Dynamics and Composition of Collapsar Disk Outflows

Abstract

We investigate mass ejection from accretion disks formed during the collapse of rapidly-rotating Wolf-Rayet stars, also known as collapsars. The neutrino-cooled, black hole (BH) accretion disk that forms at the center of the star — and the ensuing outflows – provides 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. Sufficient 56 Ni is produced to power a typical type Ic-BL supernova light curve, but the average asymptotic velocity is a factor ∼ 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. We also analyze passive tracer particles included in our simulations, used for post-processing with a larger nuclear reaction network, and we evolve models in which we modify the rotation profile of the progenitor star to maximize neutrino reprocessing of circularized mass shells. All of our models produce several M⊙ of oxygen, followed by about a solar mass of carbon, neon, and nickel, with other alpha elements produced in smaller quantities. Only one of our models, with the lowest strength of viscous angular momentum transport, yields significant amounts of first r-process peak elements, with negligible yields at higher nuclear masses. The rest of the set produces very small or negligible quantities of elements beyond the iron group. Models that produce the heaviest elements (up to A ∼ 200) do so along the proton-rich side of the valley of stability at high entropy (s/kB ∼ 80), pointing to the rapid proton capture process (rp-process) as a mechanism that operates in collapsars. The absence of neutron-rich ejecta proves to be insensitive to changes in the rotation profile of the star, suggesting that heavy r-process elements are difficult to produce in collapsars if no large-scale poloidal magnetic field is present in the disk to drive outflows during neutronization.