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Astron. Astrophys. 327, 173-182 (1997)

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5. Discussion

Cataclysmic binaries are most efficient for the investigation of accretion processes on short time scales. To understand the dynamics of accretion phenomena, the parameters of the interacting components in a CV system must be sufficiently well known. These parameters can only be derived with high precision from eclipsing binaries in which the eclipse of the hot spot and that of the white dwarf can be measured separately and radial velocity curves of both components are available. At present, only five systems fitting the required characteristics are known. HS 1804 + 6753 has proved to be a new member of this class. In this paper, relevant system parameters of HS 1804 + 6753 are given. Their relatively small errors are due to the large number of measurements involved. So far, systematic errors have not yet been considered. They may occur when measuring the radial velocities of the stellar components.

For the secondary, there is no phase shift between the mid eclipse determined from the observed radial velocity curve of the secondary and the white dwarf's ingress and egress timings calculated from the photometric data. Hence, it is assumed that for this component the mass centre and the photocentre coincide. This indicates that the heating of the secondary by the primary can be neglected.

However, for the white dwarf a phase shift of [FORMULA] is observed. This phenomenon may be due to an asymmetrical disk or an excentric photocentre of a symmetrical disk, e.g. caused by the hot spot (Marsh, 1988). Nevertheless, the derived photometric mass ratio coincides with the spectroscopic one. A possible influence of the phase shift on [FORMULA] seems to be negligible, as proved by the coincidence of the photometric and spectroscopic mass ratio. Furthermore, the similarity of the derived system velocities [FORMULA] and [FORMULA] indicates that there are probably no relevant distortions of the line profiles.

The location of the hot spot itself is dependent on the radius of the disk, and therefore depends on the activity state of the system. From a superposition of different eclipse light curves in quiescence, only a mean radius could be derived. The observed fluctuations of the eclipse egress of the white dwarf and the resulting possible period fluctuations are too small to affect the derived physical parameters.

In all calculations, the secondary is not assumed to be a lower main sequence star. This can now be proved by applying the mass-radius relation of Patterson (1984)

[EQUATION]

For [FORMULA] a radius of [FORMULA] is calculated. Therefore, the secondary star is definitely not above the main sequence and therefore not evolved.

In principle, from the radiation and the spectral type of the secondary the distance of HS 1804 + 6753 can be derived. Its spectra are preferably taken during the eclipse of both the white dwarf and the hot spot. Even during this phase interval, the observed S/N of the spectra are too low to derive the true spectral type. The only spectral signature of the secondary are Ca I absorption lines, no TiO bands could be detected (see Table 7). The relative ratios of these lines indicate a spectral type of M0, although the spectrum of the secondary is contaminated with that of the accretion disk throughout the eclipse.

The origin of the Ti II absorption lines showing the motion of the primary component is still unknown. Their equivalent widths obviously increase towards the blue spectral range. It must be proved whether they originate from the disk or from the white dwarf itself.

Furthermore, HS 1804 + 6753 is an interesting object because of its frequent outbursts. Normally, the amplitude of an outburst does not exceed [FORMULA], which is lower than expected for dwarf novae, but outbursts up to [FORMULA] have been recorded. During an outburst the hump apparently disappears and a U-shaped eclipse feature appears. For yet unknown reasons, the slope of the egress feature is steeper than that of the ingress. However, the minimum brightness during the eclipse drops to the same level in quiescence as well as in outburst, indicating that the source of the outburst is totally eclipsed by the secondary. From light curves taken during different outbursts, an investigation of the dynamic behaviour of the accretion disk is expected.

Further investigations of HS 1804 + 6753 will include spectral eclipse-mapping as well as Doppler-tomography to reveal e.g. the origin of the Ti II absorption lines. Due to the relatively high precision of the derived system parameters and the obviously high outburst frequency, HS 1804 + 6753 appears to be an outstanding system for the analysis of the involved accretion phenomena.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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