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Hot C-rich white dwarfs: testing the DB-DQ transition through pulsations

Hot DQ white dwarfs constitute a new class of white dwarf stars, uncovered recently within the framework of the SDSS project. Recently, three hot DQ white dwarfs have been reported to exhibit photometric variability with periods compatible with pulsation g-modes. Here, we present a nonadiabatic pulsation analysis of the recently discovered carbon-rich hot DQ white dwarf stars. One of our main aims is to test the convective-mixing picture for the origin of hot DQs through the pulsational properties. Our study relies on the full evolutionary models of hot DQ white dwarfs recently developed by Althaus et al. (2009), that consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Specifically, we present a stability analysis of white dwarf models from stages before the blue edge of the DBV instability strip until the domain of the hot DQ white dwarfs, including the transition DB --> hot DQ white dwarf. We found that at evolutionary phases in which the models are characterized by He-dominated atmospheres, they exhibit unstable $g$-mode pulsations typical of DBV stars, and when the models become DQ white dwarfs with carbon-dominated atmospheres, they continue being pulsationally unstable with similar characteristics than DB models, and in agreement with the periods detected in variable hot DQ white dwarfs. Our calculations provide strong support to the convective-mixing picture for the formation of hot DQs. In particular, our results point to the existence of pulsating DB white dwarfs with very thin He-rich envelopes, which after passing the DBV instability strip become variable hot DQ stars. The existence of these DB stars with very thin envelopes could be investigated through asteroseismology.