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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 KeckObservatory. The set includes 28 stars, with effective temperature ranging from4800 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 detailedabundances. For the most metal-poor star in our sample, CS 30336-049, wemeasure an abundance pattern that is very similar to stars in the range[Fe/H]$\sim-3.5$, including a normal C+N abundance. We also find that it hasvery low but measurable Sr and Ba, indicating some neutron-capture activityeven at this low of a metallicity. We explore this issue further by examiningother 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 newr-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 ``actinideboost'', i.e. much higher [Th/Eu] than expected and at a similar level to CS31082-001. Our spectra allow us to further constrain the abundance scatter atlow metallicities, which we then use to fit to the zero-metallicity Type IIsupernova yields of Heger & Woosley (2008). We find that supernovae withprogenitor masses between 10 and 20 M$_{\odot}$ provide the best matches to ourabundances.

Detailed Abundances for 28 Metal-poor Stars: Stellar Relics in the Milky Way