OPTICAL-SPECTRA OF ZETA-AURIGAE SYSTEMS .8. THE CHROMOSPHERE OF HR-6902

We analyze chromospheric spectra of the zeta Aur system HR 6902 observ ed during its 1987 and 1989 eclipses, deriving empirical column densit ies, number densities and ionization characteristics for 5 distinct he ights within the giant primary's chromosphere. Column densities for Ca II are determined from K-line profiles and for other ions by a curve- of-growth technique, modelling iteratively the scale heights of the li ne-forming regions according to the known geometry of the eclipses and dimensions of the system. We find evidence for the existence of two p lasma components in and beyond the chromosphere that are physically di fferent. The inner component has a low turbulence (v(tur) = 15 km s(-1 )); its ion column densities decrease steeply outwards and seem to out line the geometrically-thin chromosphere itself. There is some evidenc e that the geometrical extent of this component varies from one eclips e to another. The outer component has a significantly higher turbulenc e (v(tur) = 45 km s(-1)) and a scale height that is nearly an order of magnitude larger. Because its column density is much lower, we have o nly been able to observe it by our technique in the strongest chromosp heric lines (i.e. Ca II); it extends much farther outwards than the in ner component - probably into regions beyond the top of the chromosphe re, for which IUE observations give evidence of plasma at transition-r egion temperatures (approximate to 10(5)). We consider the evolutionar y status of the two components, and conclude that the primary star is an ordinary 3.9 M(.) blue-loop giant that is 2.0 10(8) years old. We c ompare the chromosphere of HR 6902 with those of other zeta Aur giants and discuss the meaning of the 'dividing lines' as tracers of the ver y different conditions which are encountered in G and K (super)giant c hromospheres.