Low-mass supernovae in the early Galactic halo: source of the double r/s-process enriched halo stars?

Several stars at the low-metallicity extreme of the Galactic halo ([Fe/H]=-2.5) show strong enhancements of both s-process and r-process elements. The presence of s-process elements in main-sequence stars is explained via mass transfer from an asymptotic giant branch (AGB) companion star in a binary system. r-process elements originate in Type II supernovae and also require mass transfer. It is, however, unclear how pollution by both an AGB star and a supernova could have occurred. Here I show that the initial-final mass relation steepens at low metallicity, due to low mass-loss efficiency. This may cause the degenerate cores of low-Z, high-mass AGB stars to reach the Chandrasekhar mass, leading to an Iben & Renzini Type 1.5 supernova. Such supernovae can explain both the enhancement patterns and the metallicity dependence of the double-enhanced halo stars. Reduced mass-loss efficiency predicts more massive remnants in metal-poor globular clusters. The evidence for a high M/L population in the cores of globular clusters is briefly discussed.