A&A 431, 1027-1035 (2005)
The X-ray activity of the slowly rotating G giant CrBP. Gondoin
European Space Agency, ESTEC - Postbus 299, 2200 AG Noordwijk, The Netherlands
(Received 10 September 2004/ Accepted 20 October 2004 )
CrB is a single G giant whose distinctive characteristics include an X-ray luminosity exceptionally high for a slowly rotating star. CrB was observed in March 2003 by the observatory. The X-ray spectra of CrB are described by a MEKAL plasma model with two components at K and 10 7 K. 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 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 CrB suggests a corona configuration with little contribution from quiet regions similar to the Sun. On the contrary the temperature K of the "cool" plasma component is reminiscent of solar type active regions, while the hot ( K) component may be caused by disruptions of magnetic fields associated with a permanent flaring activity. The analysis results of the observation of 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 < 9 days) 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 CrB and slowly rotating giants could be related to the existence of magnetic fields induced by turbulent motion.
Key words: stars: individual: CrB -- stars: activity -- stars: coronae -- stars: evolution -- stars: late-type -- X-rays: stars
© ESO 2005