Thermohaline Instabilities Induced by Heavy Element Accretion onto White Dwarfs: Consequences on the Derived Accretion Rates

TL;DR

This paper investigates how thermohaline mixing, caused by inverse μ-gradients during heavy element accretion, affects the estimated accretion rates in white dwarfs, revealing that previous models underestimated these rates especially for DA types.

Contribution

It introduces thermohaline mixing into models of white dwarf accretion, significantly altering the derived accretion rates and improving the understanding of atmospheric compositions.

Findings

Thermohaline mixing increases estimated accretion rates.

Previous models underestimated accretion rates without considering thermohaline effects.

Impacts differ between DA and DB white dwarfs.

Abstract

Heavy elements are observed in the atmospheres of many DA and DB white dwarfs, and their presence is attributed to the accretion of matter coming from debris disks. Several authors have deduced accretion rates from the observed abundances, taking into account the mixing induced by the convective zones and the gravitational settling. The obtained values are different for DA and DB white dwarfs. Here we show that an important process was forgotten in all these computations: thermohaline mixing, induced by the inverse $\mu$-gradient built during the accretion process. Taking this mixing into account leads to an increase of the derived accretion rates, specially for DA white dwarfs, and modifies the conclusions.