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| Paper: |
Metal Accretion onto White Dwarfs. III. A Still Better Approach Based on the Coupling of Diffusion with Evolution |
| Volume: |
493, 19th European Workshop on White Dwarfs |
| Page: |
121 |
| Authors: |
Brassard, P.; Fontaine, G. |
| Abstract: |
The accretion-diffusion picture is the model par excellence for
describing the presence of planetary debris polluting the atmospheres of
relatively cool white dwarfs. In the time-dependent approach used in
Paper II of this series (Fontaine et al. 2014), the basic assumption is that the
accreted metals are trace elements and do not influence the background
structure, which may be considered static in time. Furthermore, the
usual assumption of instantaneous mixing in the convection zone is
made. As part of the continuing development of our local evolutionary
code, diffusion in presence of stellar winds or accretion is now fully
coupled to evolution. Convection is treated as a diffusion process,
i.e., the assumption of instantaneous mixing is relaxed, and,
furthermore, overshooting is included. This allows feedback on the
evolving structure from the accreting metals. For instance, depending of
its abundance, a given metal may contribute enough to the overall
opacity (especially in a He background) to change the size of the
convection zone as a function of time. Our better approach also allows
to include in a natural way the mechanism of thermohaline convection,
which we discuss at some length. Also, it is easy to consider
sophisticated time-dependent models of accretion from circumstellar
disks, such as those developed by Roman Rafikov at Princeton for
instance. The current limitations of our approach are 1) the
calculations are extremely computer-intensive, and 2) we have not yet
developed detailed EOS megatables for metals beyond oxygen. |
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