Evolution of binaries containing a hot subdwarf and a white dwarf to double white dwarfs, and double detonation supernovae with hypervelocity runaway stars

¹ Max Planck Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching bei München, Germany
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge MA 02138, USA

^⋆ Corresponding author: abinaya@mpa-garching.mpg.de

Received: 10 March 2025
Accepted: 26 September 2025

Abstract

Compact binaries containing hot subdwarfs and white dwarfs have the potential to evolve into a variety of explosive transients. These systems could also explain hypervelocity runaway stars such as US 708. We use the detailed binary evolution code MESA to evolve hot subdwarf and white dwarf stars interacting in binaries. We explore their evolution toward double detonation supernovae, helium novae, or double white dwarfs. We present a grid of 3120 binary evolution models that map from initial conditions, such as the orbital period and masses of the hot subdwarf and white dwarf, to these outcomes. The minimum amount of helium required to ignite the helium shell that leads to a double detonation supernova in our grid is ≈0.05 M_⊙, likely too large to produce spectra similar to normal Type Ia supernovae, but compatible with inferred helium shell masses from some observed peculiar Type I supernovae. We also provide the helium shell masses for our double white dwarf systems, with a maximum He shell mass of ≈0.18 M_⊙. In our double detonation systems, the orbital velocity of the surviving donor star ranges from ≈450 km s⁻¹ to ≈1000 km s⁻¹. Among the surviving donors, we also estimate the runaway velocities of proto-white dwarfs, which have higher runaway velocities than hot subdwarf stars of the same mass. Our grid will provide a first-order estimate of the potential outcomes for the observation of binaries containing hot subdwarfs and white dwarfs from future missions like Gaia, LSST, and LISA.