In section 7. Stellar structure and evolution
Importance of fingering convection for accreting white dwarfs in the framework of full evolutionary calculations: the case of the hydrogen-rich white dwarfs GD133 and G29-38
The presence of heavy elements in hydrogen-layer white dwarfs (the most common variety) several decades ago led to a number of different hypotheses regarding the source of metals. One of the more popular ideas currently, is linked to the discovery of debris disks around some white dwarfs: a source of heavy elements could be the disruption of planetisimals that are close to the tidal radius of the white dwarf. In order to link the composition of disrupted planetesimals to the white dwarf surface composition, a number of processes need to be taken into consideration, especially those that influence the mixing of any accreted material. Wachlin and collaborators have carried out such a study, focussing on the most important of these: the double-diffusive instability that results from the accretion of heavy elements. They found that the accretion rates required to explain the abundances in two well-observed white dwarfs, G 29-38 and GD 133, are higher by 1.7 to 2 dex presenting a challenge to current observations, and implying the disruption of significantly more massive bodies than previously considered.