2025ApJ...992..108W

Following a Type Ia supernova (SN Ia) in a double white dwarf (WD) binary, a surviving WD companion leaves at its orbital velocity ≈ 1000-3000 km s^–1. 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_⊙ helium WD companions using 3D 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 ≈0.02 M_⊙ object. We find helium mass loss ≈ 0.005-0.06 M_⊙ with velocities ≈ 1000-4000 km s^–1, 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, ≲1600 km s^–1. We model the postexplosion evolution of the shock-heated companions using MESA, and find reasonable agreement with the hypervelocity stars D6-2, J0546+0836, J1332-3541, and SDSS J1637+3631. A surviving ≳0.3 M_⊙ helium WD can be ruled out in SN 1972E and SN 2011fe, and any surviving helium WD is likely ruled out in SN remnants 0509-67.5 and SN 1006.