Observational signatures of the surviving donor star in the double detonation model of Type Ia supernovae

TL;DR

This study uses 3D hydrodynamical simulations to explore the observable signatures of surviving helium-star companions in the double-detonation Type Ia supernova model, aiming to identify such stars in future observations.

Contribution

First comprehensive 3D impact simulations of SN ejecta with helium-star companions in the double-detonation scenario, linking impact outcomes to observable signatures and the hypervelocity star US 708.

Findings

Surviving helium-star companions become overluminous for about 1 million years.

Post-impact properties are insensitive to ejecta-donor interaction details after thermal re-equilibration.

US 708's properties suggest it could be a surviving donor remnant, but progenitor system constraints challenge this interpretation.

Abstract

The sub-Chandrasekhar mass double-detonation (DDet) scenario is a contemporary model for SNe Ia. The donor star in the DDet scenario is expected to survive the explosion and to be ejected at the high orbital velocity of a compact binary system. For the first time, we consistently perform 3D hydrodynamical simulations of the interaction of SN ejecta with a helium (He) star companion within the DDet scenario. We map the outcomes of 3D impact simulations into 1D stellar evolution codes and follow the long-term evolution of the surviving He-star companions. Our main goal is to provide the post-impact observable signatures of surviving He-star companions of DDet SNe Ia, which will support the search for such companions in future observations. We find that our surviving He-star companions become significantly overluminous for about 1e6 yr during the thermal re-equilibration phase. After the star re-establishes thermal equilibrium, its observational properties are not sensitive to the details of the ejecta-donor interaction. We apply our results to hypervelocity star US 708, which is the fastest unbound star in our Galaxy, travelling with a velocity of about 1200 km/s, making it natural candidate for an ejected donor remnant of a DDet SN Ia. We find that a He-star donor with an initial mass of >0.5 Msun is needed to explain the observed properties of US 708. Based on our detailed binary evolution calculations, however, the progenitor system with such a massive He-star donor cannot get close enough at the moment of SN explosion to explain the high velocity of US 708. Instead, if US 708 is indeed the surviving He-star donor of a DDet SN~Ia, it would require the entire pre-SN progenitor binary to travel at a velocity of about 400 km/s. It could, for example, have been ejected from a globular cluster in the direction of the current motion of the surviving donor star.