Accretion of Debris-Disk Matter onto White Dwarfs: Stellar Consequences and Derived Accretion Rates

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 observed abundances, taking into account the mixing induced by convective zones and gravitational settling. The values obtained 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 which is established during an accretion process. Taking this mixing into account leads to an increase in the derived accretion rates, especially for DA white dwarfs, and modifies the conclusions.