Search for trace amounts of hydrogen in hot DO white dwarfs
K. Werner1,21Lehrstuhl Astrophysik, Universität Potsdam, Am Neuen Palais 10, Germany
2Institut für Astronomie und Astrophysik der Universität, D-24098 Kiel, Germany
The question as to the hydrogen content in the atmospheres of helium-rich white dwarfs is of considerable importance to explain the long-standing problem of the chemical surface abundance evolution of white dwarfs along their cooling sequence. It is hoped that the H abundance gives a hint to the origin of the bifurcation of the white dwarf cooling sequence into a H-rich (DA) and a He-rich (non-DA) branch and the possible relationship between these.
We present high resolution spectra from two hot He-rich white dwarfs (RE 0503-289 and PG 1034+001) and the prototype of the PG 1159 spectral class (PG 1159-035). The spectra cover the core of the He II 6560Å line and were taken in order to detect possible weak H alpha emission components. The search was entirely negative and imposes an upper limit (approx 5% by number) for the hydrogen abundance which is tighter than determined from previous medium resolution spectroscopy. We consider implications of this result on the evolutionary scenario.
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