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

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 $\approx 0.02M_{\odot }$ object. We find helium mass loss $\approx 0.005-0.06M_{\odot }$ 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 $\gtrsim 0.3M_{\odot }$ 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.

18 pages, 5 figures; Accepted to ApJ