Quiescent and Flaring Structure in RS Canum Venaticorum Stars

Abstract

Four of the most active RS CVn stars (V711 Tau, II Peg, σ Gem, and UX Ari) have been observed for a total of 3 Ms with the Extreme Ultraviolet Explorer satellite (EUVE) between 1992 and 2000 January.Flaring and quiescent states of extreme ultraviolet spectra (λλ70-740) and light curves (λλ75-175) have been analyzed to provide emission measure distributions (EMD) for these systems in the range logT e (K)~5.6-7.4, based principally on iron lines. Flux measurements obtained with IUE and the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) complete the EMD in the lower temperature range [logT e (K)~4.0-5.6].Frequent flaring activity has been found in the systems, including an increase during the rise phase by a factor of ~9 in the flux of σ Gem, the largest flare enhancement observed with EUVE. Analyses of the EUVE emission in the active single star AB Dor and the low-rotation giant star β Cet are also included. The EMDs are remarkably similar among all the stars, showing a narrow enhancement or ``bump'' around logT e (K)~6.9. These narrow bumps are apparently unrelated to rotation rate, spectral type, binarity, or evolutionary stage.Significant material is found at logT e (K)>~7.0 for the most active stars. Modulation of the EUV flux outside of flaring occurs