Stellar atmospheres

All the work on white dwarfs conducted at Leicester depends on having access to state-of-the-art stellar atmosphere models. We have continued our longstanding collaboration with Dr Ivan Hubeny of Goddard Space Flight Center and his colleague Dr Thierry Lanz.

The work centres on the use of the TLUSTY programme written by Hubeny and developed by Lanz, with assistance from Martin Barstow at Leicester. TLUSTY is able to calculate the line blanketed atmospheric models which are essential for analysis of the spectroscopic data we have obtained, particularly for those objects containing significant quantities of heavy elements. TLUSTY is coupled with the programme SYNSPEC, which calculates the synthetic spectra from the models, needed for direct comparison with the observations. Recent progress has concentrated on improving the detail in the model calculations. For example, early models only included the 30,000 or so Fe lines observed experimentally. However, this level of blanketing was insufficient to account for the observed levels of opacity in the EUV spectra of the hottest white dwarfs. Extending the models to deal with nearly 10 million theoretically predicted transitions allowed self-consistent interpretation of the EUV, UV and optical data for the first time ( Lanz et al., ApJ 473 , pp 1089-1093, 1996 ).

An EUV count spectrum of G191-B2B covering the wavelength range 180 - 600 Angstroms. The data points ( error bars ) are compared with the predictions of a non-LTE model spectrum created using the TLUSTY code, including the effects of interstellar absorption. The discontinuity near 320 Angstroms where the medium-wavelength and long-wavelength EUVE spectrometer ranges overlap, is due to the different effective areas of the instruments, for which the data are not corrected.

The proposition that He might exist in significant quantities in the atmospheres of hot DA stars coupled with a remaining problem in predicting their soft X-ray fluxes, has prompted to extend TLUSTY to deal with inhomogeneous models. While elements such as Fe should be supported in the atmosphere by radiation pressure, He will not and is most likely found in a layered structure with H overlying a predominantly He envelope. In simple H+He models, we have seen that this structure suppresses the soft X-ray flux in the way required to explain the existing observations.