X-ray spectroscopy of the Hertzsprung-gap giant star 31 Com, observed with XMM-Newton

Abstract

We have analysed the XMM-Newton X-ray spectra of the yellow giant 31 Com with the aim of deriving information on the coronal structures of this archetypical Hertzsprung-gap star. To determine the emission measure distribution vs. temper- ature, EM(T ), and the elemental abundances of the coronal plasma, with an accurate line-based approach, we have developed a new method for simple and accurate line measurements, based on rebinning and co-adding the two RGS spectra. We have reconstructed the EM(T ) independently with both APED and CHIANTI atomic databases in order to investigate possible differ- ences in the final outcome of the analysis, and we have obtained consistent results. The derived emission measure distribution has a well defined peak at T ∼ 10 7 K and a significant amount of plasma at higher temperatures; there is also evidence for plasma at temperatures below ∼10 6.5 K, with a mean electron density of ∼3 × 10 10 cm −3 , as inferred from the line ratio of the O  triplet. We have made a global fitting of the EPIC spectra, using multi-component isothermal (3-T) model, and then com- pared the results with the EM(T ), looking for a consistent multi-temperature description of both the RGS and EPIC spectra, over the whole spectral range. While the EM(T ) and the 3-T models individually provide a good description of the data set on which they are based, none of them describes adequately the data of all the other instruments; the disagreements may be related, at least in part, to cross-calibration problems. Finally, we have used the EM(T ) to derive information about the properties of the coronal structures. Our results indicate that the corona of 31 Com is dominated by a class of magnetic loops with peak temperature ∼10 7 K and apparently more isothermal than the solar ones.