A hidden population of white dwarfs with atmospheric carbon traces in the Gaia bifurcation

Context. The high-quality photometric and astrometric capabilities of the ESA Gaia space mission have revealed a bifurcation of the white dwarf sequence on the color magnitude diagram with two branches: A and B. While the A branch consists mostly of white dwarfs with hydrogen(H)-rich atmospheres, the B branch is not completely understood. Although it has been proposed that the B branch is populated mainly by helium (He)-rich white dwarfs, this branch overlaps with a & SIM;0.8  M-& ODOT; evolutionary track with a pure He envelope, which would imply an unexpected peak in the white dwarf mass distribution.Aims. In cold He-rich white dwarfs, it is expected that the outer convective zone penetrates into deep carbon (C) -rich layers, leading to a slight C contamination in their surfaces at & SIM;10  000 K. In this paper we aim to study the Gaia bifurcation as the natural consequence of C dredge-up by convection in cold He-dominated white dwarfs.Methods. Relying on accurate atmosphere models, we provide a new set of evolutionary models for He-rich white dwarfs, which employ different prescriptions for the C enrichment. On the basis of these models, we carried out a population synthesis study of the Gaia 100 pc white dwarf sample to constrain the models that best fit the bifurcation.Results. Our study shows that He-rich white dwarf models with a slight C contamination below the optical detection limit can accurately reproduce the Gaia bifurcation. We refer to these stars as "stealth DQ" white dwarfs because they do not exhibit detectable C signatures in their optical spectra, but the presence of C in their atmosphere produces a continuum absorption favoring the emission in bluer wavelengths, thereby creating the B branch of the bifurcation. Furthermore, our study shows that the white dwarf mass distribution obtained when a stealth C contamination is taken into account presents a peak at & SIM;0.6  M-& ODOT;, which is consistent with the mass distribution for H-rich white dwarfs and with the standard evolutionary channels for their formation.Conclusions. We conclude that stealth DQ white dwarfs can account for the lower branch in the Gaia bifurcation. The C signatures of these stars could be detectable in ultraviolet (UV) spectra.