The Stellar Coronae in Active Binary Systems

Abstract

The study of stellar coronae has been traditionally limited by the small collecting area of the telescopes and the low resolution of the instruments operating in X-ray and EUV, yielding to a thermal structure in the coronae of only 2 or 3 temperatures. With the arrival of EUVE in 1992, it has been possible to give a detailed description of the coronal structures in the range log T _e (K) ~ 5.9–7.4 for a wide sample of stars, through the Emission Measure Distribution (EMD). The first systematic survey of the EMD in a sample of cool stars is presented here, involving 22 active binary systems and 6 single stars. The survey shows the persistence of a “bump” in the distribution at log T_e (K) ~ 6.9, with densities of log N _e (cm⁻³)≳ 12, in the active stars, but with a clear dependence with the stellar rotation. The EMD is interpreted in view of the actual coronal loops models, which are insufficient to explain all observational details. The technique employed in the analysis has been successfully applied in the interpretation of the spectra of Chandra and XMM-Newton, which allow to extend the analysis to temperatures up to log T _e (K) ~ 8, and to accurately calculate the coronal abundances.