Gaia reveals evidence for merged white dwarfs

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