Numerical simulations of the random angular momentum in convection II: delayed explosions of red supergiants following "failed'' supernovae

TL;DR

This study uses 3D hydrodynamical simulations to explore how convective angular momentum in supergiant envelopes can cause delayed, energetic explosions and luminous transients resembling red novae during black hole formation.

Contribution

It demonstrates that convective angular momentum can lead to nearly complete envelope ejection with observable signatures, providing a new mechanism for supernova-like transients in failed supernovae.

Findings

Nearly complete envelope ejection with >10^48 erg energy.

Outflow speeds exceeding 200 km/s.

Transient light curves with red plateau lasting hundreds of days.

Abstract

When collapse of the iron core in a massive red or yellow supergiant does not lead to an energetic supernova, a significant fraction of the convective hydrogen envelope will fall in towards the black hole formed from the collapsing core. The random velocity field in the convective envelope results in finite specific angular momentum in each infalling shell. Using 3D hydrodynamical simulations, we follow the infall of this material to small radii, resolving the circularization radii of the flow. We show that infall of the convective envelope leads to nearly complete envelope ejection in a $\gtrsim$ 10$^{48}$ erg explosion with outflow speeds of $\gtrsim$ 200 km/s. The light curve of such an explosion would show a characteristic, red plateau as the ejecta cools and a hydrogen recombination front recedes through the expanding ejecta. Adopting supernova IIp scalings, the event would have a plateau luminosity of $\gtrsim$ 10$^{40}$ erg/s and a duration of several hundreds of days. These events would appear quite similar to luminous red novae with red or yellow supergiant progenitors; some luminous red novae may, in fact, be signposts of black hole formation. The mechanism studied here produces more energetic explosions than the weak shock generated from the radiation of neutrino energy during the proto-neutron star phase. Because we cannot simulate all the way to the horizon, our results are likely lower limits on the energy and luminosity of transients produced during the collapse of a red or yellow supergiant to form a black hole.