Collapsar disk outflows II: Heavy element production

Abstract

We investigate nucleosynthesis in the sub-relativistic outflows from black hole (BH) accretion disks formed in failed supernovae from rapidly-rotating Wolf-Rayet stars. These disks reach the neutrino-cooled regime during a portion of their evolution, undergoing significant neutronization and thus having the potential to support the $r$-process. Here, we analyze the formation of heavy elements in the ejecta from global, axisymmetric, long-term, viscous hydrodynamic simulations of these systems that include neutrino emission and absorption, Newtonian self-gravity, a pseudo-Newtonian potential for the BH gravity, and a 19-isotope nuclear network. Tracer particles are used for post-processing with a larger network. In addition to analyzing models from a previous paper, we present new 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 $M_\odot$ of O, followed by about a solar mass of C, Ne, and Ni, with other alpha elements produced in smaller quantities. Only one of our models, with the lowest viscosity, yields significant amounts of first $r$-process peak elements, with negligible yields at higher nuclear masses. The rest of the set, including models with a modified rotation profile, produces very small or negligible quantities of elements beyond the iron group. Models that produce the heaviest elements (up to $A\sim200$) do so along the proton-rich side of the valley of stability at high entropy ($s/k_B\sim80$), pointing to the $rp$-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.