Shocking and Mass Loss of Compact Donor Stars in Type Ia Supernovae

TL;DR

This paper models the interaction between helium donor stars and supernova ejecta, revealing significant mass loss and long-lasting luminosity, and connects these findings to observed hypervelocity stars.

Contribution

It provides detailed hydrodynamical simulations of helium donor stars in Type Ia supernovae, explaining their mass loss and post-explosion evolution, and links these to observed hypervelocity stars.

Findings

0.01-0.02 solar masses of donor material stripped

Donor stars remain luminous and expanded for 10^5-10^6 years

Post-explosion properties match observed hypervelocity star D6-2

Abstract

Type Ia supernovae arise from thermonuclear explosions of white dwarfs accreting from a binary companion. Following the explosion, the surviving donor star leaves at roughly its orbital velocity. The discovery of the runaway helium subdwarf star US 708, and seven hypervelocity stars from Gaia data, all with spatial velocities $\gtrsim 900$ km/s, strongly support a scenario in which the donor is a low-mass helium star, or a white dwarf. Motivated by these discoveries, we perform three-dimensional hydrodynamical simulations with the $\texttt{Athena++}$ code modeling the hydrodynamical interaction between a helium star or helium white dwarf, and the supernova ejecta. We find that $\approx 0.01-0.02\,M_{\odot}$ of donor material is stripped, and explain the location of the stripped material within the expanding supernova ejecta. We continue the post-explosion evolution of the shocked donor stars with the $\texttt{MESA}$ code. As a result of entropy deposition, they remain luminous and expanded for $\approx 10^{5}-10^{6}$ yrs. We show that the post-explosion properties of our helium white dwarf donor agree reasonably with one of the best-studied hypervelocity stars, D6-2.