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

Abstract

The relevance of resonant scattering (absorption by photo-excitation and re-emission out of the line of sight) in the solar corona has always been discussed controversially. Ratios of emission lines from identical ions but different oscillator strengths are tracers for damping of resonance lines due to possible resonant scattering. We present an analysis of a large sample of 44 stellar spectra obtained with all gratings on board Chandra and XMM-Newton and systematically measure line ratios of FeXVII lines at 15.27A and 16.78A lines to the resonance line at 15.03A as well as the He-like f/r ratio of OVII and NeIX. The same ratios obtained from optically thin plasma atomic databases such as MEKAL, Chianti, and APEC are found to be discrepant, however, 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 15.27/15.03A ratio shows a regular trend suggesting blending of the 15.27A 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.