ON THE NATURE OF THE EMISSION-LINE PROFILES OF SYMBIOTIC STARS .1. ACCRETION DISKS

The Balmer lines in the spectra of symbiotic stars are often seen at high resolution to have a double-peaked structure. Following the suggestions of previous authors we attempt to model these profiles on the assumption that they originate in an accretion disc which surrounds the hot component of the system. Our theoretical model is an adaptation of that developed by Home & Marsh to model similar line profiles found in the spectra of dwarf novae. Successful modelling would constrain several important physical parameters of these systems. Application of the model to those symbiotics for which we have obtained high-resolution optical spectroscopy, which have been suggested previously to contain accretion discs, and for which reliable orbital parameters already exist, leads to acceptable fits for three stars (CH Cyg, AG Dra and T CrB). For one of the prime accretion disc candidates, CI Cyg, a good fit to the observations was found, but at an epoch at which the disc would, in fact, be expected to be eclipsed. For EG And, the weakness of the emission line does not permit firm conclusions to be drawn. Finally, the model provides poor fits to the line profile for RS Oph based on the published orbital parameters. In general, it appears unlikely that the lines originate solely from a disc, and we briefly discuss alternative models.