Description
At least a quarter of white dwarfs display signatures of atmospheric pollution of heavy metals in their spectra, caused by the accretion of planetary debris. This debris is thought to originate from planetesimals that are tidally disrupted, forming a debris disc. The material in the disc is then accreted onto the white dwarf, generating the pollution we observe. I will discuss how this accretion process is commonly modelled and whether a different choice of model, one in which the accretion rate decays exponentially, affects the interpretation of the composition of the material. I will show how our analysis of two cool, helium-atmosphere polluted white dwarfs depends on the choice of disc model, demonstrating the phases of accretion that a simplified disc model would miss. With our analysis, both white dwarfs were found to be accreting rocky planetary material with a bulk composition similar to that of the Earth, and the total amount of accreted material for each star ranged from a small moon to dwarf planet in mass. These two white dwarfs, though typical in their polluted material composition, provide an ideal starting point to test disc models, due to their low temperatures and long sinking timescales.
