Abstract

Using the 3D radiation-hydrodynamics code, CO5BOLD , I have modelled the surface convection zones of hydrogen atmosphere white dwarfs. In particular, probing the effective temperature range 11,400K – 18,000K where convection is existent, yet confined to a region small enough to allow modelling of the full vertical extent of the convective and overshoot layers. I will present the first 3D simulations of degenerate stars with passive scalar particles that provide a statistical characterisation of the macroscopic diffusion of metals in the overshoot layers below the convective zone. Our results suggest that macroscopic diffusion, driven by convective overshoot, decays over at least 2.5 pressure scale heights beneath the unstable layers, leading to much larger mixed masses. I will discuss the implications of this result for the inferred accretion rates, masses and compositions of evolved planetary systems, and recent X-ray observations with the Chandra X-ray Observatory which provide a test of these models and confirm metal-polluted white dwarfs as a new class of X-ray source. In addition, combined with photometry from the Sloan Digital Sky Survey, the Galaxy Evolution Explorer and Gaia, our CO5BOLD simulations have improved constraints on the primordial mass of hydrogen in white dwarfs. I will present these results, which have implications for e.g. nuclear burning during the asymptotic giant branch and asteroseismological studies, as well as the prospects for detecting variability at actively accreting white dwarfs.