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

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4. HZ 43

The binary system HZ 43 is a very bright extrasolar source of radiation in the extreme UV and soft X-rays. Continuum hard photons of the system are radiated by the hot hydrogen-dominated white dwarf, designated as HZ 43 A. Effective temperature and other parameters of the white dwarf were analysed in a number of papers (Paerels et al. 1986, Holberg et al. 1986, Finley et al. 1990, cf. also Barstow et al. 1995a). In this paper I shall adopt results of the recent analysis by Napiwotzki et al. (1993), who derived [FORMULA] 49 000 K and [FORMULA], based on the archival EXOSAT and ROSAT PSPC data, blue and red visual spectrograms, and the NLTE model atmospheres computed with the code by Werner (1986).

The secondary companion star (HZ 43 B) is classified as a dM3.5e star (Margon et al. 1976). Napiwotzki et al. (1993) presented a thorough spectral analysis of this star and concluded, that the cool component of the HZ 43 system should not contribute to the total EUV and X-ray luminosity at energies above 0.1 keV.

The papers quoted above point out, that the atmosphere of HZ 43 A consists of exceptionally pure H with practically no trace of helium or heavier elements. The NLTE model atmosphere analysis by Napiwotzki et al. (1993) gives only upper limits for abundances of heavier elements. The authors claim, that the number abundance of helium in HZ 43 A is less than [FORMULA]. Abundances of CNO elements are even lower, in which the highest is the upper limit of carbon abundance, [FORMULA]. Most recent EUVE observations by Barstow et al. (1995b) yield [FORMULA].

The pure H model atmosphere of HZ 43 A exhibits an insignificant effect of Compton heating in the uppermost layers, which is related to the lower [FORMULA] of this star and rather high gravity (compare to [FORMULA] curves in Fig. 1). The only clear effect of Comptonisation can be seen in the X-ray spectrum. Fig. 3 displays the X-ray part of its spectrum (solid line), which features a cutoff at [FORMULA] (0.3 keV). The spectrum of this star computed with account of coherent Thomson scattering (long-dashed line) predicts much higher flux at wavelengths [FORMULA].

[FIGURE] Fig. 3. X-ray spectra of various model atmospheres of the DA white dwarf HZ 43 A. Solid line is the Comptonized spectrum of a pure H model, whereas the dashed line is the Thomson scattering spectrum. Note the X-ray flux deficiency of the spectrum computed with Compton scattering. Dotted lines present Comptonized spectra of H-dominated atmospheres with He number abundance [FORMULA] (line a), [FORMULA] (line b), and [FORMULA] (line c), and with zero abundance of heavier elements.

The effect can be found in hydrogen-dominated atmospheres only and its existence strongly depends on the remnant abundance of heavier elements. This is illustrated by a set of models, in which small amounts of helium have been added to hydrogen (dotted lines in Fig. 3). The three lines represent X-ray spectra of hydrogen-dominated model atmospheres with [FORMULA] and [FORMULA] as in HZ 43 A, with the helium number abundance [FORMULA] (line labeled a), [FORMULA] (line b), and [FORMULA] (line c). It is evident, that even small amounts of helium efficiently increase absorption in that spectral region, which in turn reduces the significance of electron scattering. Therefore the value of the Compton parameter y is then reduced in X-rays and the effects of Comptonisation (depression of X-ray flux) quickly vanish.

The real X-ray spectrum of HZ 43 A is best represented by the solid line in Fig. 3, taking into account exceptionally low [FORMULA] (Barstow et al. 1995b) and the apparent absence of heavier elements. This is the theoretical prediction which at present cannot be verified by observations due to limited spectral resolution of X-ray observations (e.g. that of ROSAT PSPC).

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998