Woosley & Weaver 1995 and Thielemann et al. 1996) and
compared the results with a very large sample of data, one of the largest ever
used to this purpose. These data have been analysed with a new and powerful
statistical method which allows us to quantify the observational spread in
measured elemental abundances and obtain a more meaningful comparison with the
predictions from our chemical evolution model. Our analysis shows that the
``plateau'' observed for the [/Fe] ratios at low metallicities () is not perfectly constant but it shows a slope, especially for
oxygen. This slope is very well reproduced by our model with both sets of
yields. This is not surprising since realistic chemical evolution models,
taking into account in detail stellar lifetimes, never predicted a completely
flat plateau. This is due either to the fact that massive stars of different
mass produce a slightly different O/Fe ratio or to the often forgotten fact
that supernovae of type Ia, originating from white dwarfs, start appearing
already at a galactic age of 30 million years and reach their maximum at 1 Gyr.Comment: 32 pages, 9 figures, to be published in Ap
Helium variation due to self-pollution among Globular Cluster stars: consequences on the horizontal branch morphology
It is becoming clear that `self--pollution' by the ejecta of massive asymptotic giant branch stars has an important role in the early chemical evolution of globular cluster stars, producing CNO abundance spreads which are observed also at the surface of unevolved stars. Considering that the ejecta which are CNO processed must also be helium enriched, we have modelled stellar evolution of globular cluster stars by taking into account this possible helium enhancement with respect to the primordial value. We show that the differences between the main evolutionary phases (main sequence, turn--off and red giants) are small enough that it would be very difficult to detect them observationally. However, the difference in the evolving mass may play a role in the morphology of the horizontal branch, and in particular in the formation of blue tails, in those globular clusters which show strong CNO abundance variations, such as M13 and NGC 6752
The Chemical Compositions of the SRd Variable Stars. III. KK Aquilae, AG Aurigae, Z Aurigae, W Leo Minoris, and WW Tauri
Chemical compositions are derived from high-resolution spectra for five field
SRd variables. These supergiants not previously analysed are shown to be
metal-poor: KK Aql with [Fe/H] = -1.2, AG Aur with [Fe/H] = -1.8, Z Aur with
[Fe/H] = -1.4, W LMi with [Fe/H] = -1.1, and WW Tau with [Fe/H] = -1.1. Their
compositions are, except for two anomalies, identical to within the measurement
errors with the compositions of subdwarfs, subgiants, and less evolved giants
of the same [Fe/H]. One anomaly is an s-process enrichment for KK Aql, the
first such enrichment reported for a SRd variable. The second and more
remarkable anomaly is a strong lithium enrichment for W LMi, also a first for
field SRds. The Li I 6707 A profile is not simply that of a photospheric line
but includes strong absorption from red-shifted gas, suggesting, perhaps, that
lithium enrichment results from accretion of Li-rich gas. This potential clue
to lithium enrichment is discussed in light of various proposals for lithium
synthesis in evolved stars.Comment: 18 pages of text, 8 tables, 2 figures. Accepted for publication in
PAS
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