The metallicity of stars in the Galaxy ranges from [Fe/H] = -4 to +0.5 dex, and the solar iron abundance is epsilon(Fe) = 7.51 +/- 0.01 dex. The average values of [Fe/H] in the solar neighborhood, the halo, and Galactic bulge are -0.2, -1.6, and -0.2 dex respectively.

We analyze the recent thorium detection in the metal-poor halo star CS 22892-052. Sneden et al. have demonstrated that all of the stable elements with Z greater than or equal to 56, including those near the thorium nuclear region, are consistent with the solar r-process abundances. This result strongly suggests that thorium (formed in the r-process) was also produced in solar proportions in the progenitor of CS 22892-052. Theoretical calculations, presented here, that reproduce the observed stable solar system r-process abundances predict a thorium/europium (Th/Eu) ratio in close agreement with the extrapolated, corrected r-process-only ratio (0.463) at the time of the formation of the solar system, Sneden et al, found that Th/Eu = 0.219 in CS 22892-052, substantially below the current solar ratio, indicating a much greater age for this star. Ignoring additional production of thorium over time, and thus any chemical evolution effects, comparison between the observed and the solar system corrected Th/Eu ratios gives a simple radioactive-decay age for CS 22892-052 of 15.2 +/- 3.7 Gyr. Additional age estimates, based upon theoretically determined Th/Eu ratios, also suggest a range of 11.5 less than or similar to t(CS 22892-052) less than or similar to 18.8 Gyr and are thus consistent with that simple radioactive-decay age determination for this star. Since additional Galactic production of thorium leads to an increase in this decay time, this is a lower limit on the stellar age, and hence the age of the Galaxy. We evaluate the magnitude of this effect on the chronometric age determinations by employing several simple chemical evolution models, including a closed-box model and one which allows for Galactic infall. These Galactic chemical evolution models suggest an age of 17 +/- 4 Gyr for CS 22892-052. We discuss the implications and possible sources of errors and uncertainties in these age estimates.

New, improved, barium abundances for 33 extremely metal-poor halo stars from the 1995 sample of McWilliam et al. have been computed. The mean [Ba/Eu] ratio for stars with [Fe/H] less than or equal to -2.4 is -0.69 +/- 0.06 dex, consistent with pure r-process nucleosynthesis within the measurement uncertainties. Although the [Sr/Fe] and [Ba/Fe] abundance ratios span a range of 2.6 dex, the mean values are approximately constant with [Fe/H]. This is consistent with a model of chemical evolution in which the parent clouds were enriched by small numbers of supernova events. In this model, the decreasing heavy-element dispersion with increasing [Fe/H] is simply due to the averaging of element yields from many supernovae at higher [Fe/H]; however, it is necessary to increase the number of extremely metal-poor stars known in order to confirm this picture. In addition to the random Sr component from the r-process, the [Sr/Ba] ratios indicate that there is a second, also random, source of Sr from an as yet unidentified nucleosynthesis site.

We discuss how abundance ratios can help unravel the evolutionary history of the Galactic bulge. An LTE abundance analysis of a solar-[Fe/H] bulge red giant star reveals that the su-elements O, Mg, Si, Ca and Ti are enhanced by +0.3 dex, the r-process element Eu is enhanced by +0.5 dex and the Ba/Eu ratio is -0.5. These ratios are characteristic of normal halo composition, and suggest that the bulge reached solar [Fe/H] in less than 1Gyr.

In extremely metal-poor stars ([Fe/H]less than or equal to -2.5) the neutron capture elements are characterized by a 300-fold dispersion in M/Fe ratios which decreases with increasing metallicity, the median M/Fe ratio increases with increasing [Fe/H], but the average M/Fe number ratio is approximately constant. These observations are consistent with a highly dispersed intrinsic yield of neutron-capture elements in supernova (SN) events, and a progression to increasing metallicity by stochastic chemical evolution.

We report on detailed abundances of giants in the Galactic bulge, measured with the HIRES echelle spectrograph on the 10-m Keck telescope. We also review other work on the bulge field population and globular clusters using Keck/HIRES. Our new spectra have 3 times the resolution and higher S/N than previous spectra obtained with 4m telescopes. We are able to derive log g from Fe II lines and excitation temperature from Fe I lines, and do not rely on photometric estimates for these parameters. We confirm that the iron abundance range extends from -1.6 to +0.55 dex. The improved resolution and S/N of the Keck spectra give [Fe/H] typically 0.1 to 0.2 dex higher than previous studies,(1) for bulge stars more metal rich than the Sun. Alpha elements are enhanced even for stars at the Solar metallicity (as is the case for bulge globular clusters). We confirm our earlier abundance analysis of bulge giants(1) and find that Mg and Ti are enhanced relative to Ca and Si even up to [Fe/H]=+0.55. We also report the first reliable estimates of the bulge oxygen abundance. Our element ratios confirm that bulge giants have a clearly identifiable chemical signature, and suggest a rapid formation timescale for the bulge.

We investigate the impact of hyperfine splitting on stellar abundance analyses of Mn and Sc and find that incorrect hyperfine splitting treatment can lead to spurious abundance trends with metallicity. We estimate corrections to a recent study by Nissen et al. and find (1) [Mn/Fe] is described by a bimodal distribution, with Mn/Fe] similar to -0.3 for stars with [Fe/H] < -0.7 and [Mn/Fe] similar to -0.05 for stars at higher metallicity, suggestive of a transition between halo/thick-disk and thin-disk populations, and (2) the large majority of stars show nearly solar [Sc/Fe] ratios, although important deviations cannot be ruled out.

We present first results from a program to measure the chemical abundances of a large (N > 30) sample of thick disk stars with the principal goal of investigating the formation history of the Galactic thick disk. We have obtained high-resolution, high signal-to-noise spectra of 10 thick disk stars with the HIRES spectrograph on the 10 m Reck I telescope. Our analysis confirms previous studies of O and Mg in the thick disk stars, which reported enhancements in excess of the thin disk population. Furthermore, the observations of Si, Ca, Ti, Mn, Co, V, Zn, Al, and Eu all argue that the thick disk population has a distinct chemical history from the thin disk. With the exception of V and Co, the thick disk abundance patterns match or tend toward the values observed for halo stars with [Fe/H] approximate to -1. This suggests that the thick disk stars had a chemical enrichment history similar to the metal-rich halo stars. With the possible exception of Si, the thick disk abundance patterns are in excellent agreement with the chemical abundances observed in the metal-poor bulge stars, suggesting the two populations formed from the same gas reservoir at a common epoch. T

Thirteen red clump stars from Baade's window were observed with high resolution in the red part of the optical spectrum with the UVES echelle spectrograph at the Mount Paranal ESO Observatory. The model atmosphere abundance analysis placed their [Fe/H] values in a range from 0.0 to - 1.52 dex. Present results, based on direct measurements of iron abundance, confirm former suggestions that the I-band brightness of the red clump giants only weakly depends on [Fe/H].