The population of white dwarf binaries with hot subdwarf companions

TL;DR

This paper identifies and analyzes 51 close binary systems consisting of hot subdwarfs and white dwarfs, revealing their mass distribution, potential evolutionary paths, and implications for supernovae and other stellar phenomena.

Contribution

It presents the first comprehensive population study of sdB+WD binaries, including their mass distribution and future evolution predictions.

Findings

Most white dwarfs have masses below average, likely helium-core WDs.

12 systems will merge within a Hubble time, leading to various stellar outcomes.

The study constrains the formation and evolution of close sdB+WD binaries.

Abstract

Hot subdwarfs (sdBs) are core helium-burning stars, which lost almost their entire hydrogen envelope in the red-giant phase. Since a high fraction of those stars are in close binary systems, common envelope ejection is an important formation channel. We identified a total population of 51 close sdB+WD binaries based on time-resolved spectroscopy and multi-band photometry, derive the WD mass distribution and constrain the future evolution of these systems. Most WDs in those binaries have masses significantly below the average mass of single WDs and a high fraction of them might therefore have helium cores. We found 12 systems that will merge in less than a Hubble time and evolve to become either massive C/O WDs, AM\,CVn systems, RCrB stars or even explode as supernovae type Ia.