He-star donor pathway for the hypervelocity star D6-2

TL;DR

This paper models hot subdwarf + white dwarf binaries to explain hypervelocity star D6-2 as a surviving companion of Type Ia supernovae, linking stellar evolution, explosion dynamics, and observed star velocities.

Contribution

It introduces detailed binary evolution models that explain the origin, velocity, and composition of hypervelocity star D6-2 as a supernova survivor, aligning with observed supernova rates.

Findings

D6-2 likely originated from a hot subdwarf + white dwarf binary system.

Predicted ejection velocities reach up to ~1000 km/s, matching D6-2.

The supernova rate from this channel is consistent with 1% of observed Type Ia supernovae.

Abstract

Type Ia supernovae are thermonuclear explosions of white dwarfs, yet the nature of their progenitor systems remains uncertain. Recent discoveries of hypervelocity stars provide unique constraints, as these stars likely represent the surviving companions of such explosions. Using detailed binary evolution models computed with MESA and population synthesis with MSE, we investigate the outcomes of hot subdwarf + white dwarf binaries undergoing helium accretion. We find that donors can nearly exhaust their helium and form compact, C/O cores before explosion. The predicted ejection velocities span a broad distribution reaching up to $\sim 1000\,\mathrm{km\,s^{-1}}$, with D6-2 representing the extreme high-velocity tail of this population. We estimate analytically that the thin residual helium envelope can be stripped by the supernova ejecta, producing a C/O-rich surface composition consistent with the observed spectrum. The Type Ia supernova rate from this channel is ${\sim}(1.69\pm0.06)\times10^{-5}\,\mathrm{M_\odot^{-1}}$, consistent with 1% of the observed Type Ia supernova rate. Hot subdwarf + white dwarf binaries containing nearly exhausted He-star donors can therefore naturally explain the velocity and composition of D6-2 while providing a quantitatively consistent contribution to the observed Type Ia supernova rate. Our models predict a distribution of surviving donor remnants with various core He fractions and with ejection velocities extending down to $\sim 450\,\mathrm{ km\,s^{-1}}$. The orbital velocities of donor stars in this progenitor channel naturally yield orbital velocities consistent with US 708, LP 40-365 stars, and D6-2, indicating that a single class of thermonuclear supernova progenitors can account for their entire range of ejection velocities.