The Inscrutable Hot Component in the Symbiotic Binary Z Andromedae

We analyze optical and ultraviolet spectroscopic observations of the symbiotic binary star Z Andromedae. This system consists of an M3 -M4 giant and a hot component surrounded by an ionized nebula. The orbital mass function is consistent with M_g∼M_sun and M_sunh∼0.5^-1 M_sun as in other symbiotic stars. The nature of the hot component remains unclear. Most quiescent data are consistent with a luminous stellar source powered by thermonuclear burning. The decrease in the effective temperature of the hot component and the fluxes of most high ionization emission lines during outburst provides further support for a nuclear-powered hot component. However, the evolution of He II emission lines in outburst, the short outburst recurrence times, and the presence of short period oscillations in the outburst light curves are all more easily interpreted with an accretion disk model, in which a luminous disk surrounds a solar-type main sequence star. Better models for the light variations and the evolution of emission lines during outburst are needed to choose between accretion disk and thermonuclear models for the hot component.