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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]

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
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