[Paper Review] Collapse of rotating massive stars leading to black hole formation and energetic supernovae

Sho Fujibayashi, Yuichiro Sekiguchi+2 more|arXiv (Cornell University)|Dec 7, 2022

Gamma-ray bursts and supernovae5 citations

TL;DR

This paper proposes that rotating massive stars collapsing into black holes can power energetic supernovae via viscous heating in accretion disks, with explosion energies reaching >3×10⁵¹ erg and ⁵⁶Ni masses ≥0.1M☉—potentially explaining broad-lined Ic supernovae. The mechanism relies on disk outflows driven by viscous heating when neutrino cooling becomes inefficient, particularly in rapidly rotating progenitors with high mass infall rates.

ABSTRACT

We explore a possible scenario of the explosion as a result of core collapses of rotating massive stars that leave a black hole by performing a radiation-viscous-hydrodynamics simulation in numerical relativity. We take moderately and rapidly rotating compact pre-collapse stellar models derived in stellar evolution calculations as the initial conditions. We find that the viscous heating in the disk formed around the central black hole powers an outflow. For rapidly rotating models, the explosion energy is $\gtrsim 3 imes10^{51}$ erg, which is comparable to or larger than that of typical stripped-envelope supernovae, indicating that a fraction of such supernovae may be explosions powered by black-hole accretion disks. The explosion energy is still increasing at the end of the simulations with a rate of $>10^{50}$ erg/s, and thus, it may reach $\sim10^{52}$ erg. The nucleosynthesis calculation shows that the mass of $^{56}$Ni amounts to $\gtrsim 0.1M_\odot$, which, together with the high explosion energy, satisfies the required amount for broad-lined type Ic supernovae. The moderately rotating models predict small ejecta mass of order $0.1M_\odot$ and explosion energy of $\lesssim 10^{51}$ erg. Due to the small ejecta mass, these models may predict a short-timescale transient with the rise time 3$-$5 d. It can lead to a bright ($\sim10^{44}$ erg/s) transient like superluminous supernovae in the presence of dense massive circum-stellar medium. Irrespective of the models, the lowest value of the electron fraction of the ejecta is $\gtrsim 0.4$, and thus, the synthesis of heavy $r$-process elements is not found in our calculation.

Motivation & Objective

• To investigate whether viscous heating in accretion disks around black holes formed from collapsing rotating massive stars can power energetic supernovae.
• To determine the conditions under which such explosions produce high explosion energies and significant ⁵⁶Ni yields.
• To assess the viability of this mechanism as a progenitor scenario for broad-lined Ic supernovae.

Proposed method

• Performing 3D general relativistic radiation-viscous-hydrodynamics simulations of core collapse in rotating massive stars.
• Using stellar evolution models as initial conditions for 9M☉ and 20M☉ zero-age main sequence stars with moderate and rapid rotation.
• Implementing an alpha-viscosity model (ν = α_vis c_s H) to simulate turbulent angular momentum transport and viscous heating in the disk.
• Tracking neutrino cooling efficiency and comparing it with viscous heating rates to identify regions where outflows are launched.
• Calculating nucleosynthesis yields, particularly ⁵⁶Ni mass, in the ejecta from high-temperature regions of the disk outflow.
• Monitoring the time evolution of explosion energy and ejecta mass to assess whether simulations reach or exceed observed supernova energetics.

Experimental results

Research questions

• RQ1 Can viscous heating in a black hole accretion disk drive an explosion in collapsing rotating massive stars that form black holes?
• RQ2 What are the resulting explosion energies and ejecta masses for moderately versus rapidly rotating progenitors?
• RQ3 Can the ⁵⁶Ni yields from such explosions match those inferred for broad-lined Ic supernovae?
• RQ4 How does the electron fraction (Yₑ) in the ejecta affect the synthesis of r-process elements?
• RQ5 Does the explosion energy continue to increase beyond the simulation timescale, suggesting even higher energies in nature?

Key findings

• For moderately rotating 20M☉ models, the ejecta mass is ≈0.1M☉ and explosion energy is ≤10⁵¹ erg, suggesting a short-rise-time transient (3–5 days) that could appear superluminous in a dense circumstellar medium.
• For rapidly rotating models with high mass infall rates, the explosion energy reaches >3×10⁵¹ erg and is still increasing at the end of simulations, potentially reaching ~10⁵² erg.
• The ⁵⁶Ni mass in the ejecta of rapidly rotating models is ≈0.56M☉, which is sufficient to explain the typical 0.4M☉ inferred for broad-lined Ic supernovae.
• The electron fraction (Yₑ) in the ejecta is ≥0.46 in the most rapidly rotating model, ruling out significant r-process nucleosynthesis in these simulations.
• Outflows are launched primarily from the disk surface region where neutrino cooling is inefficient, despite higher Yₑ in these regions, due to strong viscous heating and low cooling efficiency.
• The simulation results represent lower bounds, as both ejecta mass and explosion energy are still increasing at t = 30 s, indicating potential for even more energetic events in nature.