Gravitational settling of 22Ne and white dwarf evolution

TL;DR

This study models the sedimentation of 22Ne in white dwarfs using a self-consistent stellar evolution code, revealing its impact on cooling delays, especially in more massive white dwarfs, and exploring potential observational detection methods.

Contribution

First self-consistent modeling of 22Ne sedimentation in white dwarfs, improving accuracy over previous simplified approaches and assessing its effects on stellar evolution and observables.

Findings

Sedimentation causes minor delay in 0.6 Msun white dwarfs.

Delay is significantly larger in 1.0 Msun white dwarfs.

Previous estimates underestimated the sedimentation delay by about a factor of 2.

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 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 differentation is minor for a typical 0.6 Msun white dwarf. For more massive white dwarfs, say M_Wd about 1.0 Msun, 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 about 2. We also perform a preliminary study of the impact of 22Ne sedimentation on the white dwarf luminosity function. Finally, we hypothesize as well 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.