Accretion from debris disks onto white dwarfs : Fingering (thermohaline) instability and derived accretion rates

TL;DR

This study investigates how fingering (thermohaline) convection affects the accretion rates of debris onto white dwarfs, revealing significant impacts on DA stars but not on DB stars, thus explaining previous discrepancies.

Contribution

It introduces the role of fingering convection in modeling accretion rates onto white dwarfs, resolving prior inconsistencies between DA and DB star observations.

Findings

Fingering convection significantly increases accretion rates in DAs.

In DB white dwarfs, fingering convection is ineffective.

The previous differences in accretion rates between DAs and DBs are explained by convection effects.

Abstract

Recent observations of a large number of DA and DB white dwarfs show evidence of debris disks, which are the remnants of old planetary systems. The infrared excess detected with \emph{Spitzer} and the lines of heavy elements observed in their atmospheres with high-resolution spectroscopy converge on the idea that planetary material accretes onto these stars. Accretion rates have been derived by several authors with the assumption of a steady state between accretion and gravitational settling. The results are unrealistically different for DA and DB white dwarfs. When heavy matter is accreted onto stars, it induces an inverse $\mu$-gradient that leads to fingering (thermohaline) convection. The aim of this letter is to study the impact of this specific process on the derived accretion rates in white dwarfs and on the difference between DA and DB. We solve the diffusion equation for the accreted heavy elements with a time-dependent method. The models we use have been obtained both with the IRAP code, which computes static models, and the La Plata code, which computes evolutionary sequences. Computations with pure gravitational settling are compared with computations that include fingering convection. The most important result is that fingering convection has very important effects on DAs but is inefficient in DBs. When only gravitational settling is taken into account, the time-dependent computations lead to a steady state, as postulated by previous authors. When fingering convection is added, this steady state occurs much later. The surprising difference found in the past for the accretion rates derived for DA and DB white dwarfs disappears. The derived accretion rates for DAs are increased when fingering convection is taken into account, whereas those for DBs are not modified. More precise and developed results will be given in a forthcoming paper.