Accretion from debris disks onto white dwarfs

Abstract

Context. 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 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. Aims. When heavy matter is accreted onto stars, it induces an inverse μ-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. Methods. 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. Results. 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. Conclusions. 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

The frequency of planetary debris around young white dwarfs

“…There has been a recent claim by Deal et al (2013) that the diffusion fluxes would change by orders of magnitude, if the thermohaline instability would be taken into account. In Xu et al (2014) we discuss some arguments, why we do not think that these calculation apply to the diffusion in white dwarfs.…”

Accretion of planetary matter and the lithium problem in the 16 Cygni stellar system

“…It occurs in particular in the case of the accretion of planetary matter (Vauclair 2004;Garaud 2011;Deal et al 2013). …”

“…Many studies and past publications assumed that the accreted matter remains inside the outer stellar convective zone, so that accretion can lead to an increase of the heavy element abundances, as well as to an increase of the lithium abundance. This assumption is not valid, as shown in detail by Vauclair (2004), Garaud (2011, and Deal et al (2013). When heavy matter falls onto the star, it creates an inverse gradient of molecular weight, which leads to a double-diffusive instability now called fingering (or thermohaline) convection.…”

Hydrodynamical instabilities induced by atomic diffusion in A stars and their consequences

“…On the other hand, heavy element accumulation leads to inverse, destabilizing µ-gradients. When the global mean molecular weight increases towards the surface, the local stellar gas is subject to thermohaline, or fingering convection (Vauclair 2004;Garaud 2011;Deal et al 2013, and references therein).…”