Gravitational Settling of 22 Ne and White Dwarf Evolution

Abstract

We study the effects of the sedimentation of the trace element 22Ne in the cooling of white dwarfs. In contrast with previous studies—which adopted a simplified treatment of the effects of 22Ne sedimentation—this is done self-consistently for the first time, using an up-to-date stellar evolutionary code in which the diffusion equation is coupled with the full set of equations of stellar evolution. Due the large neutron excess of 22Ne, this isotope rapidly sediments in the interior of the white dwarf. Although we explore a wide range of parameters, we find that when using the most reasonable assumptions concerning the diffusion coefficient and the physical state of the white dwarf interior, the delay introduced by the ensuing chemical differentiation is minor for a typical 0.6 M⊙ white dwarf. For more massive white dwarfs, say MWD ~ 1.0 M⊙, the delay turns out to be considerably larger. These results are in qualitatively good accord with those obtained in previous studies, but we find that the magnitude of the delay introduced by 22Ne sedimentation was underestimated by a factor of ~2. We also perform a preliminary study of the impact of 22Ne sedimentation on the white dwarf luminosity function. Finally, we hypothesize on the possibility of detecting the sedimentation of 22Ne using pulsating white dwarfs in the appropriate effective temperature range with accurately determined rates of change of the observed periods.