Abundance ratios in extreme metal-poor stars

In extremely metal-poor stars ([Fe/H]less than or equal to -2.5) the neutron capture elements are characterized by a 300-fold dispersion in M/Fe ratios which decreases with increasing metallicity, the median M/Fe ratio increases with increasing [Fe/H], but the average M/Fe number ratio is approximately constant. These observations are consistent with a highly dispersed intrinsic yield of neutron-capture elements in supernova (SN) events, and a progression to increasing metallicity by stochastic chemical evolution. The abundance trends indicate that the synthesis of elements heavier than barium was dominated by the r-process. The Sr/Ba ratio shows a dispersion which suggests a stochastic source of Sr in excess of the r-process value; possibly due to the alpha-rich freeze out. The iron-peak elements Cr, Mn, and Co show non-solar abundance ratios for extreme metal-poor stars, and no measurable intrinsic dispersion relative to iron. We discuss chemical evolution models which explain these observations.