Alone but not lonely: Observational evidence that binary interaction is always required to form hot subdwarf stars

TL;DR

This study provides observational evidence that binary interactions are essential in forming hot subdwarf stars, challenging the idea that single star scenarios can produce these stars without interaction.

Contribution

The paper demonstrates that hot subdwarfs are almost always formed through binary interactions, based on analysis of their companions' rotation rates and the scarcity of wide binaries.

Findings

Companions in hot subdwarf binaries show rapid rotation, indicating past interaction.

Few hot subdwarfs have wide binary companions, suggesting interaction is necessary.

Most hot subdwarfs are part of binary or hierarchical systems, not single stars.

Abstract

Hot subdwarfs are core-helium burning stars that show lower masses and higher temperatures than canonical horizontal branch stars. They are believed to be formed when a red giant suffers an extreme mass-loss episode. Binary interaction is suggested to be the main formation channel, but the high fraction of apparently single hot subdwarfs (up to 30%) has prompted single star formation scenarios to be proposed. If such formation scenarios without interaction were possible, that would also imply the existence of hot subdwarfs in wide binaries that have undergone no interaction. We probe the existence of these systems by analysing light curves from the Transiting Exoplanet Survey Satellite (TESS) for all known hot subdwarfs with a main sequence wide binary companion, and by searching for common proper motion pairs to spectroscopically confirmed hot subdwarfs. We find that (i) the companions in composite hot subdwarfs show short rotation periods when compared to field main sequence stars. They display a triangular-shaped distribution with a peak around 2.5 days, similar to what is observed for young open clusters. This observed distribution of rotation rates for the companions in known wide hot subdwarf binaries provides evidence of previous interaction causing spin-up. We also report (ii) a shortage of hot subdwarfs with candidate common proper motion companions, considering the frequency of such systems among progenitors. We identify only 16 candidates after probing 2938 hot subdwarfs with good astrometry. Out of those, at least six seem to be hierarchical triple systems, in which the hot subdwarf is part of an inner binary. These results suggest that binary interaction is always required for the formation of hot subdwarfs.