Flare X-Ray Observations of AB Doradus: Evidence of Stellar Coronal Opacity

X-ray spectra of the late-type star AB Dor obtained with the XMM-Newton satellite are analyzed. AB Dor was particularly active during the observations. An emission measure reconstruction technique is employed to analyze flare and quiescent spectra, with emphasis on the Fe XVII 15-17 Å wavelength region. The Fe XVII 16.78 Å/15.01 Å 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 Å line photons from the line of sight. The escape probability technique indicates an optical depth of ≈0.4 for the 15.01 Å line. During the flare, the electron density is 4.4 × 1010 cm-3, and the fractional Fe abundance is 0.5 ± 0.1 of the solar photospheric value. Using these parameters, a path length of ≈8000 km is derived. There is no evidence of opacity in the quiescent X-ray spectrum of the star.