The Ratio of Helium- to Hydrogen-Atmosphere White Dwarfs: Direct Evidence for Convective Mixing

TL;DR

This study analyzes the ratio of helium- to hydrogen-atmosphere white dwarfs across temperatures, providing evidence that convective mixing causes the transition, with implications for hydrogen layer masses in white dwarfs.

Contribution

It offers the most accurate and extensive analysis of white dwarf atmospheric compositions, linking the ratio changes to convective mixing processes.

Findings

The ratio increases from ~0.25 to twice that value as temperature decreases from 15,000 K to 8,000 K.

Convective mixing likely causes the transition between hydrogen and helium atmospheres.

Hydrogen layer masses are estimated to be between log M_H/M_tot = -10 and -8 for some white dwarfs.

Abstract

We determine the ratio of helium- to hydrogen-atmosphere white dwarf stars as a function of effective temperature from a model atmosphere analysis of the infrared photometric data from the Two Micron All Sky Survey combined with available visual magnitudes. Our study surpasses any previous analysis of this kind both in terms of the accuracy of the Teff determinations as well as the size of the sample. We observe that the ratio of helium- to hydrogen-atmosphere white dwarfs increases gradually from a constant value of ~0.25 between Teff = 15,000 K and 10,000 K to a value twice as large in the range 10,000 > Teff > 8000 K, suggesting that convective mixing, which occurs when the bottom of the hydrogen convection zone reaches the underlying convective helium envelope, is responsible for this gradual transition. The comparison of our results with an approximate model used to describe the outcome of this convective mixing process implies hydrogen mass layers in the range log M_H/M_tot = -10 to -8 for about 15% of the DA stars that survived the DA to DB transition near Teff ~ 30,000 K, the remainder having presumably more massive layers above log M_H/M_tot ~ -6.