Supernova Shocks Cannot Explain the Inflated State of Hypervelocity Runaways from White Dwarf Binaries

TL;DR

This study investigates whether supernova shocks can account for the inflated radii of hypervelocity white dwarf runaways, concluding that shocks alone are insufficient and additional physics are needed.

Contribution

The paper provides the first detailed long-term evolution models of hypervelocity white dwarf runaways post-supernova, showing shocks do not cause inflation over observed timescales.

Findings

Supernova shocks do not significantly inflate white dwarfs beyond a few thousand years.

Models contract back to small radii within about 10,000 years after shock.

Additional heating or physics are required to explain observed inflated states.

Abstract

Recent observations have found a growing number of hypervelocity stars with speeds of $\approx 1500-2500\,$km\,s$^{-1}$ which could have only been produced through thermonuclear supernovae in white dwarf binaries. Most of the observed hypervelocity runaways in this class display a surprising inflated structure: their current radii are roughly an order of magnitude greater than they would have been as white dwarfs filling their Roche lobe. While many simulations exist studying the dynamical phase leading to supernova detonation in these systems, no detailed calculations of the long-term structure of the runaways have yet been performed. We use an existing \textsc{Arepo} hydrodynamical simulation of a supernova in a white dwarf binary as a starting point for the evolution of these stars with the 1 dimensional stellar evolution code MESA. We show that the supernova shock is not enough to inflate the white dwarf over timescales longer than a few thousand years, significantly shorter than the $10^{5-6}$ year lifetimes inferred for observed hypervelocity runaways. Despite experiencing a shock from a supernova less than $\approx 0.02\,R_\odot$ away, our models do not experience significant interior heating, and all contract back to radii around $0.01\,R_\odot$ within about $10^4$\,years. Explaining the observed inflated states requires either an additional source of significant heating or some other physics that is not yet accounted for in the subsequent evolution.