Trace Metals in Cool DA White Dwarfs
Until recently traces of heavy elements like Ca, Mg, Fe in cool white dwarfs were almost exclusively seen in helium-rich objects. The only classical example of a hydrogen-atmosphere DA white dwarf with heavy elements was G74-7, until the discovery of metals in G29-38 (Koester et al. 1997) and G238-44 (Holberg et al. 1997). The most widely accepted explanation for these metals is the interstellar medium (ISM) accretion/diffusion scenario as discussed in great detail in Dupuis et al. (1992=D92, 1993a=D93a, 1993b=D93b). There is no obvious reason, why accretion — if that is the correct explanation — should preferentially occur for helium objects, and the observational differences can very likely be understood by a combination of several factors. Diffusion time scales — determined by the depth of the convection zone and the diffusion velocity at the bottom of this zone are much shorter for DAs with their much shallower convection at the same T eff; from the figures in D92 we can infer that at T eff = 10000 K the diffusion time scale is a few 106 years in the helium WD, but only of the order of 104 years in a DA. According to the Dupuis et al. model we expect during the accretion phase in a cloud — roughly for 106 years — the same metal abundances in H and He envelope white dwarfs, because the time scale for accretion to fill the reservoir and that for diffusion to empty it are both proportional to the convective mass.
KeywordsConvection Helium Ambi
Unable to display preview. Download preview PDF.