A study of ultraviolet spectra of zeta Aurigae/VV Cephei systems. XII. The physical state of supergiant chromospheres at the transition from coronae to cool winds.

Results are reported from an analysis of 20 IUE spectra of the Zeta Aur binaries HR 6902, HR 2554, and Tau Per during their near-eclipse phases in 1987-1988. The data are presented in tables and graphs and characterized in detail, with comparison of similar results for 22 Vul. It is shown (1) that the Fe II column densities at low heights (about 10 to the 6th km) vary by 3-4 orders of magnitude among the stars, and (2) that the changes are related to their positions on the HR diagram and thus to the onset of cool stellar winds. A new type of hot CS matter is postulated, extended like a corona but at temperatures similar to those in the solar transition region (100,000 K or less). The potential roles of thermal instabilities and ionization in this phase are explored.