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-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_ provide the best matches to our abundances.