Carbon pollution in helium-rich white dwarf atmospheres Time-dependent calculations of the dredge-up process

Calculations are presented showing that carbon diffusing upward from the core can contaminate the atmospheres of cooling He-rich white dwarfs, a process which provides a natural explanation for the presence of traces of carbon in the spectra of such stars. The computations explicitly couple diffusion and evolution, include the effects of electron degeneracy on the carbon distribution, and take into account the variable ionization state of carbon. Maximum contamination occurs at temperatures around 12,000 K, approximately the effective temperature at which the base of the helium convection zone reaches its maximum depth. Below this temperature, carbon pollution decreases with further cooling, largely because carbon sinks back into the star due to its partial recombination. The sensitivity of the theoretical results to changes in helium layer mass, stellar mass, efficiency of turbulent transport, and convective efficiency is investigated.