The proto-neutron star phase of the collapsar model and the route to long-soft gamma-ray bursts and hypernovae

Recent stellar evolutionary calculations of massive low-metallicity fast-rotating main-sequence stars yield iron cores at collapse that are endowed with high angular momentum. It is thought that high angular momentum and black hole formation are critical ingredients for the collapsar model of long-soft gamma-ray bursts (GRBs). We present two-dimensional multigroup, flux-limited-diffusion MHD simulations of the collapse, bounce, and immediate postbounce phases of a 35 M-circle dot collapsar-candidate model of Woosley & Heger. Provided that the magnetorotational instability (MRI) operates in the differentially rotating surface layers of the millisecond-period neutron star, we find that a magnetically driven explosion occurs during the proto-neutron star phase, in the form of a baryon-loaded nonrelativistic jet, and that a black hole, which is central to the collapsar model, does not form. Paradoxically, although much uncertainty surrounds stellar mass loss, angular momentum transport, magnetic fields, and the MRI, current models of chemically homogeneous evolution at low metallicity yield massive stars with iron cores that may have too much angular momentum to avoid a magnetically driven, hypernova-like explosion in the immediate postbounce phase. We surmise that fast rotation in the iron core may inhibit, rather than enable, collapsar formation, which requires a large angular momentum above the core but not in it. Variations in the angular momentum distribution of massive stars at core collapse might explain both the diversity of Type Ic supernovae/hypernovae and their possible association with a GRB. A corollary might be that, through its effect on magnetic field amplification, the angular momentum distribution, rather than the progenitor mass, is the distinguishing characteristic of these outcomes.