An alternative explanation of the EUV spectrum of the white dwarf G191-B2B invoking a stratified H+He envelope including heavier elements

Abstract

Only recently has it been possible to find an effective temperature and composition for the hot DA white dwarf G191–B2B which can consistently match the optical, far-UV and EUV data simultaneously, by using new non-LTE model calculations that include the effects of line blanketing from more than 9 million (mainly Fe and Ni) transitions. However, to maintain agreement below the He ii 228-A Lyman limit requires the inclusion of additional He ii opacity in the form of photospheric, circumstellar or interstellar material. If photospheric, the homogeneous H+He structure adopted predicted a He ii 1640-A line strength well above the IUE detection limit, while an unusally high He ionization fraction was required in this particular line of sight if the material was in the local ISM. We show that, by considering the effects of a stratified H upon He photospheric structure within the heavy-element blanketed models, a lower interstellar He ii column density is obtained, leading to a more reasonable He ionization fraction for the G191–B2B line of sight. In addition, the absence of a He ii 1640-A feature in the far-UV data is consistent with the equivalent width predicted by the stratified model. However, while this alternative interpretation offers a better description of certain aspects of the G191–B2B optical-to-EUV spectra, a new problem is introduced, namely that the He ii Lyman lines are predicted to be stronger than can be comfortably accommodated by the observational data.