ABUNDANCES OF 4 ULTRA-METAL-DEFICIENT STARS

High-resolution echelle spectra have been obtained for four of the extremely metal-deficient stars from the HK objective-prism/interference-filter survey of Beers, Preston, and Shectman. Model atmosphere analysis shows that the abundances of these objects lie in the range -4.2 < [Fe/H] < -3.2. The results are in excellent accord with the earlier investigation of two of these stars by Molaro and coworkers and confirm that the HK survey is a rich source of ultra-metal-deficient stars. Comparison of the abundances of the elements relative to iron with literature values for metal-poor stars leads to the following conclusions: (1) For material with [Fe/H] < -3.5 the alpha-rich elements show the same enrichment as found in the range -3.5 < [Fe/H] < -2.5, consistent with the view that to the lowest abundances currently observed these elements were synthesized in stars of mass 10 < M/M. < 30. (2) When odd-Z elements are compared with adjacent even-Z ones, a significant odd-even effect is seen at the lowest metallicities. The behavior of [Al/Mg] is in good accord with precepts suggested by Woosley in the context of nucleosynthesis in stars of mass 25 M.. (3) Our data strengthen earlier suggestions for a significant range in [Mn/Fe] at [Fe/H] = -4.0. (4) Strontium appears to be over-deficient with respect to iron for [Fe/H] < -3.5, in some contradistinction to its behavior for abundances greater than this limit. While further work is needed to establish the reality of the effect, we argue that this behavior may lead to a constraint on the elusive astrophysical site of the r-process. We consider the possibility presented by Mathews and Cowan that relatively low-mass Type II supernovae may be responsible for the production of the heaviest elements at the earliest times. We emphasize that the apparent difference between the behavior of Sr and Eu in extremely metal-poor stars has the potential to strongly constrain models of lower mass Type II supernovae such as those presented by Woosley and Hoffman.