Chemical Substructure in the Milky Way Halo: A New Population of Old Stars

We report the results of a coherent study of a new class of halo stars defined on the basis of the chemical compositions of three metal-poor objects (½Fe=H �’� 2) that exhibit unusually low abundances of � -element (Mg, Si, Ca) and neutron-capture (Sr, Y, Ba) material. Our analyses confirm and expand on earlier reports of atypical � - and neutron-capture abundances in BD +80 � 245, G4-36, and CS 22966-043. We also find that the latter two stars exhibit unusual relative abundance enhancements within the iron peak (Cr, Mn, Ni, Zn), along with what may be large abundances of Ga, an element not previously reported as being observed in any metal-poor star. These results provide further evidence that chemical enrichment and star formation histories varied from region to region within the Milky Way halo. Comparing the chemical abundances of the newly identified stellar population to supernova model yields, we derive supernova ratios of Type Ia versus Type II events in the range of 0:6dðNIa=NIIÞNew Popd1:3. For the Sun, we derive 0:18 � 0:01 < ðNIa=NIIÞ� < 0:25 � 0:06, supernova ratios in good agreement with values found in the literature. Given the relatively low metallicity and relatively high NIa=NII hi ratios of the low-� stars studied here, these objects may have been born from material produced in the yields of the earliest Type Ia supernova events. We also report the results of a preliminary attempt to employ the observed chemical abundances of low-metallicity stars in the identification, and possible cosmic evolution, of Type Ia supernova progenitors, and we discuss the limitations of current model yields. Subject headings: Galaxy: abundances — Galaxy: halo — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: Population II — supernovae: general On-line material: color figures, machine-readable table