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 (collapsars) and the accretion-induced collapse of rapidly rotating white dwarfs. Due to Pauli blocking as electrons become degenerate at sufficiently high accretion rates $\dot{M}$, the resulting 'self-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, super-collapsars 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 $\dot{M}$, across a vast parameter space of $\dot{M}\sim 10^{-6}-10^6{M}_{\odot }{\text{s}}^{-1}$, black hole masses of $M_{\bullet }\sim 1-10^4{M}_{\odot }$ and dimensionless spins of ${\chi }_{\bullet }\in [0,1)$, as well as $\alpha$-viscosity values of $\alpha \sim 10^{-3}-1$. We show that these accretion regimes are separated by characteristic thresholds ${\dot{M}}_{\mathrm{char}}$ that follow power laws ${\dot{M}}_{\mathrm{char}}\propto M_{\bullet }^{\alpha }{\alpha }^{\beta }$ and that can be understood based on analytic approximations we derive. We find that outflows from such disks are promising sites of r-process nucleosynthesis up to $M_{\bullet }\lesssim 3000M_{\odot }$. These give rise to lanthanide-bearing 'red' super-kilonovae transients mostly for $M_{\bullet }\lesssim 200-500M_{\odot }$ and lanthanide suppressed 'blue' super-kilonovae for larger $M_{\bullet }$. Proton-rich outflows can develop specifically for large black hole masses ($M_{\bullet }\gtrsim 100M_{\odot }$) in certain accretion regimes, which may give rise to proton-rich isotopes via the $\nu$p-process.

39 pages, 12 figures