What happens to the planetary systems at the death of their star ? Clues can be found in the study of some white dwarfs. An excess in the infrared radiation and the presence of heavy elements in their spectra show the existence of disks of planetary debris around these stars. Small bodies included in these disks are destroyed by tidal effect when their orbit approaches the white dwarf. The material resulting from their disintegration produces the observed infrared radiation and its accretion by the star accounts for the presence of heavy elements on its surface. These accreted elements diffuse rapidly towards the interior of the star because of the very strong gravity, 10,000 times higher than the solar gravity. Their real-time observation shows that accretion is occurring.
A team of IRAP has been able to assess coherently the mass of material accreted by these stars by taking into account, in the calculations, the thermohaline circulation, an hydrodynamic instability due to the inversion of the average molecular mass resulting from accretion. Accretion rates of about 3.10^14 to 10^16 kg per year are required to account for the observed abundances of heavy elements. With such accretion rates, and considering the lifetimes typical of a million years for these debris disks, the mass of the bodies destroyed by tidal effect is between that of large asteroids such as Vesta and Ceres and that of Pluto.
- Deal, M., Deheuvels, S., Vauclair, G., Vauclair, S., Wachlin, F.C. 2013, A&A, 557, L12 “Accretion from debris disks onto white dwarfs. Fingering (thermohaline) instability and derived accretion rates”