Temperature dependence of the Lyman αline wings in cool hydrogen-rich white dwarf atmospheres. Application to ZZ Ceti white dwarf spectra

Abstract

Theoretical Lyman α absorption profiles show a significant dependence on temperature of the H2 satellite at 1600 A, whereas the H + satellite at 1400 A remains unchanged. The use of opacities calculated with different approximations concerning the electric dipole transition moment (constant, dependent on interatomic distance, and modulated by the Boltzmann factor) lead to very different synthetic absorption spectra. Our new Lyman α profiles over the ZZ Ceti range of temperatures have been used to synthesize theoretical spectra which compare very well to existing HST spectra. In Allard et al. (1999) we derived a classical path expression for a pressure-broadened atomic spectral line shape which allows for an electric dipole moment that is dependent on the posi- tion of perturbers. The theory was applied to the atomic hydro- gen Lyman lines broadened by collisions with neutral and ion- ized atomic hydrogen. The far wings of the Lyman series lines exhibit line satellites, enhancements that may be associated with quasi-molecular states of H2 and H + . We showed that the strengths of these features depend on the values of the electric dipole moments at the internuclear separations responsible for the satellites. Profiles were computed with and without spatial dependence of the dipole moment, and were compared with as- tronomical and laboratory observations. We demonstrated that the dependence of the electric dipole transition moment on in- teratomic separation is an important factor which cannot be ne-