ON THE CHROMOSPHERIC STRUCTURE OF ZETA-AURIGAE

Archival IUE observations of two atmospheric eclipses and optical spectra for a third are combined to produce a reasonably consistent picture of the chromosphere of zeta Aur, although the mass in the chromosphere was probably somewhat different in different years. Column densities of matter along different lines of sight through the chromosphere are measured by using both Lyalpha wings and many lines of ionized metals, and they agree to within the errors of measurement. The chromosphere is seen to extend at least one, probably two, stellar radii above the K4-5 Ib-II supergiant's surface. Excitation temperature increases with height and attains values in the range 6500-12,000 K over much of the chromosphere. Turbulence of the gas is near 20 km s-1 and may increase with height. The pressure of motions at this level is nearly enough to explain the chromosphere's observed radial extent. Electron density, derived from ionization of Fe, Mg, C, N, and Ca, seems to be near 10(9) cm-3 throughout most of the chromosphere. Values of electron density derived from Fe I/Fe II, Mg I/Mg II, and N I/N II are roughly consistent. However, densities derived from C I/C II are about an order of magnitude lower than the others, and this may reflect significant photoionization of neutral carbon by chromospheric Lyalpha. Ca is primarily doubly ionized at greater than 15 R. above the surface, as can be deduced for the similar star 31 Cyg at heights above 60 R. in its 1951 eclipse. Balmer lines are formed in a shell extending from 30-40 R. above the K supergiant's surface to 125 R. above the surface. The equivalent widths in this shell correspond to a radial optical depth tau0 congruent-to 80-210 at line center for Halpha. Hydrogen in the upper chromosphere is probably primary neutral, but the evidence for this is circumstantial. We also discuss support of the chromosphere by magnetic fields and find this predicts a high degree of clumping of the gas.