On the galactic chemical evolution of sulfur

Sulfur abundances have been determined for ten stars to resolve a debate in the literature on the Galactic chemical evolution of sulfur in the halo phase of the Milky Way. Our analysis is based on observations of the S I lines at 9212.9, 9228.1, and 9237.5 A for stars for which the S abundance was obtained previously from much weaker S I lines at 8694.0 and 8694.6 A. In contrast to the previous results showing [S/Fe] to rise steadily with decreasing [Fe/H], our results show that [S/Fe] is approximately constant for metal-poor stars ([Fe/H] < -1) at [S/Fe] = +0.3. Thus, sulfur behaves in a similar way to the other alpha elements, with an approximately constant [S/Fe] for metallicities lower than [Fe/H] = -1. We suggest that the reason for the earlier claims of a rise of [S/Fe] is partly due to the use of the weak S I 8694.0 and 8694.6 A lines and partly uncertainties in the determination of the metallicity when using Fe I lines. The S I 9212.9, 9228.1, and 9237.5 A lines are preferred for an abundance analysis of sulfur for metal-poor stars.Comment: Accepted by A&A, 12 pages. Full article with figures in A&

Finding cool subdwarfs using a V-J reduced proper-motion diagram: Stellar parameters for 91 candidates

We present the results of a search for cool subdwarfs for which our candidates were drawn from a V-J reduced proper-motion diagram constructed by Salim & Gould (2002). Kinematic (U, V, and W) and self-consistent stellar parameters (Teff, log g, [Fe/H], and V_t) are derived for 91 candidate subdwarfs based on high resolution spectra. The observed stars span 3900K < Teff < 6200K and -2.63 < [Fe/H] < 0.25 including only 3 giants (log g < 4.0). Of the sample, 77 stars have MgH lines present in their spectra. With more than 56% of our candidate subdwarfs having [Fe/H] < -1.5, we show that the V-J reduced proper-motion diagram readily identifies metal-poor stars.Comment: PASP (in press

A Preliminary Seismic Analysis of 51 Peg: Large and Small Spacings from Standard Models

We present a preliminary theoretical seismic study of the astronomically famous star 51 Peg. This is done by first performing a detailed analysis within the Hertzsprung-Russell diagram (HRD). Using the Yale stellar evolution code (YREC), a grid of stellar evolutionary tracks has been constructed for the masses 1.00 M_sun, 1.05 M_sun and 1.10 M_sun, in the metallicity range Z=0.024-0.044, and for values of the Galactic helium enrichment ratio DY/DZ in the range 0-2.5. Along these evolutionary tracks, we select 75 stellar model candidates that fall within the 51 Peg observational error box in the HRD (all turn out to have masses of 1.05 M_sun and 1.10 M_sun. The corresponding allowable age range for these models, which depends sensitively on the parameters of the model, is relatively large and is ~2.5 - 5.5 Gyr. For each of the 75 models, a non-radial pulsation analysis is carried out, and the large and small frequency spacings are calculated. The results show that just measuring the large and small frequency spacings will greatly reduce the present uncertainties in the derived physical parameters and in the age of 51 Peg. Finally we discuss briefly refinements in the physics of the models and in the method of analysis which will have to be included in future models to make the best of the precise frequency determinations expected from space observations.Comment: 22 pages, 5 figures, 3 tables. Accepted for publicaton by Ap

Searching for Earth analogues around the nearest stars: the disk age-metallicity relation and the age distribution in the Solar Neighbourhood

