We discuss the detailed composition of 28 extremely metal-poor (EMP) dwarfs, 22 of which are from the Hamburg/ESO Survey (HES), based on Keck echelle spectra. Our sample has a median [Fe/H] of - 2.7 dex, extends to - 3.5 dex, and is somewhat less metal-poor than was expected from [Fe/H](HK, HES) determined from low-resolution spectra. Our analysis supports the existence of a sharp decline in the distribution of halo stars with metallicity below [Fe/H] = -3.0 dex. So far no additional turnoff stars with [Fe/H] < -3.5 have been identified in our follow-up efforts. For the best-observed elements between Mg and Ni, we find that the abundance ratios appear to have reached a plateau, i.e., [X/Fe] is approximately constant as a function of [Fe/H], except for Cr, Mn, and Co, which show trends of abundance ratios varying with [Fe/H]. These abundance ratios at low metallicity correspond approximately to the yield expected from Type II supernovae (SNe) with a narrow range in mass and explosion parameters; high-mass Type II SN progenitors are required. The dispersion of [X/Fe] about this plateau level is surprisingly small and is still dominated by measurement errors rather than intrinsic scatter. These results place strong constraints on the characteristics of the contributing SNe. The dispersion in neutron-capture elements and the abundance trends for Cr, Mn, and Co are consistent with previous studies of evolved EMP stars. We find halo-like enhancements for the alpha-elements Mg, Ca, and Ti, but solar Si/Fe ratios for these dwarfs. This contrasts with studies of EMP giant stars, which show Si enhancements similar to other alpha-elements. Sc/Fe is another case where the results from EMP dwarfs and from EMP giants disagree; our Sc/Fe ratios are enhanced compared to the solar value by similar to 0.2 dex. Although this conflicts with the solar Sc/Fe values seen in EMP giants, we note that alpha-like Sc/Fe ratios have been claimed for dwarfs at higher metallicity. Two dwarfs in the sample are carbon stars, while two others have significant C enhancements, all with C-12/C-13 similar to 7 and with C/N between 10 and 150. Three of these C-rich stars have large enhancements of the heavy neutron capture elements, including lead, which implies a strong s-process contribution, presumably from binary mass transfer; the fourth shows no excess of Sr or Ba.

We outline the results of a study of the chemical composition of 14 stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph). For the Sgr dSph stars with [Fe/H >= - 1 the abundances are highly unusual, showing a striking enhancement in heavy s-process elements, increasing with [Fe/H], deficiencies of the alpha-elements (O, Si, Ca, and Ti), deficiencies of Al and Na, and deficiencies of the iron-peak elements Mn and Cu. Our abundances suggest that the composition of the metal-rich Sgr dSph stars is dominated by the ejecta of an old metal-poor population, including products of AGB stars and type la supernovae (SN).

We present copper abundances for 14 red giant stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph), showing that [Cu/Fe] is deficient by similar to 0.5 dex, relative to the Galactic trend. This is most easily understood as due to an enhanced contribution of iron peak nucleosynthesis products from Type Ia supernovae (SNe Ia). The deficient [Cu/Fe] ratios might also be the result of a metallicity-dependent yield from SNe Ia, similar to previous conclusions for Mn, although SN Ia nucleosynthesis predictions suggest a negligible Cu yield. The enhanced SN Ia products, suggested by our low [Cu/Fe] ratios, fit a leaky box chemical evolution scenario for the Sgr dSph, where ejecta from the old, metal-poor, population overwhelmed nucleosynthesis products from younger generations, resulting in young stars with uncharacteristic compositions. The only other system known to have unusually low [Cu/Fe] is the Galactic globular cluster omega Cen, which, like the Sagittarius dwarf galaxy, has strong enhancements of s-process elements. Thus, our copper abundances lend support to the idea that omega Cen is the remaining nucleus of an accreted dwarf galaxy.

