Mass Loss and Subsequent Thermal Evolution of Surviving Helium White Dwarfs Shocked by Thermonuclear Supernovae

TL;DR

This study investigates how helium white dwarf companions are affected by supernova ejecta, revealing significant mass loss, shock heating effects, and their subsequent thermal evolution, with implications for observed hypervelocity stars and supernova remnants.

Contribution

The paper presents detailed 3D hydrodynamical simulations of supernova ejecta interacting with helium white dwarf companions, including their thermal evolution, which is a novel comprehensive approach.

Findings

Lower-mass WDs experience more impact and mass loss.

A relation between mass loss and ejecta pressure ratio is established.

Helium mass loss can produce observable nebular emission lines.

Abstract

Following a type Ia supernova (SN Ia) in a double white dwarf (WD) binary, a surviving WD companion leaves at its orbital velocity $\approx 1$,000 - 3,000 km/s. The Gaia mission has discovered seven such hypervelocity WDs with inflated radii indicative of shock heating by SN ejecta. We study the interaction between SN ejecta and Roche lobe-filling 0.08 - 0.45 $M_{\odot}$ helium WD companions using three-dimensional hydrodynamical simulations with Athena++. Given the importance of the later thermal evolution, we include an accurate equation-of-state for the degenerate helium WD donor. We show that a lower-mass, larger-radius WD companion is more strongly impacted by SN ejecta and undergoes substantial mass loss. We find a tight relation between the fractional mass loss and the ratio between the ejecta ram pressure and donor volume-averaged pressure, which can be used for predicting mass loss in other systems. In the most extreme case, the companion becomes a very inflated $\approx0.02\,M_{\odot}$ object. We find helium mass loss $\approx 0.005 - 0.06\,M_{\odot}$ with velocities $\approx $ 1,000$-$4,000 km/s, which may lead to emission lines in the nebular phase. The surviving helium WD receives a kick velocity, but its final velocity is essentially determined by its orbital velocity $\lesssim$ 1,600 km/s. We model the post-explosion evolution of the shock-heated companions using MESA, and find reasonable agreement with the hypervelocity stars D6-2, J0546+0836, J1332-3541 & SDSS J1637+3631. A surviving $\gtrsim 0.3\,M_{\odot}$ helium WD can be ruled out in SN1972E & SN2011fe, and any surviving helium WD is likely ruled out in SN remnants 0509-67.5 & SN1006.