Formation and evolution of hypernova progenitors in massive binary systems

Abstract

The massive stellar progenitor of a hypernova explosion and an associated gamma-ray burst must satisfy two primary constraints: (1) the outer layers of the stellar core must possess sufficient angular momentum to form a centrifugally supported torus about the collapsed central object (a Kerr black hole); and, (2) the envelope of the star must not be excessively massive or distended, so that the energetic, ultrarelativistic outflow generated by the central engine in the core of the star does not risk being smothered before it can escape from the star and expand outward to produce a gamma-ray burst. Physical processes which occur during the evolution of an isolated massive star will tend to diminish its initial angular momentum content, rendering it difficult for such a star to become a hypernova progenitor since its core will likely no longer spin rapidly enough to support a torus about its collapsed core. However, a substantial fraction of massive stars are members of binary systems. Tidal locking, mass transfer, or stellar merger in an evolved massive binary system could possibly lead to the transfer of orbital angular momentum to the core of one of the stars (or to the core of a merged star, if a common envelope develops), of sufficient magnitude to produce the core of a successful hypernova progenitor (constraint 1). Further interaction between the stars or their compact remnants could lead to the loss of the hydrogen and possibly helium envelopes of one of the stars (constraint 2). We have developed a new one-dimensional stellar evolution code that includes the effects of rotation on equilibrium stellar structure, and calculates the transport of angular momentum through the stellar interior due to convection, dynamical and secular shear instabilities, and gravity (buoyancy) waves.

(cont.) We have used this code to calculate a variety of evolutionary sequences involving the transfer of mass from one component of the binary system to the other. We have also calculated an evolutionary sequence ending in the merger of one component of the system with the core of the other, induced by a prior common-envelope phase. We find that over a wide range of initial binary system parameters, the initially less massive component of the system can accrete a substantial amount of mass and angular momentum from the initially more massive component. The accreted angular momentum is efficiently transported inward from the surface of the accreting star toward its core by a combination of convection and dynamical and secular shear instabilities. If accretion commences while the accretor is still on the main sequence, we find that the inward-progressing wave of angular momentum can penetrate the core of the mass-gaining star, contributing to its store of rotational angular momentum without the need for gravity wave transport of angular momentum across the core- envelope interface. These stars end their evolution (just prior to core carbon ignition) as red supergiants, with cores endowed with sufficient angular momentum to give rise to a hypernova explosion. We also find that a subsequent common-envelope phase with the compact remnant of the primary might result in the ejection of the accretor's red-giant envelope, leaving either a bare helium or carbon-oxygen star. Such a star would be expected to explode in a Type lb or Ic supernova/hypernova.