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| Paper: |
The Origin and Evolution of DQ White Dwarfs: The Carbon Pollution Problem Revisited |
| Volume: |
372, 15th European Workshop on White Dwarfs |
| Page: |
19 |
| Authors: |
Brassard, P.; Fontaine, G.; Dufour, P.; Bergeron, P. |
| Abstract: |
We summarize some of the results we obtained using detailed evolutionary calculations that take into account diffusion and mass loss to revisit the question of the carbon pollution observed in the atmospheres of DQ white dwarfs and in some DB stars. Our basic premise is that gravitational settling of C and O is slowed down by dying residual winds in the hot PG1159 phase and it is this competition that is ultimately responsible for the very existence of the PG1159 stars themselves and their progenies, the DO, DB, and DQ white dwarfs. Unlike some computations by other groups, we find that the carbon abundance pattern (N(C)/N(He) vs. Teff) uncovered recently by Dufour, Bergeron, & Fontaine (2005) in DQ stars can quite naturally be explained in terms of the dredge-up model developed by Pelletier et al. (1986). In particular, we recover quite well the observed trend of a monotonic decrease of the N(C)/N(He) ratio with decreasing effective temperature in those stars. The tight observational sequence found by Dufour et al. (and confirmed recently by Koester & Knist 2006) allows us to pin down accurately the masses of the He-dominated envelopes in DQ stars. The bulk of them appear to have envelopes containing about 10−2 to 10−3 of the total mass, in agreement with the expectations of "born again" post-AGB models such as those proposed by Herwig et al. (1999). In addition, we can account qualitatively for the pollution observed in the atmospheres of DB white dwarfs in terms of primordial carbon whose settling is slowed down, in the hotter phases of their evolution, by the presence of residual winds, in agreement with our original premise. We thus reaffirm the natural connection between PG1159, DO, DB, and DQ white dwarfs. |
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