160 research outputs found
Improved Color-Temperature Relations and Bolometric Corrections for Cool Stars
We present new grids of colors and bolometric corrections for F-K stars
having 4000 K < Teff < 6500 K, 0.0 < log g < 4.5 and -3.0 < [Fe/H] < 0.0. A
companion paper extends these calculations into the M giant regime. Colors are
tabulated for Johnson U-V and B-V; Cousins V-R and V-I; Johnson-Glass V-K, J-K
and H-K; and CIT/CTIO V-K, J-K, H-K and CO. We have developed these
color-temperature (CT) relations by convolving synthetic spectra with
photometric filter-transmission-profiles. The synthetic spectra have been
computed with the SSG spectral synthesis code using MARCS stellar atmosphere
models as input. Both of these codes have been improved substantially,
especially at low temperatures, through the incorporation of new opacity data.
The resulting synthetic colors have been put onto the observational systems by
applying color calibrations derived from models and photometry of field stars
which have Teffs determined by the infrared-flux method. The color calibrations
have zero points and slopes which change most of the original synthetic colors
by less than 0.02 mag and 5%, respectively. The adopted Teff scale (Bell &
Gustafsson 1989) is confirmed by the extraordinary agreement between the
predicted and observed angular diameters of the field stars. We have also
derived empirical CT relations from the field-star photometry. Except for the
coolest dwarfs (Teff < 5000 K), our calibrated, solar-metallicity model colors
are found to match these and other empirical relations quite well. Our
calibrated, 4 Gyr, solar-metallicity isochrone also provides a good match to
color-magnitude diagrams of M67. We regard this as evidence that our calibrated
colors can be applied to many astrophysical problems, including modelling the
integrated light of galaxies. (abridged)Comment: To appear in the March 2000 issue of the Astronomical Journal. 72
pages including 16 embedded postscript figures (one page each) and 6 embedded
postscript tables (18 pages total
Quantum Smoluchowski equation: Escape from a metastable state
We develop a quantum Smoluchowski equation in terms of a true probability
distribution function to describe quantum Brownian motion in configuration
space in large friction limit at arbitrary temperature and derive the rate of
barrier crossing and tunneling within an unified scheme. The present treatment
is independent of path integral formalism and is based on canonical
quantization procedure.Comment: 10 pages, To appear in the Proceedings of Statphys - Kolkata I
Modeling Mid-Ultraviolet Spectra. I. Temperatures of Metal-Poor Stars
Determining the properties of old stellar systems using evolutionary
population synthesis requires a library of reliable model stellar fluxes.
Empirical libraries are limited to spectra of stars in the solar neighborhood,
with nearly solar abundances and abundance ratios. We report here a first step
towards providing a flux library that includes nonsolar abundances, based on
calculations from first principles that are calibrated empirically. We have
started with main-sequence stars, whose light dominates the mid-ultraviolet
spectrum of an old stellar system. We have calculated mid-ultraviolet spectra
for the Sun and nine nearby, near-main-sequence stars spanning metallicities
from less than 1/100 solar to greater than solar, encompassing a range of
light-element abundance enhancements. We first determined temperatures of eight
of the stars by analyzing optical echelle spectra together with the
mid-ultraviolet. Both could be matched at the same time only when models with
no convective overshoot were adopted, and only when an approximate chromosphere
was incorporated near the surface of relatively metal-rich models. Extensive
modifications to mid-UV line parameters were also required, notably the manual
assignment of approximate identifications for mid-UV lines missing from
laboratory linelists. Without recourse to additional missing opacity, these
measures suffice to reproduce in detail almost the entire mid-UV spectrum of
solar-temperature stars up to one-tenth solar metallicity, and the region from
2900A to 3100A throughout the entire metallicity range. Ramifications for
abundance determinations in individual metal-poor stars and for age-metallicity
determinations of old stellar systems are briefly discussed, with emphasis on
the predictive power of the calculations.Comment: Proof revision -Minor changes to revised version submitted to
Astrophysical Journal May 1, 2001. 29 pages, 4 figures (Fig. 3 with 5 panels
and Fig. 4 with 6 panels). Figures 1 and 2 are .gif; postscript versions of
Figures 1 and 2 are available from
http://www.astro.virginia.edu/~rtr/uv/index.htm
Metal enrichment processes
There are many processes that can transport gas from the galaxies to their
environment and enrich the environment in this way with metals. These metal
enrichment processes have a large influence on the evolution of both the
