White dwarf-main-sequence binaries from Gaia EDR3: The unresolved 100 pc volume-limited sample

Abstract

We use the data provided by the Gaia Early Data Release 3 to search for a highly complete volume-limited sample of unresolved binaries consisting of a white dwarf and a main-sequence companion (i.e. WDMS binaries) within 100 pc. We select 112 objects based on their location within the Hertzsprung-Russell diagram, of which 97 are new identifications. We fit their spectral energy distributions (SED) with a two-body fitting algorithm implemented in VOSA (Virtual Observatory SED Analyser) to derive the effective temperatures, luminosities, and radii (hence surface gravities and masses) of both components. The stellar parameters are compared to those from the currently largest catalogue of close WDMS binaries, from the Sloan Digital Sky Survey (SDSS). We find important differences between the properties of the Gaia and SDSS samples. In particular, the Gaia sample contains WDMS binaries with considerably cooler white dwarfs and main-sequence companions (some expected to be brown dwarfs). The Gaia sample also shows an important population of systems consisting of cool and extremely low-mass white dwarfs, not present in the SDSS sample. Finally, using a Monte Carlo population synthesis code, we find that the volume-limited sample of systems identified here seems to be highly complete (≃ 80 ± 9 per cent); however, it only represents ≃9 per cent of the total underlying population. The missing ≃91 per cent includes systems in which the main-sequence companions entirely dominate the SEDs. We also estimate an upper limit to the total space density of close WDMS binaries of ≃ (3.7 ± 1.9) × 10-4 pc-3.