ILLUMINATION IN SYMBIOTIC BINARY STARS - NON-LTE PHOTOIONIZATION MODELS .1. HYDROSTATIC CASE

We describe a non-LTE photoionization code that calculates the atmosph eric structure and emergent spectrum of a red giant illuminated by the hot component of a symbiotic binary system. The model assumes hydrost atic, radiative, and statistical equilibrium for the red giant atmosph ere and solves the radiative transfer equation with a local escape pro bability method. We compute non-LTE level populations for a variety of ions and predict the variation of emission-line fluxes as function of the temperature and luminosity of the hot component. Our models produ ce strong emission lines only when the hot component has a high effect ive temperature, T-h greater than or similar to 100,000 K, for hot com ponent luminosities, L(h) greater than or similar to 630 L.. Predicted electron densities and temperatures for the photoionized atmosphere a gree with observations. The models also produce reasonably large conti nuum variations that are consistent with the light curves of some symb iotic stars. However, predictions for most optical and ultraviolet emi ssion-line fluxes fall well below those observed in typical symbiotic stars. We conclude that the hot component must illuminate a red giant wind to reproduce observed line fluxes. Hydrostatic red giant atmosphe res simply do not have enough material beyond the photosphere to accou nt for the emission features observed in most symbiotics. Illumination can modify the structure of a red giant atmosphere even when the emit ted spectrum changes very little. Energetic photons from the hot compo nent cause the atmosphere to expand by several percent for large hot c omponent luminosities. This expansion is insufficient to increase the red giant mass-loss rate, except in systems where the giant already fi lls or nearly fills its Roche lobe.