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

1. Introduction

Accretion phenomena are of relevance in a wide range of astrophysical objects. Here we consider the case of wind accretion within a binary system, where the mass losing object is smaller than its Roche lobe. If the relative velocity between the accretor and the medium is supersonic, a shock front forms around the accreting star. This shock front limits the accretion wake, a region of highly increased density.

Observational signs for accretion wakes were reported in  Aur systems and X-ray binaries. For Cen X-3 (Pounds et al. 1975) and Vela X-1 (Watson & Griffiths 1977) minima in the X-ray light curve out of eclipse have been found. Jackson (1975) presented a simple model, where such a luminosity decrease was attributed to a dense accretion wake trailing the neutron star. Kaper et al. (1994) and Feldmeier et al. (1996) later realized the importance of the termination of the radiative force driving the wind of the primary as the wind enters the highly ionized region around the neutron star.  Aur systems consist of a mass losing cool supergiant and a hot main sequence star. In these systems the light of the hot star probes the absorption column as a function of phase. Extended UV observations with IUE (International Ultraviolet Explorer) have provided evidence for accretion wakes in  Aur (Chapman 1981), 22 Vul (Ahmad & Parsons 1985), 32 Cyg (Ahmad 1986), 31 Cyg (Ahmad 1989) and AL Vel (Eaton 1994).

The eclipsing symbiotic binary RW Hya is a detached system consisting of a non-pulsating mass losing M-giant and a hot white dwarf on circular orbits with a period of (Schild et al. 1996; Kenyon & Mikolajewska 1995). Thus, we are looking at a system where wind accretion onto a white dwarf is possibly going on. Wind accretion is thought to be a necessary condition for the occurrence of symbiotic novae, however, up to now it has not been directly observed.

In this Paper we present observational evidence suggesting that the white dwarf in RW Hya is trailed by an accretion wake. In Sect. 2 we compile the available UV data of the system. In Sect. 3, we analyze the UV light curve which reveals at a high column density in the line of sight to the white dwarf. We associate this with wind accretion on the white dwarf. A wind or the radiation field from the white dwarf could prevent accretion. In Sect. 4 we put an upper limit on a radiation driven wind from the hot white dwarf in RW Hya. In Sect. 5 we present a hydrodynamical accretion simulation, which we compare in Sect. 6 with the observed light curve.

© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000
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