Understanding the spectral evolution of hot hybrid white dwarfs

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The vast majority of all stars end their life as white dwarfs. Depending on the evolutionary history, we find two different classes, one with hydrogen-rich and one with helium-rich atmospheres. They are born very hot (effective temperature Teff~200,0000 K) and cool down to a few 1000 K within billions of years. Along this cooling sequence, the white dwarfs change their spectral appearance, reflecting the chemical composition of their atmosphere. This spectral evolution is determined by the interplay of several effects, for instance, radiative levitation and gravitational settling of elements, mass-loss and accretion of matter from external sources. Our project aims to understand the spectral evolution of a small subgroup of the hydrogen-rich white dwarfs, the so-called hybrid white dwarfs which exhibit both, hydrogen and helium in their spectra. Though small in number, they represent an evolutionary phase run through by the majority (~75%) of all white dwarfs. We will perform a quantitative analysis of ultraviolet and optical spectra of a carefully selected sample of 36 hot (Teff>60,000 K) white dwarfs which allows, together with distances precisely measured by Gaia, to locate them in the Hertzsprung-Russell diagram and to derive their stellar parameters (mass, radius, luminosity). We aim at the abundance measurement of light and heavy metals in order to shed light on the question, when and how the hybrid white dwarfs transform into helium-free objects because of gravitational settling of elements. The results will help to clarify the relative importance of the different physical processes acting on helium and metal abundances. Together with future element diffusion calculations this will also enable to use hot white dwarfs as "detectors" to determine the composition of extrasolar planetary material that can be accreted onto the young white dwarfs.