Accretion from debris disks onto white dwarfs (Deal et al.)

Vol. 557
In section 1. Letters

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

Thermohaline mixing, also called double-diffusive convection and "salt fingering", is a relatively new import from geophysics into astrophysics. Its role in stellar interiors is becoming increasingly evident. The process involves mass (and energy) transport in the presence of molecular weight gradients and modifies the usual treatment of convection by allowing for an additional temperature-dependent diffusivity of a "solvent" (that in turbulence would also be called a passive scalar quantity). This modifies the usual convection criterion based on the Rayleigh number for a uniform medium thereby introducing a new dimensionless parameter, the Lewis number. The authors apply this process in a new context, the accumulation of material from circumstellar disks on the surface of white dwarfs, and compare their results with gravitational settling alone. For DB white dwarfs, the process is unimportant, and for DAs it dominates. A steady state can be reached but only at late times and for high-mass accretion rates, and the heavy elements reach much greater depth than predicted with only gravitational settling. A comprehensive exploration of the model is promised in a future paper, so stay tuned.