Black Dwarf Supernova in the Far Future

TL;DR

In the distant future, black dwarfs with iron cores may collapse as supernovae due to decreasing Chandrasekhar limits, representing a rare final astrophysical transient before heat death.

Contribution

This paper models the structure and evolution of black dwarfs with iron cores, predicting their collapse and supernovae in the far future universe.

Findings

Estimated delay time to supernova: 10^1100 years

Approximately 1% of all stars will undergo this collapse

Black dwarf supernovae resemble accretion-induced collapse and core-collapse supernovae

Abstract

In the far future long after star formation has ceased the universe will be populated by sparse degenerate remnants, mostly white dwarfs, though their ultimate fate is an open question. These white dwarfs will cool and freeze solid into black dwarfs while pycnonuclear fusion will slowly process their composition to iron-56. However, due to the declining electron fraction the Chandrasekhar limit of these stars will be decreasing and will eventually be below that of the most massive black dwarfs. As such, isolated dwarf stars with masses greater than $\sim 1.2 M_\odot$ will collapse in the far future due to the slow accumulation of iron-56 in their cores. If proton decay does not occur then this is the ultimate fate of about $10^{21}$ stars, approximately one percent of all stars in the observable universe. We present calculations of the internal structure of black dwarfs with iron cores as a model for progenitors. From pycnonuclear fusion rates we estimate their lifetime and thus delay time to be $10^{1100}$ years. We speculate that high mass black dwarf supernovae resemble accretion induced collapse of O/Ne/Mg white dwarfs while later low mass transients will be similar to stripped-envelope core-collapse supernova, and may be the last interesting astrophysical transients to occur prior to heat death.