Detailed abundances for 28 metal-poor stars: Stellar relics in the milky way

We present the results of an abundance analysis for a sample of stars with -4 < [Fe/H] < -2. The data were obtained with the HIRES spectrograph at Keck Observatory. The set includes 28 stars, with effective temperature ranging from 4800 to 6600 K. For 13 stars with [Fe/H] < -2.6, including nine with [Fe/H] < -3.0 and one with [Fe/H] = -4.0, these are the first reported detailed abundances. For the most metal-poor star in our sample, CS 30336-049, we measure an abundance pattern that is very similar to stars in the range [Fe/H] similar to -3.5, including a normal C + N abundance. We also find that it has very low but measurable Sr and Ba, indicating some neutron-capture activity even at this low of a metallicity. We explore this issue further by examining other very neutron capture-deficient stars and find that, at the lowest levels, [Ba/Sr] exhibits the ratio of the main r-process. We also report on a new r-process-enhanced star, CS 31078-018. This star has [Fe/H] = -2.85, [Eu/Fe] = 1.23, and [Ba/Eu] - -0.51. CS 31078-018 exhibits an actinide boost,'' i. e., much higher [Th/Eu] than expected and at a similar level to CS 31082 001. Our spectra allow us to further constrain the abundance scatter at low metallicities, which we then use to fit to the zero-metallicity Type II supernova yields of Heger & Woosley (2008). We find that supernovae with progenitor masses between 10 and 20 M-circle dot provide the best matches to our abundances.