Collapsar disk outflows I: Viscous hydrodynamic evolution in axisymmetry

Abstract

We investigate mass ejection from accretion disks formed during the collapse of rapidly-rotating Wolf-Rayet stars. The neutrino-cooled, black hole (BH) accretion disk system that forms at the center of the star -- and the ensuing outflows -- provide the conditions for these systems to be candidate $r$-process element production sites and potential progenitors of broad-lined Type Ic (Ic-BL) supernovae. Here we present global, long-term axisymmetric hydrodynamic simulations of collapsar disks that include angular momentum transport through shear viscosity, neutrino emission and absorption, a 19-isotope nuclear reaction network and nuclear statistical equilibrium solver, a pseudo-Newtonian BH with mass and spin modified by accreted matter, and self-gravity. Starting from a stellar profile collapsed in spherical symmetry, our models capture disk formation self-consistently, and are evolved until after the shock wave -- driven by disk winds -- reaches the surface of the star. None of our models achieve sufficient neutronization to eject significant amounts of $r$-process elements (detailed nucleosynthesis calculations will follow in a companion paper). Sufficient $^{56}$Ni is produced to power a typical type Ic-BL supernova light curve, but the average asymptotic velocity is a factor $\sim 2-3$ times too slow to account for the typical line widths in type Ic-BL supernova spectra. The gap in neutrino emission between BH formation and shocked disk formation, and the magnitude of the subsequent peak in emission, would be observable diagnostics of the internal conditions of the progenitor in a galactic collapsar. Periodic oscillations of the shocked disk prior to its expansion are also a potential observable through their impact on the the neutrino and gravitational wave signals.