We are therefore interested in objects which mark the transition from the immediate pre-WD stage to the beginning of the WD cooling sequence. They comprise the hottest white dwarfs and their progenitors (either central stars of planetary nebulae or subdwarf O stars). We perform quantitative spectral analyses in order to clarify their evolutionary history. We find their position in the HRD and conclude on interior nucleosynthesis and mixing processes from element abundance determinations. To this end, we perform spectroscopic observations and stellar atmosphere modeling.
Many WDs are “contaminated” by heavy elements. In hot WDs, metal “clouds” are supported by radiative levitation. In cool WDs this process cannot work. Convection can be responsible, that mixes heavy elements from the WD interior into the atmosphere. For some WDs it is believed that on-going external pollution occurs. Matter is accreted at a more or less constant rate, perhaps stemming from a debris disk that is fed by tidally disrupted asteroids. Hence, the abundance determination of trace elements in WDs allows us to investigate the physics of diffusion in stars, and to determine the composition of the accreted material, i.e., the composition of solid bodies in extrasolar systems.
Besides spectroscopic observations, we perform time-dependent photometry of (pre-) WDs with our institute’s 80 cm reflector. Often, these observations are part of world-wide network campaigns for day- or week-long uninterrupted photometry. They aim at asteroseismic investigations of pulsating stars.