Flare X-ray observations of AB Dor: evidence for Stellar Coronal Opacity

Abstract

X-ray spectra of the late-type star AB Dor, obtained with the XMM-Newton satellite are analysed. AB Dor was particularly active during the observations. An emission measure reconstruction technique is employed to analyse flare and quiescent spectra, with emphasis on the Fe XVII 15-17 A wavelength region. The Fe XVII 16.78 A/15.01 A line ratio increases significantly in the hotter flare plasma. This change in the ratio is opposite to the theoretical predictions and is attributed to the scattering of 15.01 A line photons from the line-of-sight. The escape probability technique indicates an optical depth of about 0.4 for the 15.01 A line. During the flare, the electron density is 4.4*10^10 cm^-3 and the fractional Fe abundance is 0.5 +/- 0.1 of the solar photospheric value. Using these parameters a path length of about 8,000 km is derived. There is no evidence for opacity in the quiescent X-ray spectrum of the star.

arXiv comment13 pages, 3 figures, to be published in ApJ Letters

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• X-ray optical depth diagnostics of T Tauri accretion shocks
• Geometry Diagnostics of a Stellar Flare from Fluorescent X-rays
• On X-ray Optical Depth in the Coronae of Active Stars
• X-ray emission processes in stars
• Line intensity enhancements in stellar coronal X-ray spectra due to opacity effects
• Soft X-ray emissions of Si IX in Procyon
• A search for line intensity enhancements in the far-UV spectra of active late-type stars arising from opacity