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

TL;DR

This study uses detailed binary evolution models to explore how hot subdwarf and white dwarf binaries can lead to various explosive phenomena, including supernovae and hypervelocity stars, informing future observational searches.

Contribution

It provides a comprehensive grid of binary evolution outcomes, including helium shell masses and velocities, for systems containing hot subdwarfs and white dwarfs, advancing understanding of their explosive potential.

Findings

Minimum helium shell for double detonation: ~0.05 M_sun

Maximum helium shell in double white dwarf systems: ~0.18 M_sun

Survivor velocities range from 450 to 1000 km/s

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 towards double detonation supernovae, helium novae, or double white dwarfs. Our grid of 3120 models maps from initial conditions such as orbital period and masses of 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 $\approx 0.05 \, \mathrm{M_{\odot}}$, 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 $\approx 0.18\,\mathrm{M_{\odot}}$. In our double detonation systems, the orbital velocity of the surviving donor star ranges from $\approx 450 \, \mathrm{km\,s^{-1}}$ to $\approx 1000 \, \mathrm{km\,s^{-1}}$. 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.