On the Balmer Line Problem

K. Werner

Lehrstuhl Astrophysik der Universität Potsdam, Germany

The so-called Balmer line problem describes the failure to achieve a consistent fit to the Balmer line spectrum of any hot sdO star whose effective temperature exceeds about 70,000K. Results of Teff determinations drastically deviate, up to a factor of 2, depending on which Balmer line is examined.

This problem was uncovered a few years ago during non-LTE analyses of very hot subdwarfs and central stars of planetary nebulae. Since then, it cast severe doubt upon non-LTE model atmosphere analysis techniques as a whole. In this Letter we show that the problem is due to the neglect or improper inclusion of metal opacities in the models. On hand of the specific case of BD+28 4211, which is a IUE standard sdO star, we show that the Balmer line problem can be solved when surface cooling by photon escape from the Stark wings of CNO lines is accounted for.

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