The Spectral Evolution of Convective Mixing White Dwarfs, the non-DA Gap, and White Dwarf Cosmochronology

TL;DR

This paper models the spectral evolution of white dwarfs and explains the non-DA gap as a natural consequence of convective mixing, impacting white dwarf cooling ages and Galactic history studies.

Contribution

It introduces a Monte Carlo model that reproduces the non-DA gap through monotonic atmospheric evolution, advancing understanding of white dwarf spectral changes.

Findings

The non-DA gap arises naturally from the model.

Convective mixing influences white dwarf luminosity functions.

White dwarf cooling ages can be refined using this model.

Abstract

The spectral distribution of field white dwarfs shows a feature called the "non-DA gap". As defined by Bergeron et al., this is a temperature range (5100K--6100K) where relatively few non-DA stars are found, even though such stars are abundant on either side of the gap. It is usually viewed as an indication that a significant fraction of white dwarfs switch their atmospheric compositions back and forth between hydrogen-rich and helium-rich as they cool. In this paper, we present a Monte Carlo model of the Galactic disk white dwarf population, based on the spectral evolution model of Chen and Hansen. We find that the non-DA gap emerges naturally, even though our model only allows white dwarf atmospheres to evolve monotonically from hydrogen-rich to helium-rich through convective mixing. We conclude by discussing the effects of convective mixing on the white dwarf luminosity function and the use thereof for Cosmochronology.