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LTE or NLTE for the analysis of hot white dwarf and subdwarf B stars?
Received 5 January 1996 / Accepted 13 November 1996
We evaluate the influence of NLTE effects on hydrogen and helium lines of high gravity stars. This investigation covers the classes of hydrogen-rich DA and DAO white dwarfs and helium-rich DO white dwarfs. Furthermore, model atmospheres of hydrogen-rich subdwarf B (sdB) stars are investigated. Implications for model atmosphere analyses of such stars are discussed.
Pure hydrogen atmospheres of DA white dwarfs are well represented by LTE calculations for effective temperatures up to 80000 K. As soon as traces of helium are present, however, drastically larger NLTE effects on the Balmer lines occur, which persist down to effective temperatures as low as 40000 K. However, a simple recipe that yields reliable results, is to neglect the traces of helium in the LTE models for the analysis of DA white dwarfs. Since DAO stars, especially the hotter ones, tend to have lower gravities than the DA white dwarfs, NLTE effects can be very pronounced in DAO stars. Even at higher gravity moderate NLTE deviations are always present for the helium lines and should be taken into account for an accurate analysis of DAO stars.
Similar effects are present in the model atmospheres of sdB stars. However, due to the even lower gravities and, hence, lower densities in their photospheres, NLTE should be taken into account down to effective temperatures of about 30000 K, i.e. for the sdOB stars. Since there exist discrepancies between the results of different groups performing LTE analyses of sdB stars, we also discuss the impact of metal line blanketing.
The class of hot, helium-rich white dwarfs (DO) always displays moderate to strong deviations from LTE. Very strong effects are found in the 60000 K region. This implies that even for the so-called cool DO white dwarfs ( K) the use of NLTE model atmospheres is highly recommended.
Some calculations were performed to test whether NLTE line formation calculations of hydrogen and helium on LTE atmospheres can yield reliable results. While this method considerably improved sdB and DAO models this was not the case for either DA model with trace helium or a DO model. This demonstrates that a careful check for the validity of line formation calculations has to be performed for each application.
Key words: white dwarfs subdwarfs stars: atmospheres
Send offprint requests to: Ralf Napiwotzki
Online publication: June 30, 1998