We present abundance results from our Keck/HIRES observations of giants in the Galactic Bulge. We confirm that the metallicity distribution of giants in the low-reddening bulge field Baade's Window can be well-fit by a closed-box enrichment model. We also confirm previous observations that find enhanced [Mg/Fe], [Si/Fe] and [Ca/Fe] for all bulge giants, including those at super-solar metallicities. However, we find that the [O/Fe] ratios of metal-rich bulge dwarfs decrease with increasing metallicity, contrary to what is expected if the enhancements of the other a-elements is due to Type II supernovae enrichment. We suggest that the decrease in oxygen production may be due to mass loss in the pre-supernova evolution of metal-rich progenitors.

We present lithium abundances for 28 halo subgiants based on high resolution, high signal-to-noise ratio spectra. Excluding the known lithium-rich subgiant BD +23 3912, the maximum abundances are log epsilon(Li) = 2.35. While subgiants evolve from stars hotter than the main sequence turn-off with shallower convection zones that may have depleted lithium to a lesser degree, lithium abundances in halo subgiants are not in agreement with the primordial value as predicted from standard big bang nucleosynthesis combined with recent results from WMAP.

We have measured the chemical composition of 14 stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) using high S/N Keck HIRES echelle spectra. For the Sgr dSph stars with [Fe/H]>=-1 the abundances are highly unusual, showing a striking enhancement in heavy s-process elements, increasing with [Fe/H], deficiencies of the a-elements (O, Si, Ca, and Ti), deficiencies of Al and Na, and deficiencies of the odd-numbered iron-peak elements Mn and Cu. Our abundances suggest that the composition of the metal-rich Sgr dSph stars is dominated by the ejecta of an old, metal-poor population, including products of AGB stars and type Ia supernovae (SN).

We present the first results of a new abundance survey of the MilkyWay bulge based on Keck HIRES spectra of 27 K giants in the Baade'sWindow (l = 1 degrees, b= -4 degrees) field. The spectral data used in this study are of much higher resolution and signal-to noise ratio than previous optical studies of Galactic bulge stars. The [Fe/H] values of our stars, which range between -1.29 and + 0.51, were used to recalibrate large low-resolution surveys of bulge stars. Our best value for the mean [Fe/H] of the bulge is -0.10 +/- 0.04. This mean value is similar to the mean metallicity of the local disk and indicates that there cannot be a strong metallicity gradient inside the solar circle. The metallicity distribution of stars confirms that the bulge does not suffer from the so-called G dwarf problem. This paper also details the new abundance techniques necessary to analyze very metal-rich K giants, including a new Fe line list and regions of low blanketing for continuum identification.

We present u'g'r'i'BV photometry and optical spectroscopy of the Type Ib/Ic SN 2005bf covering the first similar to 100 days following discovery. The u'g'BV light curves displayed an unprecedented morphology among Type Ib/Ic supernovae, with an initial maximum some 2 weeks after discovery and a second, main maximum about 25 days after that. The bolometric light curve indicates that SN 2005bf was a remarkably luminous event, radiating at least 6.3 x 10(42) ergs s(-1) at maximum light and a total of 2.1 x 10(49) ergs during the first 75 days after the explosion. Spectroscopically, SN 2005bf underwent a unique transformation from a Type Ic-like event at early times to a typical Type Ib supernova at later phases. The initial maximum in u'g'BV was accompanied by the presence in the spectrum of high-velocity (> 14,000 km s(-1)) absorption lines of Fe II, Ca II, and H I. The photospheric velocity derived from spectra at early epochs was below 10,000 km s(-1), which is unusually low compared with ordinary Type Ib supernovae. We describe one-dimensional computer simulations that attempt to account for these remarkable properties. The most favored model is that of a very energetic (2 x 10(51) ergs), asymmetric explosion of a massive ( 8.3 M-circle dot) Wolf-Rayet WN star that had lost most of its hydrogen envelope. We speculate that an unobserved relativistic jet was launched producing a two-component explosion consisting of ( 1) a polar explosion containing a small fraction of the total mass and moving at high velocity and ( 2) the explosion of the rest of the star. At first, only the polar explosion is observed, producing the initial maximum and the high-velocity absorption-line spectrum resembling a Type Ic event. At late times, this fast- moving component becomes optically thin, revealing the more slowly moving explosion of the rest of the star and transforming the observed spectrum to that of a typical Type Ib supernova. If this scenario is correct, then SN 2005bf is the best example to date of a transition object between normal Type Ib/Ic supernovae and gamma-ray bursts.

