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