On the Balmer Line Problem

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 ~70,000 K. Results of T eff 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 has 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. In the specific case of BD +28°4211, which is an 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.