On X-ray Optical Depth in the Coronae of Active Stars

TL;DR

This study analyzes high-resolution X-ray spectra of active stars to assess coronal optical depth, revealing significant resonant scattering in some cases and providing insights into coronal structure and heating.

Contribution

It presents the first systematic investigation of optical depth effects in stellar coronae using high-resolution spectra, identifying cases of significant resonant scattering and estimating coronal structure sizes.

Findings

Significant optical depth observed in some active stars like II Peg, IM Peg, and EV Lac.

Coronal structures are small, with filling factors of a few percent of the stellar surface.

Most stars show negligible optical depth, consistent with theoretical predictions.

Abstract

We have investigated the optical thickness of the coronal plasma through the analysis of high-resolution X-ray spectra of a large sample of active stars observed with the High Energy Transmission Grating Spectrometer on Chandra. In particular, we probed for the presence of significant resonant scattering in the strong Lyman series lines arising from hydrogen-like oxygen and neon ions. The active RS CVn-type binaries II Peg and IM Peg and the single M dwarf EV Lac show significant optical depth. For these active coronae, the Lya/Lyb ratios are significantly depleted as compared with theoretical predictions and with the same ratios observed in similar active stars. Interpreting these decrements in terms of resonance scattering of line photons out of the line-of-sight, we are able to derive an estimate for the typical size of coronal structures, and from these we also derive estimates of coronal filling factors. For all three sources we find that the both the photon path length as a fraction of the stellar radius, and the implied surface filling factors are very small and amount to a few percent at most. The measured Lya/Lyb ratios are in good agreement with APED theoretical predictions, thus indicating negligible optical depth, for the other sources in our sample. We discuss the implications for coronal structuring and heating flux requirements. For the stellar sample as a whole, the data suggest increasing quenching of Lya relative to Lyb as function of both L_x/L_bol and the density-sensitive MgXI forbidden to intercombination line ratio, as might generally be expected.