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Astron. Astrophys. 354, 1014-1020 (2000)

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3. Observed UV light curve

Below [FORMULA] the observed continuum flux is dominated by radiation from the white dwarf. Shortly before and after geometric eclipse by the M-giant, the white dwarf continuum is strongly attenuated. The spectral characteristic of the flux reduction indicates Rayleigh scattering by neutral hydrogen in the M-giant wind and line blanketing by Fe II absorption lines (Dumm et al. 1999; see also Vogel 1991; Shore & Aufdenberg 1993). In addition to the attenuation close to the photospheric eclipse by the M-giant, RW Hya shows this phenomenon also at [FORMULA], when the binary system is approximately at quadrature. In Fig. 1 we compare the IUE spectrum taken at this phase with the unaffected spectrum at phase [FORMULA] obtained with HST. The strength of the attenuation requires a column density of neutral hydrogen of [FORMULA].

[FIGURE] Fig. 1. IUE and HST GHRS UV spectra of RW Hya. Upper spectrum taken at [FORMULA] (HST). Lower spectrum taken at [FORMULA] (IUE) showing the effect of Rayleigh scattering due to a high column density of neutral hydrogen. Flux in units of [FORMULA] erg/(cm2 s Å).

The flux variation [FORMULA], where Rayleigh scattering is the dominant attenuation source, is shown in Fig. 2. At phases [FORMULA] 0.77 and 0.81 we still detect weak attenuation but the spectrum taken at [FORMULA] is not affected. The flux reduction at quadrature lasts thus for [FORMULA], and is well isolated from the normal photospheric eclipse by the M-giant. The orbit of RW Hya is known accurately, we therefore exclude significant errors in the phases.

[FIGURE] Fig. 2. Flux variation in RW Hya at [FORMULA], in units of [FORMULA] erg/(cm2 s Å). The horizontal arrow marks the range where the M-giant geometrically occults the white dwarf. The error bars of the three HST measurements have significantly smaller error bars than the IUE data, and are not plotted. For clarity a dotted line connecting the measurements around [FORMULA] is plotted.

In Sect. 5 we shall model this observational finding with an accretion wake trailing the white dwarf. A necessary requirement for such a model to be appropriate is that the white dwarf radiation field cannot prevent accretion, and that the white dwarf has no significant wind.

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© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000