http://arxiv.org/abs/2205.14158
Collapsar disks have been proposed to be rich factories of heavy elements, but the major question of whether their outflows are neutron-rich, and could therefore represent significant sites of the rapid neutron-capture (r-) process, or dominated by iron-group elements, remains unresolved. We present global models of collapsars that start from a stellar progenitor and self-consistently describe the evolution of the disk, its composition, and its outflows in response to the imploding stellar mantle using energy-dependent M1 neutrino transport and an alpha-viscosity to capture turbulent angular-momentum transport. We find that a neutron-rich, neutrino-dominated accretion flow (NDAF) is established only marginally–either for very short times or very low viscosities–because the disk tends to disintegrate into an advective disk (ADAF) soon after its formation, launching powerful outflows but preventing it from developing a hot and dense, and therefore neutron-rich core. Viscous outflows disrupt the star within ~100 s with explosion energies close to that of hypernovae. If viscosity is neglected, a stable NDAF with disk mass of about 1 Msun is formed but is unable to release neutron-rich ejecta, while it produces a relatively mild explosion powered by a neutrino-driven wind blown off its surface. With ejecta electron fractions close to 0.5, all models presumably produce large amounts of 56Ni. Our results suggest that neutron-rich disks, and correspondingly r-process viable outflows, do not occur as readily as in remnant disks of neutron-star mergers. A weak effective viscosity generated by magnetohydrodynamic turbulence would improve the prospects for obtaining neutron-rich ejecta.
O. Just, M. Aloy, M. Obergaulinger, et. al.
Tue, 31 May 22
39/89
Comments: 7 pages, 4 figures, 1 table, submitted to ApJL
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