<i>Gaia</i> reveals evidence for merged white dwarfs

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.Comment: MNRAS, in pres

25 years of using the spectroscopic method: where do we stand?

The 21st European Workshop on White Dwarfs was held in Austin, TX from July 23rd to 27th of 2018The spectroscopic technique, which consists in fitting high signal-to-noise optical spectra with the predictions of state-of-the-art model atmospheres, has routinely been used for over 25 years to measure the atmospheric parameters of DA white dwarfs, and DB stars as well. We review the technique, the progress we have made, and more importantly, we attempt to assess its validity and also try to identify some of the current challenges in the physics of white dwarf atmospheres related to the use of the spectroscopic technique.Astronom