Are stellar coronae optically thin in X-rays?

Abstract

Abstract. 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 45 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 Fe  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 O  and Ne  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 Fe  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 Fe . 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.

Anomalous X-ray line ratios in the cTTS TW Hydrae

“…A comparative analysis of the line ratios λλ15.26/15.01 and (3D/3C) λλ15.26/16.78 (3D/3F) has been carried out by Ness et al (2003) to investigate opacity effects. For both ratios TW Hya shows significantly larger ratios possibly indicating resonant line scattering effects.…”

A brief review of the intensity of lines 3C and 3D in neon-like Fe XVII

“…In the analysis of Capella data, the presence of the line coincidence of the Na-like line "C" and the Fe XVII line 3D was pointed out by Behar et al [65] independently of the measurements by Brown et al [56]. Ness et al [60] did an extensive study of the ratio measured from 24 different stellar coronae using measurements from both the RGS and HETG, and found R to be in the same range as measured in the corona of the Sun. Although many sources have ratios that are comparable to those measured at the LLNL EBIT and thus no significant resonance scattering is taking place, resonant scattering of line 3C has been observed (see for example [63]).…”

“…Using the reflection grating spectrometer (RGS) on XMM-Newton and the transmission grating spectrometers (HETG and LETG) on Chandra, high resolution spectra of Fe XVII have been measured from a variety of extra-Solar sources, particularly stellar coronae, and lines 3C and 3D have been measured and resolved, in most cases for the first time [60][61][62][63][64].…”

X-ray optical depth diagnostics of T Tauri accretion shocks

“…In fact, extensive investigation of coronal spectra for optical depth effects has shown coronae to be effectively optically-thin in almost all cases (e.g. Ness et al 2003;Testa et al 2004bTesta et al , 2007, with only a few notable exceptions (see below).…”