Remnants of Subdwarf Helium Donor Stars Ejected from Close Binaries with Thermonuclear Supernovae

Some binary systems composed of a white dwarf (WD) and a hot subdwarf (sdB) helium star will make contact within the helium burning lifetime of the sdB star. The accreted helium on the WD inevitably undergoes a thermonuclear instability, causing a detonation that is expected to transition into the WD core and lead to a thermonuclear supernova (SN) while the donor orbits nearby with high velocity. Motivated by the recent discovery of fast moving objects that occupy unusual locations on the HR diagram, we explore the impact of the thermonuclear SNe on the donors in this specific double detonation scenario. We use MESA to model the binary up to the moment of detonation, then 3D Athena++ to model the hydrodynamic interaction of the SN ejecta with the donor star, calculating the amount of mass that is stripped and the entropy deposited in the deep stellar interior by the strong shock that traverses it. We show that these donor remnants are ejected with velocities primarily set by their orbital speeds: 700-900 kms(-1). We model the long-term thermal evolution of remnants by introducing the shock entropy into MESA models. In response to this entropy change, donor remnants expand and brighten for timescales ranging from 10(6) to 10(8) yr, giving ample time for these runaway stars to be observed in their inflated state before they leave the galaxy. Even after surface layers are stripped, some donors retain enough mass to resume core helium burning and further delay fading for more than 10(8) yr.