2003ApJS..145..147S

A survey of 28 stars (22 active binary systems, plus six single stars or wide binaries for comparison) using extreme ultraviolet spectra has been conducted to establish the structure of stellar coronae in active binary systems from the emission measure distribution (EMD), electron densities, and scale sizes. Observations obtained by the Extreme Ultraviolet Explorer satellite (EUVE) during 9 years of operation are included for the stars in the sample. EUVE data allow a continuous EMD to be constructed in the range logT_e(K)∼5.6-7.4, using iron emission lines. These data are complemented with IUE observations to model the lower temperature range [logT_e(K)∼4.0-5.6]. Inspection of the EMD shows an outstanding narrow enhancement, or ``bump'' peaking around logT_e(K)∼6.9 in 25 of the stars, defining a fundamental coronal structure. The emission measure per unit stellar area decreases with increasing orbital (or photometric) periods of the target stars; stars in binaries generally have more material at coronal temperatures than slowly rotating single stars. High electron densities (N_e≳10¹² cm^–3) are derived at ∼logT_e(K)∼7.0 for some targets, implying small emitting volumes.

The observations suggest the magnetic stellar coronae of these stars are consistent with two basic classes of magnetic loops: solar-like loops with maximum temperature around logT_e(K)∼6.3 and lower electron densities (N_e≳10⁹-10^10.5 cm^–3), and hotter loops peaking around logT_e(K)∼6.9 with higher electron densities (N_e≳10¹² cm^–3). For the most active stars, material exists at much higher temperatures [logT_e(K)≥6.9] as well. However, current ab initio stellar loop models cannot reproduce such a configuration. Analysis of the light curves of these systems reveals signatures of rotation of coronal material, as well as apparent seasonal (i.e., year-to-year) changes in the activity levels.