Size of coronal structures in active stellar coronae from the detection of X-ray resonant scattering

We have analyzed high-resolution X-ray spectra of a large sample of active stars observed with the High Energy Transmission Grating Spectrometer on Chandra in order to investigate the properties of optical thickness of the coronal plasma. The analysis of Lyman series lines arising from hydrogen-like oxygen and neon ions shows in the coronae of the active RS CVn-type binaries II Peg and IM Peg significant decrements in the Ly alpha/Ly beta ratios as compared with theoretical predictions and with the same ratios observed in similar active binaries. We interpret these depletions in terms of resonance scattering of line photons out of the line-of-sight. These observations present the first strong evidence for this effect in active stellar coronae. The net line photon loss implies a non-uniform and asymmetric surface distribution of emitting structures on these stars. Escape probability arguments imply typical line-of-sight sizes of the coronal structures that dominate the X-ray emission of 10(10) cm at temperatures of 3 x 10(6) K and 10(10) cm at 10(7) K. These sizes are an order of magnitude larger than predicted by simple quasi-static coronal loop models, but are still very small compared to the several 10(11) cm radii of the stars.