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Astron. Astrophys. 318, L9-L12 (1997)

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3. The DA white dwarf

We determine the properties of the white dwarf using the Ly [FORMULA] line profile and the FUV continuum. A grid of FUV synthetic spectra, based on hydrogen line-blanketed model atmospheres, was computed using Schoning & Butler's (1995) stark-broadened H I line profiles. We then normalized the synthetic spectra to the observed spectrum between 1500 and 1800 Å. The fit is restricted to wavelengths between 1150 and 1310 Å and excludes bad pixels. Despite the low quality of the spectrum, a correlation holds between [FORMULA] and [FORMULA]. Figure 2 (top) shows the formal solution, but a wide range of acceptable solutions is also defined by the 1 [FORMULA] countour (66%, Fig. 2 bottom). Table 2 summarizes the properties of the white dwarf; the mass (M) and absolute magnitude ([FORMULA]) are extracted from the measured temperature and surface gravity using Wood's (1995) mass-radius relation. An estimate of the distance (d) is also given using the inferred apparent magnitude V ([FORMULA]).

[FIGURE] Fig. 2. (top) Far ultraviolet spectroscopy of the hot white dwarf in the binary EUVE J0702+129. The red wing of Ly [FORMULA] was unfortunately saturated, but the continuum at longer wavelengths and the blue wing of Ly [FORMULA] are well exposed. (bottom) [FORMULA] confidence contours at 66%, 90%, and 99% in the ([FORMULA], [FORMULA]) plane. A narrow correlation holds between [FORMULA] and [FORMULA]. Bad pixels are marked with crosses.


Table 2. Range of properties of the white dwarf.

Additional properties of the white dwarf are obtained from an analysis of the EUV count rates. Figure 3 shows solutions to the EUV measurements in the ([FORMULA], [FORMULA]) plane assuming a pure hydrogen composition. Upper limits (99%) to the temperature and neutral hydrogen column density in the local interstellar medium (ISM) are set at 41,000 K and [FORMULA], respectively. A joint FUV/EUV solution (1 [FORMULA]), assuming a pure hydrogen atmosphere, gives [FORMULA] [FORMULA] K and [FORMULA] [FORMULA]. Higher temperatures imply atmospheric impurities, such as helium. Therefore, assuming a pure hydrogen atmosphere, the white dwarf is located at a distance of [FORMULA] pc and the neutral hydrogen column density in the ISM is [FORMULA]. The low interstellar column density excludes appreciable interstellar reddening.

[FIGURE] Fig. 3. Analysis of EUVE and ROSAT WFC count rates using pure hydrogen model atmospheres. Confidence contours are given at 68%, 90% and 99%. A joint FUV/EUV solution (see Fig. 2) suggests a temperature of [FORMULA] K, a surface gravity [FORMULA] [FORMULA], and a pure hydrogen atmosphere.
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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998