Gas Dynamics of the Nickel-56 Decay Heating in Pair-instability Supernovae

Ke-Jung Chen,Daniel J Whalen,S E Woosley

doi:10.3847/1538-4357/ab9819

Abstract

Abstract Very massive 140–260 M ⊙ stars can die as highly energetic pair-instability supernovae (PI SNe) with energies of up to 100 times those of core-collapse SNe that can completely destroy the star, leaving no compact remnant behind. These explosions can synthesize 0.1–30 M ⊙ of radioactive 56Ni, which can cause them to rebrighten at later times when photons due to 56Ni decay diffuse out of the ejecta. However, heat from the decay of such large masses of 56Ni could also drive important dynamical effects deep in the ejecta that are capable of mixing elements and affecting the observational signatures of these events. We have now investigated the dynamical effect of 56Ni heating on PI SN ejecta with high-resolution two-dimensional hydrodynamic simulations performed with the CASTRO code. We find that expansion of the hot 56Ni bubble forms a shell at the base of the silicon layer of the ejecta ∼200 days after the explosion but that no hydrodynamical instabilities develop that would mix 56Ni with the 28Si/16O-rich ejecta. However, while the dynamical effects of 56Ni heating may be weak they could affect the observational signatures of some PI SNe by diverting decay energy into internal expansion of the ejecta at the expense of rebrightening at later times.

Highlights

Observations by Humphreys & Davidson (1979), Davidson & Humphreys (1997), and Crowther et al (2010) indicate that stars with masses >100 Me can form in the local universe
Cosmological simulations suggest that the initial mass function (IMF) of primordial stars is topheavy and that many of them would have died with masses above 100 Me (e.g., Hirano et al 2014)
We find that 56Ni bubble dynamics does not affect the spectra of pair-instability supernovae (PI SNe) but can reduce bolometric luminosities during rebrightening

Summary

Introduction

Observations by Humphreys & Davidson (1979), Davidson & Humphreys (1997), and Crowther et al (2010) indicate that stars with masses >100 Me can form in the local universe. Very massive stars (VMS; 140–260 Me) are thought to explode as pair-instability supernovae (PI SNe; Woosley et al 2002). When the core of a VMS evolves to temperatures above 109 K, thermal photons in the tail of their Maxwellian distribution (hν 1 MeV) begin to freeze out as electron—positron (e−–e+) pairs through collisions with nuclei. Pair-production occurs at the expense of thermal pressure support of the core against gravity and it begins to contract and become hotter. Rising temperatures and densities in the core eventually ignite explosive oxygen and silicon burning that can completely disrupt the star. PI SNe can produce 1052–1053 erg of energy and 0.1–30 Me of 56Ni and may be the most energetic thermonuclear explosions in the universe. Unlike core-collapse (CC) SNe, whose central engines are not fully understood, PI

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