galaxies and their environment. Various processes can contribute to the gas
transfer: ram-pressure stripping, galactic winds, AGN outflows, galaxy-galaxy
interactions and others. We review their observational evidence, corresponding
simulations, their efficiencies, and their time scales as far as they are known
to date. It seems that all processes can contribute to the enrichment. There is
not a single process that always dominates the enrichment, because the
efficiencies of the processes vary strongly with galaxy and environmental
properties.Comment: 18 pages, 8 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 17; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Spectroscopic Signatures of Convection in the Spectrum of Procyon. Fundamental Parameters and Iron Abundance
We have observed the spectrum of Procyon A (F5IV) from 4559 to 5780 A with a
S/N of ~ 1e3 and a resolving power of 2e5. We have measured the line bisectors
and relative line shifts of a large number of Fe I and Fe II lines, comparing
them to those found in the Sun. A three-dimensional(3D) hydrodynamical model
atmosphere has been computed and is tested against observations. The model
reproduces in detail most of the features observed, although we identify some
room for improvement. At all levels, the comparison of the 3D time-dependent
calculations with the observed spectral lines shows a much better agreement
than for classical homogeneous models, making it possible to refine previous
estimates of the iron abundance, the projected rotational velocity, the
limb-darkening, and the systemic velocity of the Procyon binary system.
The difference between the iron abundance determined with the 3D model and
its
1D counterpart is <~ 0.05 dex. We find consistency between the iron abundance
derived from Fe I and Fe II lines, suggesting that departures from LTE in the
formation of the studied lines are relatively small. The scatter in the iron
abundance determined from different lines still exceeds the expectations from
the uncertainties in the atomic data, pointing out that one or more components
in the modeling can be refined further.Comment: 30 pages, 19 figures; uses emulateapj.sty (included); to appear in
ApJ (Feb 2002
The Chemical Composition of Carbon-Rich, Very Metal-Poor Stars: A New Class of Mildly Carbon-Rich Objects Without Excess of Neutron-Capture Elements
We report on an analysis of the chemical composition of five carbon-rich,
very metal-poor stars based on high-resolution spectra. One star, CS22948-027,
exhibits very large overabundances of carbon, nitrogen, and the neutron-capture
elements, as found in the previous study of Hill et al.. This result may be
interpreted as a consequence of mass transfer from a binary companion that
previously evolved through the asymptotic giant branch stage. By way of
contrast, the other four stars we investigate exhibit no overabundances of
barium ([Ba/Fe]<0), while three of them have mildly enhanced carbon and/or
nitrogen ([C+N]+1). We have been unable to determine accurate carbon and
nitrogen abundances for the remaining star (CS30312-100). These stars are
rather similar to the carbon-rich, neutron-capture-element-poor star
CS22957-027 discussed previously by Norris et al., though the carbon
overabundance in this object is significantly larger ([C/Fe]=+2.2). Our results
imply that these carbon-rich objects with ``normal'' neutron-capture element
abundances are not rare among very metal-deficient stars. One possible process
to explain this phenomenon is as a result of helium shell flashes near the base
of the AGB in very low-metallicity, low-mass (M~< 1M_sun) stars, as recently
proposed by Fujimoto et al..
The moderate carbon enhancements reported herein ([C/Fe]+1) are similar to
those reported in the famous r-process-enhanced star CS22892-052. We discuss
the possibility that the same process might be responsible for this similarity,
as well as the implication that a completely independent phenomenon was
responsible for the large r-process enhancement in CS22892-052.Comment: 53 pages, 8 figures, to appear in Ap
HE1327-2326, an unevolved star with [Fe/H]<-5.0. I. A Comprehensive Abundance Analysis
We present the elemental abundances of HE1327-2326, the most iron-deficient
star known, determined from a comprehensive analysis of spectra obtained with
the Subaru Telescope High Dispersion Spectrograph.Comment: 53 pages, 14 figures, 10 tables, to appear in Ap
Chemical Abundances from Inversions of Stellar Spectra: Analysis of Solar-Type Stars with Homogeneous and Static Model Atmospheres
Spectra of late-type stars are usually analyzed with static model atmospheres
in local thermodynamic equilibrium (LTE) and a homogeneous plane-parallel or
spherically symmetric geometry. The energy balance requires particular
attention, as two elements which are particularly difficult to model play an
important role: line blanketing and convection. Inversion techniques are able
to bypass the difficulties of a detailed description of the energy balance.
