A<i>Gaia</i>Data Release 2 catalogue of white dwarfs and a comparison with SDSS

Abstract

We present a catalogue of white dwarf candidates selected from the second\ndata release of $Gaia$ (DR2). We used a sample of spectroscopically confirmed\nwhite dwarfs from the Sloan Digital Sky Survey (SDSS) to map the entire space\nspanned by these objects in the $Gaia$ Hertzsprung-Russell diagram. We then\ndefined a set of cuts in absolute magnitude, colour, and a number of $Gaia$\nquality flags to remove the majority of contaminating objects. Finally, we\nadopt a method analogous to the one presented in our earlier SDSS photometric\ncatalogues to calculate a probability of being a white dwarf\n($P_{\\mathrm{WD}}$) for all $Gaia$ sources which passed the initial selection.\nThe final catalogue is composed of $486\\,641$ stars with calculated\n$P_{\\mathrm{WD}}$ from which it is possible to select a sample of $\\simeq\n260\\,000$ high-confidence white dwarf candidates in the magnitude range\n$8&lt;G&lt;21$. By comparing this catalogue with a sample of SDSS white dwarf\ncandidates we estimate an upper limit in completeness of $85$ per cent for\nwhite dwarfs with $G \\leq 20$ mag and $T_{\\mathrm{eff}} &gt; 7000$ K, at high\nGalactic latitudes ($|b|&gt;20^{\\circ}$). However, the completeness drops at low\nGalactic latitudes, and the magnitude limit of the catalogue varies\nsignificantly across the sky as a function of $Gaia$'s scanning law. We also\nprovide the list of objects within our sample with available SDSS spectroscopy.\nWe use this spectroscopic sample to characterise the observed structure of the\nwhite dwarf distribution in the H-R diagram.\n