A multi-frequency study of symbiotic stars
Multi-frequency observations using a wide variety of telescopes and instrumentation have been used to throw light upon some of the physical characteristics of symbiotic stars. A detailed understanding of these enigmatic binaries requires the planning of large, simultaneous observing campaigns to obtain the maximum possible range of diagnostic tools - line ratios, profiles and intensities and continuum flux densities. This then is the general theme of the work presented in this thesis. The results of a low dispersion, optical spectroscopic survey of northern sky symbiotics are presented and reference is made to the high dispersion counterpart. Preliminary analysis has diagnosed that the dominant excitation mechanism operating in several symbiotic binaries is radiative. Simultaneous optical/ultraviolet and optical/infrared observations of AX Persei and CH Cygni, respectively, are discussed. For AX Per, a simple model is proposed. This explains the observed line profiles, the variability of optical and ultraviolet line ratios and the recent V-band light curve in terms of the gradual obscuration of the outbursting primary component by the large, dusty envelope of the late-type secondary. Light curves have been used to estimate both the distance to the system and the physical extent of the emission regions. Revised positional coordinates are also given. For CII C yg , optical/infrared spectroscopy has been used to classify the cool secondary star. Near-simultaneous optical and radio observations of a sample of symbiotic stars have been used to demonstrate an apparent division between D- and S-type symbiotics. The use of Baimer line ratios as a primary indicator of interstellar extinction has been investigated and rejected. Estimates of 5-GHz optical depth, turnover frequency, angular diameter and mass-loss rate are calculated and an extinction-based method of distance determination is discussed. The ionized regions of D-type syrnbiotics have been shown to be more extensive and of lower density than those in S-type systems thus confirming the hypothesis that the S-type are more likely to be suffering Roche-lobe overflow than the D-type symbiotics. Pioneering submillimetre/millimetre continuum measurements of symbiotic stars are also described. The data for two D-type symbiotics provide evidence for an extra emission component contributing to the observed flux density at submillimetre wavelengths. This has been interpreted as either cold dust emitting at a temperature of 10-20 K or as emission from an optically thick, ionized stellar wind within the cavity excavated by the last outbursts in these systems. Finally, I outline the possibility of further developing the multi-frequency element of this work, of utilizing new or improved observational facilities and of beginning new projects not directly related to this thesis.
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