Empirical velocity laws for cool giants. I. The symbiotic binary EG Andromedae.

Abstract

An empirical velocity law is presented for the wind of a cool giant. The analysis is performed for the symbiotic binary EG And. The system consists of an M2.4 III giant and a hot subdwarf. The UV radiation of the compact hot component is Rayleigh-scattered in the wind of the cool giant. The column density for Rayleigh scattering varies with the orbital phase, allowing the determination of an empirical density structure. From this, the velocity law is inferred via the continuity equation. The derived velocity law suggests a rather rapid increase to the terminal velocity at a few stellar radii. The analysis of the wind structure is performed within a binary model for the symbiotic system which reproduces the nebular emission.