The Capella Giants and Coronal Evolution across the Hertzsprung Gap

We present an analysis of the coronal line Fe XXI λ1354 observed in medium-resolution Hubble Space Telescope/Space Telescope Imaging Spectrograph echelle spectra of Capella obtained on 1999 September 12. These data were collected in support of a multiwavelength spectroscopic campaign designed to provide detailed temperature and density diagnostics over a wide range of temperatures, and they are accompanied by simultaneous Chandra, Extreme Ultraviolet Explorer, BeppoSAX, and Very Large Array observations. The line Fe XXI λ1354 formed at 10⁷ K is crucial to the interpretation of these data sets since it allows the deconvolution of the coronal flux of the two binary components. While earlier observations implied comparable coronal emission from both stars, our data indicate that nearly all the Fe XXI flux in the C I/Fe XXI blend at 1354 Å arises from the rapidly rotating G1 III secondary. This suggests a significant decline in the hottest part of the corona of the G8 III primary over the past 5 years. For the first time, we can assign the variability of the high-temperature X-rays to the cooler He core burning star in the system. This result suggests that nascent coronae in the early Hertzsprung gap phase apparently do not vary and that cyclic activity and strong variability are seen only in later evolutionary stages when dynamos are more developed in deeper convective envelopes.