Are stellar coronae optically thin in X-rays? A systematic investigation of opacity effects

The relevance of resonant scattering in the solar corona has always been discussed controversially. Ratios of emission lines from identical ions but different oscillator strengths have been used in order to estimate damping of resonance lines due to possible resonant scattering, i.e., absorption by photo-excitation and re-emission out of the line of sight. The analysis of stellar spectra in analogy to previous works for the Sun is possible now with XMM-Newton and Chandra grating spectra and requires this issue to be considered again. In this work we present a sample of 44 X-ray spectra obtained for 26 stellar coronae with the RGS on board XMM-Newton and the LETGS and HETGS on board Chandra. We use ratios of the Fexvii lines at 15.27 Å and 16.78 Å lines to the resonance line at 15.03 Å as well as the He-like f/r ratio of Ovii and Ne ix to measure optical depth effects and compare them with ratios obtained from optically thin plasma atomic databases such as MEKAL, Chianti, and APEC. From the Fexvii line ratios we find no convincing proof for resonance line scattering. Optical depths are basically identical for all kinds of stellar coronae and we conclude that identical optical depths are more probable when effects from resonant scattering are generally negligible. The 15.27/15.03 Å ratio shows a regular trend suggesting blending of the 15.27 Å line by a cooler Fe line, possibly Fexvi. The He-like f/r ratios for O and Ne show no indication for significant damping of the resonance lines. We mainly attribute deviations from the atomic databases to still uncertain emissivities which do not agree well with laboratory measurements and which come out with differing results when accounting for one or the other side effect. We attribute the discrepancies in the solar data to geometrical effects from observing individual emitting regions in the solar corona but only overall emission for stellar coronae including photons eventually scattered into the line of sight.