EUVE Science Highlights - August 31, 1998
EUVE Science Highlights - August 31, 1998
|
Text only version of this page |
Slide 1 of 2
Dr. Martin Barstow (University of Leicester) and Dr. Ivan Hubeny (NASA/GSFC) investigate, in a paper soon to be published in the Monthly Notices of the Royal Astronomical Society, the challenging EUVE spectrum of the hot white dwarf G191-B2B. This star is a bright DA white dwarf that has been well studied at visible and ultraviolet wavelengths. It is often used as a flux standard for UV and FUV observations. Sitting at the top of the white dwarf cooling sequence, it is an important template for the class of DA (hydrogen dominated atmosphere) stars. EUV and soft X-ray observations have revealed much lower flux than predicted from pure hydrogen atmosphere indicating the presence of additional absorbers in the atmosphere of G191-B2B. This opacity is now attributed to a cocktail of elements (C, N, O, Si, S, P, Fe, and Ni) in which Fe plays a dominant role. However, the EUVE spectrum of G191-B2B still remains partly unexplained. The best efforts to model this spectrum based on LTE and non-LTE model atmospheres are imperfect, unknown opacity sources are still missing because the predicted spectra show too much flux below 200 Å. It has been suggested that helium might be the key to this problem. By adding a modest abundance of helium in the photosphere, it is possible to quench the short wavelength flux below the HeII photoionization edge at 228 angstroms. However the required He abundance is still too high and would be detected in the UV (at 1640 angstroms). One can also increase the interstellar column density of HeII, but this would imply an ionization fraction of 80% for helium in the local ISM, which is uncomfortably large. In this work, Dr. Barstow and Dr. Hubeny investigate the alternative possibility that the atmosphere of G191-B2B is stratified with a thin layer of hydrogen floating on the top of the helium envelope. The atmospheric abundance profiles of hydrogen and helium are those obtained from the assumption of diffusive equilibrium in the atmosphere. They find that a fit of EUVE G191-B2B spectrum using stratified atmosphere is somewhat more successful because they result into a lower value of the HeII column density and do not imply a detectable HeII 1640 absorption line. The authors stress that there is now a problem with the HeII Lyman series predicted to be too strong compared with the observation.