Assuming that the atmosphere is in hydrostatic equilibrium and LTE, it is
possible to constrain its structure from spectroscopic observations. Among the
most serious approximations still implicit in the method is a static and
homogeneous geometry. In this paper, we take advantage of a realistic
three-dimensional radiative hydrodynamical simulation of the solar surface to
check the systematic errors incurred by an inversion assuming a plane-parallel
horizontally-homogeneous atmosphere. The thermal structure recovered resembles
the spatial and time average of the three-dimensional atmosphere. Furthermore,
the abundances retrieved are typically within 10% (0.04 dex) of the abundances
used to construct the simulation. The application to a fairly complete dataset
from the solar spectrum provides further confidence in previous analyses of the
solar composition. There is only a narrow range of one-dimensional thermal
structures able to fit the absorption lines in the spectrum of the Sun. With
our carefully selected dataset, random errors are about a factor of two smaller
than systematic errors. A small number of strong metal lines can provide very
reliable results. We foresee no major difficulty in applying the technique to
other similar stars, and obtaining similar accuracies, using spectra with a
resolving power about 50,000 and a signal-to-noise ratio as low as 30.Comment: 65 pages, figures included; uses aastex; to appear in The
Astrophysical Journa
Abundances of 30 elements in 23 metal-poor stars
We report the abundances of 30 elements in 23 metal-poor ([Fe/H] <-1.7)
giants. These are based on 7774 equivalent widths and spectral synthesis of 229
additional lines. Hyperfine splitting is taken into account when appropriate.
Our choice of model atmospheres has the most influence on the accuracy of our
abundances. We consider the effect of different model atmospheres on our
results. In addition to the random errors in Teff, log g, and microturbulent
velocity, there are several sources of systematic error. These include using
Teff determined from FeI lines rather than colors, ignoring NLTE effects on the
FeI/FeII ionization balance, using models with solar [alpha/Fe] ratios and
using Kurucz models with overshooting. Of these, only the use of models with
solar [alpha/Fe] ratios had a negligible effect. However, while the absolute
abundances can change by > 0.10 dex, the relative abundances, especially
between closely allied atoms such as the rare earth group, often show only
small (<0.03 dex) changes. We found that some strong lines of FeI, MnI and CrI
consistently gave lower abundances by ~0.2 dex, a number larger than the quoted
errors in the gf values. After considering a model with depth-dependent
microturbulent velocity and a model with hotter temperatures in the upper
layers, we conclude that the latter did a better job of resolving the problem
and agreeing with observational evidence for the structure of stars. The error
analysis includes the effects of correlation of Teff, log g, and microturbulent
velocity errors, which is crucial for certain element ratios, such as [Mg/Fe].
The abundances presented here are being analyzed and discussed in a separate
series of papers.Comment: 27 pages, 9 figures, Table 2 included separately, to published in
ApJ
Oxygen in the Very Early Galaxy
Oxygen abundances in a sample of ultra-metal-poor subdwarfs have been derived
from measurements of the oxygen triplet at 7771--5 A and OH lines in the near
UV performed in high-resolution and high signal-to-noise spectra obtained with
WHT/UES, KeckI/HIRES, and VLT/UVES. Our Fe abundances were derived in LTE and
then corrected for NLTE effects following Thevenin and Idiart (1999). The new
oxygen abundances confirm previous findings for a progressive linear rise in
the oxygen-to-iron ratio with a slope -0.33+-0.02 from solar metallicity to
[Fe/H] -3. A slightly higher slope would be obtained if the Fe NLTE corrections
were not considered. Below [Fe/H]= -2.5 our stars show [O/Fe] ratios as high as
~ 1.17 (G64-12), which can be interpreted as evidence for oxygen overproduction
in the very early epoch of the formation of the halo, possibly associated with
supernova events with very massive progenitor stars. We show that the arguments
against this linear trend given by Fulbright and Kraft (1999), based on the LTE
Fe analysis of two metal-poor stars cannot be sustained when an NLTE analysis
is performed. Using 1-D models our analysis of three oxygen indicators
available for BD +23 3130 gives consistent abundances within 0.16 dex and
average [O/Fe] ratio of 0.91.Comment: 45 pages, 11 figures, accepted for publication in The Astrophysical
Journa
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