Astron. Astrophys. 324, 97-108 (1997)

1. Introduction

The families of binary systems referred to as symbiotic and barium stars (as well as their relatives the CH stars and Tc-poor S stars) share much in common in terms of their orbital characteristics and stellar components. Both types of binaries are typified by rather long orbital periods of hundreds to thousands of days and consist of cool giant components (with temperatures corresponding to spectral types G, K or M) and low-mass companions (Kenyon 1994; McClure 1984). These companions are white dwarfs (WD) in most cases (McClure & Woodsworth 1990; Brown et al. 1990; Iben & Tutukov 1996; Jorissen & Van Eck 1997). In the symbiotic binaries, the wind from the cool giant is strong, and the WD is heated by the accretion of some fraction of that wind. The wind is in turn ionized by the radiation field of the hot companion, giving rise to the visual emission lines which partially define a symbiotic system. Presumably, the barium and CH stars consist of binaries in which the cool giant does not have a mass loss rate large enough to trigger significant symbiotic activity (see Jorissen 1997). On the other hand, the cooler analogs of the barium stars, the Tc-poor S stars, display mild symbiotic activity (e.g. HD 35155 and HR 363; Brown et al. 1990; Ake et al. 1991; Jorissen et al. 1996; Jorissen 1997) due to the increased mass loss rates in this more luminous phase of giant branch evolution.

An additional characteristic that originally defined barium stars and their ilk is an overabundance of the elements heavier than iron that are synthesized by the s-process (see Käppeler et al. 1989 for a review). In an earlier paper (Smith et al. 1996 - hereafter referred to as Paper I), we demonstrated that the cool component in the yellow symbiotic system AG Dra is enriched in s-process elements, thus linking yellow symbiotic stars to barium stars. The subclass of yellow symbiotic stars is characterized by the cool component being a G or K giant rather than a M giant as in red symbiotic stars. As most normal G or K giants do not exhibit large enough mass loss rates to drive the symbiotic phenomenon, while the cooler and more luminous M giants do, the evolutionary status of the yellow symbiotics remained unclear (Schmid & Nussbaumer 1993).

In Paper I we showed that AG Dra is metal-poor ([Fe/H] , where [X/H] = log (N (X)/N (H)) - log (N (X)/N (H)) ). Because the giant branch in a metal-poor population is translated towards higher luminosities for a given , it was argued that AG Dra has a larger mass loss rate than a more metal-rich giant of similar . As discussed above, it is this large mass loss rate that drives the symbiotic phenomenon in a metal-poor binary such as AG Dra. By contrast, in a barium system involving a higher-metallicity yellow giant, the mass loss rate is insufficient to trigger the symbiotic activity. If these ideas are correct, abundance peculiarities similar to those of AG Dra are expected to be observed in other yellow symbiotic systems as well.

One such yellow symbiotic system is . Allen (1980) assigns the spectral type G to the giant, while Schulte-Ladbeck (1988) classifies it as approximately G8 based on near-infrared spectra. The cool giant in this system has moreover been suspected to exhibit enhanced heavy-element lines (Jorissen 1989), suggestive of the barium syndrome. This suspicion is tested in this paper from a detailed abundance analysis of the cool component (Sects. 2, 3 and 4). These results are then compared to those obtained for AG Dra in Paper I.

Unlike AG Dra, does not seem to undergo outbursts, though it does exhibit photometric variations. The variations in the Strömgren y and b bands are interpreted by Niehues, Bruch & Duerbeck (1992) as due to the tidal deformation suffered by the giant star (ellipsoidal variable; Sect. 5.3). This property offers the possibility to narrow down the range of admissible parameters for the binary system, when combined with the newly derived orbital parameters presented in Sect. 5.1. Finally, it is shown in Sect. 5.4 that no consistent solution can be found satisfying the constraints from the orbital parameters, the ellipsoidal variations and the evolutionary tracks, with the usual Roche lobe geometry.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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