Carbon in the cool DC and C2 white dwarfs - Dredge-up in compositionally stratified envelopes

The problem of the origin of the carbon detected in cool DC and C2 white dwarfs is reexamined. The conditions under which the helium-rich atmosphere can be contaminated by carbon dredged up from the core are investigated. It is found that trace amounts of carbon can indeed be brought up by the helium convection zone from the tail of the carbon equilibrium abundance profile. The predicted behavior of the carbon abundance as a function of effective temperature for Te less than or equal to 20,000 K is calculated and shown to be consistent with recent carbon abundance determinations based on spectrophotometry. The sensitivity of the results to several parameters and uncertainties is briefly discussed.