The XMM-Newton view of stellar coronae: High-resolution X-ray spectroscopy of Capella

We present the high-resolution RGS X-ray spectrum of the stellar binary Capella observed by the XMM-Newton satellite. A multi-thermal approach has first been applied to fit the data and derive elemental abundances. Using the latter, the emission measure distribution has been reconstructed using a Chebychev polynomial fit. Its shape is found to display a sharp peak around 7 MK, consistent with previous EUVE and ASCA results. A smaller but significant amount of emission measure is required around 1.8 MK in order to explain the O Vii He-like triplet and the C Vi Lyalpha line. We have applied the temperature diagnostics of dielectronic recombination satellite lines to the He-like O Vii triplet to constrain the cool plasma temperature, and have obtained a lower limit consistent with the global reconstruction of the emission measure distribution. We have used line ratios from the forbidden, intercombination, and resonance lines of the O Vii triplet to derive an average density for the cool coronal plasma (n_e <1 10¹⁰ cm^-3). Implications for the coronal structure of Capella are discussed. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA).