Evolution models of helium white dwarf--main-sequence star merger remnants: the mass distribution of single low-mass white dwarfs

TL;DR

This study models the formation of single low-mass white dwarfs via helium white dwarf and main-sequence star mergers, predicting their mass distribution, formation rates, and comparing with observations to understand their origins.

Contribution

It provides the first detailed stellar evolution models of merger remnants, linking pre-merger masses to final white dwarf masses and estimating their contribution to the white dwarf population.

Findings

Mass distribution of merger remnants spans 0.37 to 0.5 solar masses.

Galactic formation rate of these white dwarfs is approximately 8.7×10^-3 per year.

Merger channel accounts for about 2% of all white dwarf formation.

Abstract

It is not known how single white dwarfs with masses less than 0.5Msolar -- low-mass white dwarfs -- are formed. One way in which such a white dwarf might be formed is after the merger of a helium-core white dwarf with a main-sequence star that produces a red giant branch star and fails to ignite helium. We use a stellar-evolution code to compute models of the remnants of these mergers and find a relation between the pre-merger masses and the final white dwarf mass. Combining our results with a model population, we predict that the mass distribution of single low-mass white dwarfs formed through this channel spans the range 0.37 to 0.5Msolar and peaks between 0.45 and 0.46Msolar. Helium white dwarf--main-sequence star mergers can also lead to the formation of single helium white dwarfs with masses up to 0.51Msolar. In our model the Galactic formation rate of single low-mass white dwarfs through this channel is about 8.7X10^-3yr^-1. Comparing our models with observations, we find that the majority of single low-mass white dwarfs (<0.5Msolar) are formed from helium white dwarf--main-sequence star mergers, at a rate which is about $2$ per cent of the total white dwarf formation rate.