1996A&A...311..631S

We analyze chromospheric spectra of the ζ Aur system HR6902 observed during its 1987 and 1989 eclipses, deriving empirical column densities, number densities and ionization characteristics for 5 distinct heights within the giant primary's chromosphere. Column densities for CaII are determined from K-line profiles and for other ions by a curve-of-growth technique, modelling iteratively the scale heights of the line-forming regions according to the known geometry of the eclipses and dimensions of the system. We find evidence for the existence of two plasma components in and beyond the chromosphere that are physically different. The inner component has a low turbulence (v_tur=15km/s); its ion column densities decrease steeply outwards and seem to outline the geometrically-thin chromosphere itself. There is some evidence that the geometrical extent of this component varies from one eclipse to another. The outer component has a significantly higher turbulence (v_tur=45km/s) and a scale height that is nearly an order of magnitude larger. Because its column density is much lower, we have only been able to observe it by our technique in the strongest chromospheric lines (i.e. CaII); it extends much farther outwards than the inner component - probably into regions beyond the top of the chromosphere, for which IUE observations give evidence of plasma at transition-region temperatures (≃10⁵). We consider the evolutionary status of the two components, and conclude that the primary star is an ordinary 3.9M_☉ blue-loop giant that is 2.0x10⁸years old. We compare the chromosphere of HR6902 with those of other ζ Aur giants and discuss the meaning of the `dividing lines' as tracers of the very different conditions which are encountered in G and K (super)giant chromospheres.