Modelling the balmer line profiles in the spectra of symbiotic stars

Abstract

The Balmer emission lines in the spectra of symbiotic stars are often seen to have a double-peaked morphology when observed at high resolution. In this respect, they bear striking similarities to the Balmer line profiles in the spectra of cataclysmic variables, (Cv's) where their origin is ascribed to emission from an accretion disc.
The role which accretion discs play within the class of symbiotic stars however, is much more uncertain and modelling the lines on this basis could, if successful, help to strengthen the evidence for their presence in symbiotic systems. This is the main motivation for the work described in this thesis, which is a direct offshoot of the Manchester Echelle/European Southern Observatory High Resolution Spectroscopic Survey of symbiotic stars (see van Winckel et al., 1993; Ivison, Bode & Meaburn., 1994)
We first provide a critique of previously developed, alternative Balmer line profile models and offer our reasons for opting in favour of the accretion disc hypothesis. We then describe two models for computing synthetic line profiles from accretion discs, which were originally developed to simulate line profiles in CV spectra. One of these models (Home & Marsh, 1986) is adapted by us for the purpose of modelling line profiles in the Manchester survey spectra. In its basic form, this model produces line profiles with emission peaks of equal height, whereas virtually all of the survey spectra showed unequal peak heights. We ascribe this to the effects of absorption, possibly due to the outflowing wind of the cool giant, which is universally accepted as an integral part of all symbiotic systems.
The model is used to produce fits to a total of 21 spectra; i.e. all of those spectra in the Manchester survey which were considered suitable for modelling in this way. Only the higher emission peak is modelled, it being assumed that this is the least affected by the aforementioned absorption. Some of the fits are very good indeed while others are not so convincing. This was in part due to the poorer quality of some of the spectra but also possibly to the effects of absorption.
We then investigate the effect on line profiles from an accretion disc, of absorption by a surrounding wind. This is done by constructing a simple wind model which is then incorporated into the Home & Marsh model. Model fits are produced for the best quality spectra from the Manchester survey.
Finally, future developments to the model are discussed as well as further observations which would help to guide this promising line of investigation.