Abstract

Many isolated, old white dwarfs (WDs) show surprising evidence of metals in their photospheres. Given that the timescale for gravitational sedimentation is astronomically short, this is taken as evidence for ongoing accretion, likely of tidally disrupted planetesimals. These polluted WDs require detailed modeling of surface mixing to understand accretion rates and compositions of parent bodies that supply polluting material. Much modeling of the pollution process relies on assuming an equilibrium between diffusive sedimentation and metal accretion supplied to the WD’s surface convective envelope. However, recent work is calling attention to the importance of additional mixing processes such as convective overshoot and the thermohaline instability. We present results from a large grid of MESA models indicating that this additional mixing can lead to inferred accretion rates that are several orders of magnitude higher than previous estimates. WDs with hydrogen atmospheres at effective temperatures above 10,000 K require rates as large as 10^13 g/s to explain observed Calcium abundances. Mixing other than diffusion may also inhibit chemical separation, resulting in systematic effects on inferences of compositions for the rocky bodies that are the ultimate source of polluting metals.