![[Spectrum]](Images/hp_fig1.gif)
White dwarfs of spectral type DA have atmospheres consisting of extremely pure hydrogen; correspondingly, the only spectral lines visible are the Balmer lines in the optical and the Lyman lines in the ultraviolet range. Only in the very hottest objects, at very high resolution and signal-to-noise can very weak lines of C, N, Si, Fe and Ni be detected. In these hot white dwarfs the metals are to a certain extent kept in the atmosphere against gravitational settling by radiative levitation.
In cooler DA, below an effective Temperature of 20000 K, additional features were noted on the red wing of the Ly alpha line (similar features for Ly beta are marked by the arrows in the spectrum above). When the transition of the hydrogen atom takes place during a close encounter with another hydrogen atom or a proton, the difference in potential energy between the initial and final state of the radiator results in an energy shift of the emitted/absorbed photon. If the interaction between emitter and perturber occurs at a distance, where the potential difference is stationary with respect to internuclear distance (the most likely scenario), so called satellites are formed. The combination of radiator and perturber resembles a hydrogen molecule, which has led to the name "quasi molecular satellites". The same applies to other Lyman lines and, indeed, HUT observations of WD2032+248 (Wolf 1346) revealed distinct features in the Ly beta profile at 1060 and 1078 Å.
The spectrum above shows the same features in a far UV spectrum of WD0413-077 (better known as 40 Eri B), obtained with the ORFEUS-SPAS II platform in December 1996. The red line shows a new theoretical model calculated by D. Koester and N. Allard for an effective Temperature 16500 K and a surface gravity log g = 7.75. The excellent agreement provides yet another example of the level of accuracy theoretical atmosphere models of DA white dwarfs have reached.