2 - The Quiescent Phase - Theory

Summary

Introduction

The composite nature of a typical symbiotic star is shown effectively in Figure 1.1, which exhibits the spectroscopic features commonly associated with these interesting variables, namely (i) a bright red continuum, (ii) strong TiO absorption bands, (iii) prominent high ionization emission lines, and (iv) a weak blue continuum. Berman (1932) and Hogg (1934) were the first to suggest that these phenomena are most naturally explained by a binary star. Both authors suggested that the behavior of an “average” star with combination spectrum could be understood if an M-type giant had a faint O- or B-type companion star similar to that known to exist in o Ceti. This faint star would give rise to the weak blue continuum, and surrounding nebulosity would provide an obvious emission-line region. The small-scale fluctuations observed in these lines and in the continuum are a result of binary motion, while the larger flares are caused by instabilities in the hot source itself.

A few years later, Kuiper (1940) proposed an alternative binary model, suggesting that a system composed of a normal main sequence star and a Roche lobe-filling companion might explain such peculiar stars as Z And and (3 Lyr. Matter lost by the giant falls onto its fainter companion, giving rise to a hot emission region. In this case, flares and other types of random variability are a result of instabilities in the mass-losing star, rather than in the hot, compact star. This explanation gained some support with the detection of gas streams from the giant components in AX Mon and 17 Lep (Cowley 1964 and references therein).