The XMM-Newton view of stellar coronae: Coronal structure in the Castor X-ray triplet

Abstract

We present results of a comprehensive X-ray investigation of the multiple stellar system Castor AB + YY Gem. The observations were obtained with the XMM-Newton observatory. We report the first spatially separated detection and identification of both Castor A and B as frequently flaring X-ray sources. Spectral modeling of the YY Gem and Castor coronal sources indicates a broad plasma distribution between 2-15 MK. Elemental abundances are found to be below solar photospheric values, with the abundance of Ne being highest (~0.7 times solar photospheric). The abundances increase during a large flare, most notably for Fe. The Ovii He-like line triplet indicates average coronal densities of the cooler plasma of a few times 1010 cm-3 for YY Gem and Castor. The YY Gem light curve shows three deep eclipses, indicating that the coronae of both binary components are similarly active and are relatively compact. A 3-D light curve inversion shows that coronal active regions follow density scale heights compatible with spectroscopically measured temperatures. The dominant densities also agree with spectroscopic values. Most active regions are located between degrees latitude, in agreement with findings from previous Doppler imaging. We also report the tentative detection of line broadening due to orbital motion.