Astronomy and Astrophysics, volume 431, 1027-1035 (2005/3-1)
The X-ray activity of the slowly rotating G giant δ CrB.
Abstract (from CDS):
δ CrB is a single G giant whose distinctive characteristics include an X-ray luminosity exceptionally high for a slowly rotating star. δ CrB was observed in March 2003 by the XMM-Newton observatory. The X-ray spectra of δ CrB are described by a MEKAL plasma model with two components at 6.5x106K and 107K. Series of lines of highly ionized Fe and several lines of the Ly series are visible in RGS spectra, most notably from O and Ne. The oxygen abundance is similar to the average abundance of the other elements but the Ne/O ratio found for δ CrB seems higher than in the solar photosphere, reminiscent of a similar anomaly observed in a subset of solar flares and in active stellar coronae. The spectral fitting of the EPIC and RGS spectra of δ CrB suggests a corona configuration with little contribution from quiet regions similar to the Sun. On the contrary the temperature T≃6.5x106K of the ``cool'' plasma component is reminiscent of solar type active regions, while the hot (T≃107K) component may be caused by disruptions of magnetic fields associated with a permanent flaring activity. The analysis results of the XMM-Newton observation of δ CrB were compared with those of other single G giants with similar spectral type, mass and evolutionary status but with higher rotation rates. The comparison suggests that rapid rotation (P<9days) could increase the surface coverage with active regions and the flaring rate on G giants as expected from classical, helicity related, dynamo-driven activity. We argue that the X-ray emission of δ CrB and slowly rotating giants could be related to the existence of magnetic fields induced by turbulent motion.