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

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6. Conclusions

In UV spectra of RW Hya, taken around [FORMULA], there is evidence for a high column density of neutral hydrogen along the line of sight to the hot white dwarf. We have demonstrated that this observation can qualitatively be interpreted in terms of a wind accretion model with an associated accretion wake. If the high column density is indeed due to accretion, we have found the first direct observational detection of wind accretion onto a white dwarf in a detached binary system.

Our wind accretion simulation shows a pronounced peak in column density in the line of sight to the accretor for a relatively short phase interval. This peak is due to the high density ridge behind the inner shock of the accretion wake. In our model the height of the peak, i.e. the column density, depends on the mass-loss rate of the M-giant. The high densities increase the recombination rate, and lead to a narrow cone of neutral hydrogen. This cone of neutral hydrogen leads to Rayleigh attenuation if viewed from a favourable angle. The amount of neutral hydrogen formed in the wake strongly depends on the luminosity of the accretor and also on the density structure of the unresolved area inside the computational radius of the accretor [FORMULA]. If about half of the total column density of hydrogen through the wake is neutral hydrogen, the width of the wake of neutral hydrogen would be in good agreement with the observed UV-light curve.

In our model, the orbital phase [FORMULA] at which the line of sight to the accretor passes trough the inner ridge depends on the ratio of the unperturbed wind velocity in the neighbourhood of the accretor to the orbital velocity of the accretor. Negligible orbital velocity imply [FORMULA], whereas a large orbital velocity results in [FORMULA]. In our model the unperturbed wind velocity at the position of the accretor is [FORMULA], which is [FORMULA] of the orbital velocity, leading to [FORMULA]. If the observed high column density of neutral hydrogen at [FORMULA] is indeed due to an accretion wake, this indicates that the unperturbed M-giant wind velocity at the distance of the white dwarf is smaller than [FORMULA].

Our interpretation of the occultation around quadrature assumes that the white dwarf has no wind and no radiation field, of sufficient strength to prevent accretion. This assumption cannot be verified with current observations. A wind, as predicted by the theory of radiation driven winds, would not be detected in the HST spectra analyzed in Sect. 4, but could still prevent accretion. However, the colliding wind model `weak' ([FORMULA]) of Walder (1998), which is representative for the colliding wind model predicted by the wind momentum-luminosity relation, does not lead to an isolated high column density at [FORMULA].

According to the evolutionary model for a [FORMULA] post-AGB single star (Vassiliadis & Wood 1994) the time for a white dwarf to cool from [FORMULA] down to [FORMULA] is [FORMULA]. This is short compared to the time it takes for an early type [FORMULA] M-giant to become a planetary nebula. Thus, it is likely that the white dwarf was previously less luminous than it is now in its symbiotic binary phase. We expect that the white dwarf in a detached binary system has accreted hydrogen and helium rich M-giant matter in the pre-symbiotic phase. If the white dwarf enters the symbiotic phase, it then has a hydrogen and helium rich surface layer. For such a white dwarf which undergoes mass-loss at [FORMULA], it is likely that hydrogen and helium in the wind decouple from the metallic ions (Springmann & Pauldrach 1992, Porter & Skouza 1999). In this case the metallic ions can freely accelerate, leaving hydrogen and helium with no further acceleration. If this happens before escape velocity is reached, hydrogen and helium will fall back onto the white dwarf (Porter & Skouza 1999). This leads to a drastic decrease of the hot wind momentum, making it unlikely that the hot wind can prevent accretion. The white dwarf radiation field will also act only upon the in-falling metallic ions in the M-giant wind, with little pressure on the in-falling hydrogen and helium ions. To put stringent upper limit on the mass-loss rate of the white dwarf, new high resolution, high signal to noise UV spectra are needed.

The interplay of accretion and nova-like outbursts in symbiotic systems is an interesting but unsolved question. Since the beginning of the [FORMULA] century neither a nova-like outburst, nor any Z Andromeda type activity has been recorded for RW Hya. Spectroscopic and photometric variability is strictly periodic with the orbital period, indicating that it arises from viewing angle effects alone. RW Hya thus belongs to the class of very stable symbiotic binary systems. It appears to be in a quiet stage which is compatible with either a post-outburst plateau luminosity phase, or burning of the accreted hydrogen under steady-state conditions (Sion & Starrfield 1994). The accretion rate of [FORMULA] in our simulation is sufficient to power the hot component via steady-state thermonuclear burning The total nuclear-burning luminosity of [FORMULA] falls in the range of hot component luminosities derived from observations (Kenyon & Mikolajewska 1995; Schild at al. 1996), but one has to keep in mind that the conditions for steady-state burning on a low-mass hot white dwarf strongly depend on its stellar parameters (Sion & Starrfield 1994).

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© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000