The effect of $^{22}$Ne diffusion in the evolution and pulsational properties of white dwarfs with solar metallicity progenitors

TL;DR

This study investigates how $^{22}$Ne sedimentation affects the cooling and pulsational properties of white dwarfs with Solar metallicity progenitors, revealing a delay in cooling and significant period differences in pulsations.

Contribution

It provides detailed evolutionary models showing the impact of $^{22}$Ne sedimentation on white dwarf cooling times and pulsation periods, especially for massive white dwarfs.

Findings

$^{22}$Ne sedimentation delays white dwarf cooling by about 1 Gyr at low luminosities.

Sedimentation causes period differences in ZZ Ceti stars larger than observational uncertainties.

Effects are more pronounced in more massive white dwarfs.

Abstract

Because of the large neutron excess of $^{22}$Ne, this isotope rapidly sediments in the interior of the white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of $^{22}$Ne in the inner regions of the star, modifies the Brunt-V\"ais\"al\"a frequency, thus altering the pulsational properties of these stars. In this work, we discuss the impact of $^{22}$Ne sedimentation in white dwarfs resulting from Solar metallicity progenitors ($Z=0.02$). We performed evolutionary calculations of white dwarfs of masses $0.528$, $0.576$, $0.657$ and $0.833$ M$_{\sun}$, derived from full evolutionary computations of their progenitor stars, starting at the Zero Age Main Sequence all the way through central hydrogen and helium burning, thermally-pulsing AGB and post-AGB phases. Our computations show that at low luminosities ($\log(L/L_{\sun})\la -4.25$), $^{22}$Ne sedimentation delays the cooling of white dwarfs with Solar metallicity progenitors by about 1~Gyr. Additionally, we studied the consequences of $^{22}$Ne sedimentation on the pulsational properties of ZZ~Ceti white dwarfs. We find that $^{22}$Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.