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 ] \sim - 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 .
We find that supernovae with progenitor masses between 10 and 20 M _ { \odot } provide the best matches to our abundances .