(the following information provided by Dr. Pierre Chayer)
Drs. Stephane Vennes, Jean Dupuis, and Stuart Bowyer from the Center for Extreme Ultraviolet Astrophysics, observed and analyzed the EUVE spectra of the hot DAO white dwarf EUVE J0720-317, member of a pre-cataclysmic binary system, and the nearby hot white dwarf EUVE J0723-277.
They noted that the column densities in the line of sight of EUVE J0720-317 and EUVE J0723-277 are very different even though these two stars are less than 5 degrees apart on the celestial sphere. The measured interstellar medium column densities of neutral hydrogen N(HI), neutral helium N(HeI), and ionized helium N(HeII) in the line of sight of EUVE J0720-317 are respectively 2.1e18 cm-2, 1.3e18 cm-2, and smaller than 0.5e18 cm-2, although in the line of sight of EUVE J0723-277 the column densities are respectively 0.9e18 cm-2, 0.1e18 cm-2, and 0.2e18 cm-2. They point out that the N(HeI) to N(HI) ratio, along the sight line of EUVE J0720-317, is unusually high with a value of 0.6 compared to the typical values of 0.05 to 0.08 observed in the line of sight of other hot white dwarfs.
They propose two possibilities to explain the high N(HeI) to N(HI) ratio toward EUVE J0720-317. They suggest that this line of sight might very well represent a direction in the local interstellar medium where the hydrogen is strongly ionized. For example, if (N HI + N HII)/(N HeI + N HeII) is equal to 10, they derive a ionization fraction of 90% for hydrogen and 30% for helium. The other possibility comes from the fact that EUVE J0720-317 is member of a pre-cataclysmic binary that went through a common envelope evolution phase in the past.
Dr. Vennes and his group put forward the idea that some fossil circumbinary material might attenuate the EUV emission produced by the white dwarf. The results of these observations were presented by Dr. Dupuis at the 10th European Workshop on White Dwarfs in June 1996, in Blanes, Spain.
See Attached PostScript Figure
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