Accretion Regimes of Neutrino-cooled Flows onto Black Holes

Abstract Neutrino-cooled accretion disks can form in the aftermath of neutron-star mergers as well as during the collapse of rapidly rotating massive stars and the accretion-induced collapse of rapidly rotating white dwarfs. Due to Pauli blocking as electrons become degenerate at sufficiently high accretion rates M ̇ , the resulting neutronization of the dissociated accreting plasma makes these astrophysical systems promising sources of rapid neutron-capture nucleosynthesis (the r -process). We present a one-dimensional, general-relativistic, viscous-hydrodynamic model of neutrino-cooled accretion disks around black holes. With collapsars, supercollapsars, and very massive star collapse in mind, we chart the composition of the accretion flow and systematically explore different radiatively efficient and inefficient accretion regimes with increasing M ̇ , across a vast parameter space of M ̇ ∼ 1 0 − 6 − 1 0 6 M ⊙ s − 1 , black hole masses of M • ∼ 1–10 4 M ⊙ , and dimensionless spins of χ • ∈ [0, 1), as well as α -viscosity values of α ∼ 10 −3 –1. We show that these accretion regimes are separated by characteristic thresholds M ̇ char that follow power laws M ̇ char ∝ M • α α β and that can be understood based on analytic approximations we derive. We find that outflows from such disks are promising sites for r -process nucleosynthesis up to M • ≲ 3000 M ⊙ . These give rise to lanthanide-bearing “red” super-kilonovae transients mostly for M • ≲ 200–500 M ⊙ and lanthanide-suppressed “blue” super-kilonovae for larger M • . Proton-rich outflows can develop specifically for large black hole masses ( M • ≳ 100 M ⊙ ) in certain accretion regimes, which may give rise to proton-rich isotopes via the neutrino-induced proton-capture process (the ν p-process).