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Astron. Astrophys. 340, 617-625 (1998)

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5. PG 0824+289

This star is a spectroscopic binary (hot DA plus dwarf carbon star) accompanied by a visually separated M3-4 dwarf (Heber et al. 1993, Green & Margon 1994). The DA white dwarf belongs to the sample recently analysed by Wolff et al. (1995) on the basis of ROSAT spectral data. A model atmosphere analysis of available spectral observations yielded [FORMULA] 36 000 K at the assumed [FORMULA], with the estimated error [FORMULA] of several thousands K. The model atmosphere analysis was performed with pure hydrogen LTE models (Koester et al. 1979). Following a brief discussion Wolff et al. (1995) concluded, that the pure H atmosphere of the DA white dwarf satisfactorily represents the X-ray flux of PG 0824+289, and that both the dC and the dM dwarfs do not contribute to the X-ray luminosity of the whole system.

Model atmosphere computations with Compton scattering did not produce any significant [FORMULA] rise at the outer boundary, and significant effects of noncoherent scattering appear in the X-ray spectrum alone. Fig. 4. displays X-ray branch of the theoretical Comptonized spectrum of PG 0824+289 (solid line). The continuum flux exhibits a cutoff at [FORMULA] (0.3 keV), and a deficiency of flux at [FORMULA] as compared to the Thomson scattering model (long-dashed line). The effect of flux depression and cutoff in X-ray domain is very similar to that in HZ 43 A.

[FIGURE] Fig. 4. Same as in Fig. 3 for the DA white dwarf PG 0824+289. Spectra of pure H models only are indicated in the Figure.

The value of [FORMULA] equals to -0.10 at the cutoff [FORMULA]. This value is also similar to that in HZ 43 at the corresponding cutoff.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998