We have carried out a detailed abundance analysis using high-dispersion spectra from HIRES at Keck for a sample of 16 carbon stars found among candidate extremely metal-poor (EMP) stars from the Hamburg/ESO Survey (HES). We find that the Fe metallicities for the cooler C stars (T-eff similar to 5100 K) have been underestimated by a factor of similar to 10 by the standard HES tools. The results presented here provided crucial supporting data used recently by Cohen et al. to derive the frequency of C stars among EMP stars. C enhancement in these EMP C stars appears to be independent of Fe metallicity and approximately constant at similar to 1/5 the solar epsilon(C). The C enhancement shows some evidence of decreasing with decreasing T-eff (increasing luminosity), presumably due to mixing and dredge-up of C-depleted material. The mostly low C-12/C-13 ratios (similar to 4) and the high N abundances in many of these stars suggest that material that has been through proton burning via the CN cycle comprises most of the stellar envelope. C enhancement in this sample is associated with strong enrichment of heavy nuclei beyond the Fe peak for 12 of the 16 stars. The remaining C stars from the HES, which tend to be the most Fe-poor, show no evidence for enhancement of the heavy elements. Very high enhancements of lead are detected in some of the C stars with highly enhanced Ba. The strong lead lines, the high Ba/Eu ratios, and the high ratios of abundances of the diagnostic elements in the first and second s-process peaks demonstrate that the s-process is responsible for the enhancement of the heavy elements for the majority of the C stars in our sample. The low C-12/C-13 ratios and large C and N enhancements of the EMP C stars are more extreme than those of intrinsic asymptotic giant branch C stars of near-solar Fe metallicity, but closer to the composition of CH stars. Our subsample of EMP C stars without s-process enhancement is reminiscent of the R-type C stars in the solar neighborhood; thus, we expect that they are formed by similar mechanisms. We suggest that both the s-process-enhanced and Ba-normal C stars result from phenomena associated with mass transfer in binary systems. This leads directly to the progression from C stars to CH stars and then to Ba stars as the Fe metallicity increases.

We present an abundance analysis for the extremely metal-poor (EMP) star HE 1424-0241 based on high dispersion spectra from HIRES at Keck. This star is a giant on the lower red giant branch with [Fe/H] similar to -4.0 dex. Relative to Fe, HE 1424-0241 has normal Mg, but it shows a very large deficiency of Si, with is an element of(Si)/is an element of(Fe) similar to and that of all previously known EMP giants or dwarfs. It also has a moderately large 1/10 is an element of(Si)/is an element of(Mg) similar to 1/25 deficiency of Ca and a smaller deficit of Ti, combined with enhanced Mn and Co and normal or low C. We suggest that in HE 1424-0241 we see the effect of a very small number of contributing supernovae, and that the SNe II contributing to the chemical inventory of HE 1424-0241 were biased in progenitor mass or in explosion characteristics so as to reproduce its abnormal extremely low Si/Mg ratio. HE 1424-0241 shows a deficiency of the explosive alpha-burning elements Si, Ca, and Ti coupled with a ratio [Mg/Fe] normal for EMP stars; Mg is produced via hydrostatic alpha-burning. The latest models of nucleosynthesis in SNe II fail to reproduce the abundance ratios seen in HE 1424-0241 for any combination of the parameter space of core-collapse explosions they explore.