Formation and Evolution of Hypernova Progenitors in Massive Binary Systems

Abstract

If long γ-ray bursts are produced by hypernovae, a problem that must be confronted is how the core of the hypernova progenitor retains or acquires sufficient angular momentum to produce the requisite axisymmetric collapse. Physical processes during the evolution of an isolated massive star will tend to extract any initial angular momentum from the stellar core, rendering it difficult for such a star to become a hypernova. However, a substantial fraction of massive stars are members of binary systems. Tidal locking, mass transfer, or stellar merger in an evolved massive binary may lead to the transfer of orbital angular momentum to the core of one of the stars (or the merged star), sufficient to produce the progenitor of a hypernova. We have developed a new binary stellar-evolution code that includes the effects of mass and angular-momentum transfer between the component stars and the subsequent transport of angular momentum through one of the stars. This transport is affected by dynamical and secular shear instabilities, convective motions, the critical layer instability, and gravity waves. Our code treats in a self-consistent way the dynamical distortion of the star resulting from the induced rapid differential rotation. The results of our numerical computations indicate that late main-sequence or early post-mainsequence accretion from a binary companion onto a star with an initial mass ≥ 20Msun may produce a stellar core that is rotating sufficiently rapidly when it collapses to provide the initial conditions necessary for a hypernova event. Our results also indicate that the merger of a late post-main-sequence star with its binary companion, as considered by Ivanova, Podsiadlowski & Spruit (2002), may also lead to a hypernova event in the stellar core but is unlikely to produce an observable γ-ray burst.