Accretion Regimes of Neutrino-Cooled Flows onto Black Holes

TL;DR

This paper models neutrino-cooled accretion disks around black holes, exploring different accretion regimes and their potential for r-process nucleosynthesis and kilonovae transients across a broad parameter space.

Contribution

It presents a comprehensive general-relativistic, viscous-hydrodynamic model of accretion disks, characterizing accretion regimes and thresholds with analytic approximations for the first time.

Findings

Accretion regimes are separated by characteristic thresholds following power laws.

Outflows can produce r-process elements up to black hole masses of ~3000 solar masses.

Different kilonovae types ('red' and 'blue') are associated with specific black hole mass ranges.

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 $\dot{M}_{\rm char}$ that follow power laws $\dot M_{\rm char}\propto M_{\bullet}^\alpha \alpha^\beta$ 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 $M_\bullet \lesssim 3000 M_\odot$. These give rise to lanthanide-bearing 'red' super-kilonovae transients mostly for $M_\bullet \lesssim 200-500 M_\odot$ and lanthanide suppressed 'blue' super-kilonovae for larger $M_\bullet$. Proton-rich outflows can develop specifically for large black hole masses ($M_\bullet \gtrsim 100 M_\odot$) in certain accretion regimes, which may give rise to proton-rich isotopes via the $\nu$p-process.