Capella: Coronal Variability from EUVE

Extreme ultraviolet spectra (λ λ 70--740) of the bright spectroscopic binary system Capella (α Aur, HD 34029, G8 III+G0 III) were obtained at several different orbital phases using the spectrometers on the Extreme Ultraviolet Explorer Satellite (EUVE) from 1992 through 1999. The most recent spectrum was obtained in September 1999 in conjunction with the Chandra Emission Line Project. All spectra show rich emission dominated by strong iron emission lines: Fe IX, Fe XV--XXIV (excepting Fe XVII). The emission measure for the system generally maintains a continuous distribution of plasma temperatures between 10⁶ and 10^7.4K, with a local maximum at 10^6.8K. This enhancement of the emission measure over a restricted temperature region appears to be a stable feature of the corona in Capella. Such ``bumps'' in the emission measure are associated with rapidly rotating individual stars or stellar systems which for one eclipsing systen, 44i Boo, can be identified (Brickhouse and Dupree 1998, ApJ, 502, 918) with a high latitude dense feature. No short term variations in the Capella EUVE spectra are found that might be attributed to flaring. However, the relative fluxes of high ion species vary by factors of 3 to 4 indicating a modification of the apparent structure of the corona of the Capella system when viewed at different aspects. This research is supported in part by NASA Grant NAG5-7224 to the SAO.