Gaia Reveals Evidence for Merged White Dwarfs

TL;DR

Gaia data reveals a bifurcation in white dwarf sequences, indicating a significant population of massive white dwarfs likely formed through mergers, providing new insights into white dwarf evolution.

Contribution

This study is the first to directly detect a population of massive white dwarfs formed via mergers using Gaia's volume-limited sample.

Findings

Bifurcation observed in Gaia and SDSS white dwarf sequences.

36% of white dwarfs have helium atmospheres.

Evidence for merger-origin white dwarfs in the local volume.

Abstract

We use Gaia Data Release 2 to identify 13,928 white dwarfs within 100 pc of the Sun. The exquisite astrometry from Gaia reveals for the first time a bifurcation in the observed white dwarf sequence in both Gaia and the Sloan Digital Sky Survey (SDSS) passbands. The latter is easily explained by a helium atmosphere white dwarf fraction of 36%. However, the bifurcation in the Gaia colour-magnitude diagram depends on both the atmospheric composition and the mass distribution. We simulate theoretical colour-magnitude diagrams for single and binary white dwarfs using a population synthesis approach and demonstrate that there is a significant contribution from relatively massive white dwarfs that likely formed through mergers. These include white dwarf remnants of main-sequence (blue stragglers) and post-main sequence mergers. The mass distribution of the SDSS subsample, including the spectroscopically confirmed white dwarfs, also shows this massive bump. This is the first direct detection of such a population in a volume-limited sample.