ASTRONOMY COLLOQUIUM

White dwarfs are the stellar remnants left behind by the majority of stars. In 30-50% of the population, material such as asteroids, comets, moons and even planets from their disrupted planetary systems can be identified by metal absorption lines detected in the white dwarf atmosphere. These stellar remnants thus offer the unique window through which to infer the bulk composition of exoplanetary material, but this method depends upon accurate models of the white dwarf atmosphere. Most white dwarfs have surface convection zones, and one key uncertainty here and throughout stellar evolution theory is in the treatment of convection, in particular convective overshoot, which provides enhanced mixing outside of the convection zone. I will present the first 3D radiation-hydrodynamic simulations of white dwarfs with passive scalar particles that provide a characterisation of the additional mixing caused by convective overshoot. As a test of these models, I have recently confirmed with the Chandra X-ray Observatory that the canonical metal-polluted white dwarf, G29–38, is a source of soft X-rays. This discovery provides the first independent measurement of the accretion rate at a metal-polluted white dwarf, and opens the opportunity for a new window through which to study these systems.  I will present this result, along with recent and ongoing X-ray observations which provide a further test of these models and confirm metal-polluted white dwarfs as a new class of soft X-ray source.