s/alpha/Fe Abundance Ratios in Halo Field Stars: Is there a Globular Cluster Connection?

Abstract

We try to understand the s- and r-process elements vs Ti/Fe plots derived by Jehin et al. (1999) for mildly metal-poor stars within the framework of the analytical semi-empirical models for these elements by Pagel & Tautvaisiene (1995, 1997). Jehin et al. distinguished two Pop II subgroups: IIa with alpha/Fe and s-elements/Fe increasing together, which they attribute to pure SNII activity, and IIb with constant alpha/Fe and a range in s/Fe which they attribute to a prolonged accretion phase in parent globular clusters. However, their sample consists mainly of thick-disk stars with only 4 clear halo members, of which two are `anomalous' in the sense defined by Nissen & Schuster (1997). Only the remaining two halo stars (and one in Nissen & Schuster's sample) depart significantly from Y/Ti (or s/alpha) ratios predicted by our model.