Binary merger progenitors for γ-ray bursts and hypernovae

The collapsar model, the now leading model for the engine behind gamma-ray bursts and hypernovae, requires that a star collapses to form a black hole surrounded by an accretion disk of high angular momentum material. The current best theoretical stellar models, however, do not retain enough angular momentum in the core of the star to make a centrifugally supported disk. In this paper, we present the first calculations of the helium star / helium star merger progenitors for the collapsar model. These progenitors invoke the merger of two helium cores during the common-envelope inspiral phase of a binary system. We find that in some cases, the merger can produce cores that are rotating 3 - 10 times faster than single stars. He star / He star gamma-ray burst progenitors have a very different redshift distribution than their single-star gamma-ray burst progenitors, and we discuss how gamma-ray burst observations can constrain these progenitors.