Evidence of spectral evolution on the white dwarf sample from the Gaia mission

Abstract

Since the Gaia data release 2, several works have been published describing a bifurcation in the observed white dwarf colour−magnitude diagram for GBP − GRP > 0. Some possible explanations in the literature include the existence of a double population with different initial mass functions or two distinct populations, one formed by hydrogen-envelope and one formed by helium-envelope white dwarfs. We propose instead spectral evolution to explain the bifurcation. From a population synthesis approach, we find that spectral evolution occurs for effective temperatures below 11 000 K and masses mainly between 0.64 M and 0.74 M, which correspond to around 16 per cent of all DA white dwarfs. We also find that the Gaia white dwarf colour–magnitude diagram indicates a star formation history that decreases abruptly for objects younger than 1.4 Gyr and a top-heavy initial mass function for the white dwarf progenitors.