Soft X-ray characteristics of white dwarfs observed by EXOSAT
Grewing, M.; Heise, J.; Bleeker, J. A. M.; Brinkman, A. C.; Paerels, F.; Gronenschild, E.; Wulf-Mathies, C.; Beuerman, K.
Netherlands
Abstract
EXOSAT has observed 19 hot white dwarfs with alleged strong soft X-ray emission. Positive detection of a large fraction of this sample was obtained, among these practically all hot DA dwarfs. High-resolution spectral data, acquired with the 500 1/mm grating spectrometer, indicates no traces of He in the atmosphere of HZ43, i.e. n(He)/n(H) <= 10-5 at a photospheric temperature of 60000 K (log g = 8). In contrast, the hot DA1 dwarf Feige 24 shows the presence of an appreciable He-abundance (n(He)/n(H) >= 10-3) however no simple homogeneously mixed H/He atmosphere can explain the observed spectral shape.