August 11-15, 2014


Fingering convection and its consequences for accreting white dwarfs

Sylvie Vauclair (IRAP/OMP/Toulouse)

G. Vauclair,IRAP/OMP/Toulouse, France M. Deal, IRAP/OMP/Toulouse and LUPM/Montpellier, France F. Wachlin,University of La Plata, Argentina

It has been realized for a decade that planetary material accreted onto stars cannot stay in their outer layers due to fingering (thermohaline) convection. This effect has first been studied for young stars which accrete matter from protoplanetary disks. It is much more efficient than atomic diffusion, although the two processes may compete in later evolution stages, because of the diffusion-induced mu-gradient which may stabilize some layers. The same physics must apply to white dwarfs in case of accretion of matter from a debris disk composed of heavy elements. Preliminary computations show that the fingering instability is very efficient in DAs but not in DBs because of the helium environment. We started a series of numerical simulations to study this effect in various cases. We will show that in order to reproduce a given observed abundance in a DA white dwarf , the accretion rate properly derived with fingering convection exceeds by orders of magnitude the one estimated by assuming depletion by gravitational settling only. The results have strong implications on the estimate of the masses of the disrupted bodies at the origin of the debris disk.

Mode of presentation: oral