August 11-15, 2014

Abstract

Metal Accretion onto White Dwarfs. III. A Still Better Approach Based on the Coupling of Diffusion with Evolution

Pierre Brassard (Université de Montréal)

Gilles Fontaine, Université de Montréal

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, 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.

Mode of presentation: poster