The Flare-Ona of 31 Comae
Session 5 -- Chromospheres/Coronae/Active Binaries
Display presentation, Monday, 30, 1994, 9:20-6:30

## [5.03] The Flare-Ona of 31 Comae

31 Comae (HD111812: G0~III) is a rapidly rotating moderate mass giant in the middle of the Hertzsprung gap. It is a luminous source of soft X-rays and C~IV~$\lambda$1549, although it is a member of the X-ray deficient'' class of F/G giants (and early-F dwarfs) identified by Simon \& Drake (1989, ApJ, 346, 303).
We discuss the nature of the corona of 31~Com based on an 80~ks pointing by the Extreme Ultraviolet Explorer (EUVE), and historical FUV and X-ray exposures from IUE and ROSAT, respectively. We provide a context for the 31~Com energy distribution by applying an identical analysis to archival EUVE, IUE, and ROSAT/PSPC spectra of the evolved stars $\alpha$~CMi (F5~IV), $\alpha$~Aur (G8~III + G0~III), and HR1099 (K0~IV [+ G5~V]).
31~Com shows a remarkably hot coronal energy distribution that rivals that of the hyperactive RS~CVn system HR1099 (and the dMe flare star AU~Mic). It is decidedly different from soft coronal sources like the Sun and $\alpha$~CMi. The hot coronal structure is reminiscent of a continuously flaring plasma. However, the overt signatures of impulsive outbursts -- common on RS~CVn's and dMe flare stars -- are absent. We propose that the G0~III giants at the red edge of the Simon \& Drake X-ray deficiency boundary are undergoing a transient evolutionary episode of intense magnetic activity. We believe that the surface manifestations include widespread compact flaring bright points'' commensurate with the shallow unstable envelopes of these newly convective stars. The flare-ona'' of 31~Com suggested by the EUVE allows us to interpret the peculiar 80--300~\AA\ spectrum of $\alpha$~Aur reported by Dupree and collaborators (1993, ApJL, 418, L41). The enhanced emission measure at and above $10^7$~K very likely is from the active G0~III secondary, while the pronounced peak in EM near $5\times10^6$~K is from the more evolved (post helium flash) G8~III primary.