On stellar coronae and solar active regions

Based on Yohkoh Soft X-Ray Telescope (SXT) observations of the Sun near pea k activity level obtained on 1992 January 6, we search for coronal structur es that have emission measure distributions EM(T) that match the observed s tellar coronal emission measure distributions derived for the intermediate- activity stars epsilon Eri (K2 V) and xi Boo A (G8 V) from Extreme Ultravio let Explorer spectroscopic observations. We find that the temperatures of t he peaks of the observed stellar distributions EM(T), as well as their slop es in the temperature range are very similar to those 6.0 less than or simi lar to log T less than or similar to 6.5, obtained for the brightest of the solar active regions in the 1992 January 6 SXT images. The observed slopes correspond approximately to EM proportional to T-beta with beta similar to 4, which is much steeper than predicted by static, uniformly heated loop m odels. Plasma densities in the coronae of epsilon Eri and xi Boo A are also observed to be essentially the same as the plasma densities typical of sol ar active regions. These data provide the best observational support yet ob tained for the hypothesis that solar-like stars up to the activity levels o f epsilon Eri (K2 V) and xi Boo A are dominated by active regions similar t o, though possibly considerably larger than, those observed on the Sun. The surface filling factor of bright active regions needed to explain the obse rved stellar emission measures is approximately unity. We speculate on the scenario in which small-scale "nanoflares" II dominate the heating of activ e regions up to activity levels similar to those of epsilon Eri (K2 V) and xi Boo A. At higher activity levels still, the interactions of the active r egions themselves may lead to increasing flaring on larger scales that is r esponsible for heating plasma to the observed coronal temperatures of T gre ater than or similar to 10(7) K on very active stars. Observations of X-ray and EUV light curves using more sensitive instruments than are currently a vailable, together with determinations of plasma densities over the full ra nge of coronal temperatures (10(6)-10(7) K and higher), will be important t o confirm flare heating hypotheses and to elicit further details concerning coronal structures at solar-like active region temperatures (T less than o r similar to 5 x 10(6) K)and the temperatures that characterize the most ac tive stars (T greater than or similar to 10(7) K).