The chemical composition of Earth's atmosphere has undergone substantial evolution over the course of its history. It is possible, even likely, that terrestrial planets in other planetary systems have undergone similar changes; consequently, the age distribution of nearby stars is an important consideration in designing surveys for Earth-analogues. Valenti & Fischer (2005) provide age and metallicity estimates for 1039 FGK dwarfs in the Solar Neighbourhood. Using the Hipparcos catalogue as a reference to calibrate potential biases, we have extracted volume-limited samples of nearby stars from the Valenti-Fischer dataset. Unlike other recent investigations, our analysis shows clear evidence for an age-metallicity relation in the local disk, albeit with substantial dispersion at any epoch. The mean metallicity increases from -0.3 dex at a lookback time of ~10 Gyrs to +0.15 dex at the present day. Supplementing the Valenti-Fischer measurements with literature data to give a complete volume-limited sample, the age distribution of nearby FGK dwarfs is broadly consistent with a uniform star-formation rate over the history of the Galactic disk. In striking contrast, most stars known to have planetary companions are younger than 5 Gyrs; however, stars with planetary companions within 0.4 AU have a significantly flatter age distribution, indicating that those systems are stable on timescales of many Gyrs. Several of the older, lower metallicity host stars have enhanced [alpha/Fe] ratios, implying membership of the thick disk. If the frequency of terrestrial planets is also correlated with stellar metallicity, then the median age of such planetary system is likely to be ~3 Gyrs. We discuss the implications of this hypothesis in designing searches for Earth analogues among the nearby stars.Comment: Accepted for publication in Ap

A Consistency Test of Spectroscopic Gravities for Late-Type Stars

Chemical analyses of late-type stars are usually carried out following the classical recipe: LTE line formation and homogeneous, plane-parallel, flux-constant, and LTE model atmospheres. We review different results in the literature that have suggested significant inconsistencies in the spectroscopic analyses, pointing out the difficulties in deriving independent estimates of the stellar fundamental parameters and hence,detecting systematic errors. The trigonometric parallaxes measured by the HIPPARCOS mission provide accurate appraisals of the stellar surface gravity for nearby stars, which are used here to check the gravities obtained from the photospheric iron ionization balance. We find an approximate agreement for stars in the metallicity range -1 <= [Fe/H] <= 0, but the comparison shows that the differences between the spectroscopic and trigonometric gravities decrease towards lower metallicities for more metal-deficient dwarfs (-2.5 <= [Fe/H] <= -1.0), which casts a shadow upon the abundance analyses for extreme metal-poor stars that make use of the ionization equilibrium to constrain the gravity. The comparison with the strong-line gravities derived by Edvardsson (1988) and Fuhrmann (1998a) confirms that this method provides systematically larger gravities than the ionization balance. The strong-line gravities get closer to the physical ones for the stars analyzed by Fuhrmann, but they are even further away than the iron ionization gravities for the stars of lower gravities in Edvardsson's sample. The confrontation of the deviations of the iron ionization gravities in metal-poor stars reported here with departures from the excitation balance found in the literature, show that they are likely to be induced by the same physical mechanism(s).Comment: AAS LaTeX v4.0, 35 pages, 10 PostScript files; to appear in The Astrophysical Journa

The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables

Abundance analyses from high-resolution optical spectra are presented for 19 Type II Cepheids in the Galactic field. The sample includes both short-period (BL Her) and long-period (W Vir) stars. This is the first extensive abundance analysis of these variables. The C, N, and O abundances with similar spreads for the BL Her and W Vir show evidence for an atmosphere contaminated with $3\alpha$-process and CN-cycling products. A notable anomaly of the BL Her stars is an overabundance of Na by a factor of about five relative to their presumed initial abundances. This overabundance is not seen in the W Vir stars. The abundance anomalies running from mild to extreme in W Vir stars but not seen in the BL Her stars are attributed to dust-gas separation that provides an atmosphere deficient in elements of high condensation temperature, notably Al, Ca, Sc, Ti, and $s$-process elements. Such anomalies have previously been seen among RV Tau stars which represent a long-period extension of the variability enjoyed by the Type II Cepheids. Comments are offered on how the contrasting abundance anomalies of BL Her and W Vir stars may be explained in terms of the stars' evolution from the blue horizontal branch.Comment: 41 pages including 11 figures and 4 tables; Accepted for publication in Ap