The Structure of Stellar Coronae in Active Binary Systems

A survey of 28 stars (22 active binary systems, plus 6 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 log Te(K) ∼5.6–7.4, using iron emission lines. These data are complemented with IUE observations to model the lower temperature range (log Te[K] ∼4.0–5.6). Inspection of the EMD shows an outstanding narrow enhancement, or “bump” peaking around log Te(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 (Ne & 10 12 cm) are derived at ∼log Te(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 log Te(K) ∼6.3 and lower electron densities (Ne & 10 9 − 10 cm), and hotter loops peaking around log Te(K) ∼6.9 with higher electron densities (Ne & 10 12 cm). For the most active stars, material exists at much higher temperatures (log Te[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. Subject headings: stars: coronae — stars: flares — stars: individual — x-rays: stars