On the formation of hot DQ white dwarfs

TL;DR

This study presents comprehensive evolutionary models explaining the origin of hot DQ white dwarfs, supporting the diffusive/convective-mixing formation scenario and linking them to PG1159 stars, with implications for their temperature and surface composition.

Contribution

First full evolutionary calculations from the born-again stage to white dwarf cooling, supporting the diffusive/convective-mixing scenario for hot DQ formation.

Findings

Hot DQ stage is short-lived.

Effective temperature range depends on residual helium and initial stellar mass.

Correlation between temperature and surface carbon abundance is predicted.

Abstract

We present the first full evolutionary calculations aimed at exploring the origin of hot DQ white dwarfs. These calculations consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Our calculations provide strong support to the diffusive/convective-mixing picture for the formation of hot DQs. We find that the hot DQ stage is a short-lived stage and that the range of effective temperatures where hot DQ stars are found can be accounted for by different masses of residual helium and/or different initial stellar masses. In the frame of this scenario, a correlation between the effective temperature and the surface carbon abundance in DQs should be expected, with the largest carbon abundances expected in the hottest DQs. From our calculations, we suggest that most of the hot DQs could be the cooler descendants of some PG1159 stars characterized by He-rich envelopes markedly smaller than those predicted by the standard theory of stellar evolution. At least for one hot DQ, the high-gravity white dwarf SDSS J142625.70+575218.4, an evolutionary link between this star and the massive PG1159 star H1504+65 is plausible.