Accretion of grains and element abundances in cool, helium-rich white dwarfs

The problem posed by the occasional presence of metal features in the spectra of cool, helium-rich white dwarfs within the framework of the accretion-diffusion picture of Vauclair, Vauclair, and Greenstein (1979) is considered. A model where metals from the interstellar medium are accreted in the form of grains by a slowly rotating, weakly magnetic white dwarf, and where the propeller mechanism of Illarionov and Sunyaev (1975) provides the screening mechanism required to prevent the accretion of interstellar hydrogen, is explored. The rate of rotation and surface magnetic field strength required in this model appears consistent with the sparse upper limits available for white dwarfs. For typical silicate grains, the propeller mechanism is found to be operative only for effective temperatures between about 8000 and about 15,000 K. This result is in qualitative agreement with the observed concentration of cool, helium-rich white dwarfs with metal features in that effective temperature range.