IUE observations of the atmospheric eclipsing binary system ζ Aurigae

Abstract

ζ Aurigae is an eclipsing binary star system with a property shared by only a few other systems. It consists of two stars in orbit around one another: a cool supergiant star (spectral type K2 II) and a hot main sequence (spectral type B8 v) star. The supergiant star is nearly 200 times larger than the Sun, while the main sequence star is only about seven times larger than the Sun. The Earth lies in the orbital plane of the two stars, and thus they periodically eclipse one another. When the relatively small B star goes into eclipse behind the cool supergiant star, its light passes through extended outer gaseous layers—the outer atmosphere—of the cool star. Thus the B star is an astrophysical light source which serves as a probe of the atmosphere of the K star. The eclipses, which occur every 972 days, have been well studied at visible wavelengths^1,2. We report here IUE observations of the ζ Aur stellar system carried out between September 1979 and January 1980. A preliminary look at the data shows the advantage of UV observations of the system. There is strong evidence for cool, outflowing material near the primary star and hotter material farther from the primary perhaps in a shockwave. The onset of the atmospheric phase of the eclipse of the system—difficult to predict because of the irregularities of the supergiant atmosphere—began between 15 September and 1 November 1979.