EDP Sciences
Free access
Volume 431, Number 3, March I 2005
Page(s) 1027 - 1035
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20041991

A&A 431, 1027-1035 (2005)
DOI: 10.1051/0004-6361:20041991

The X-ray activity of the slowly rotating G giant $\delta$ CrB

P. Gondoin

European Space Agency, ESTEC - Postbus 299, 2200 AG Noordwijk, The Netherlands
    e-mail: pgondoin@rssd.esa.int

(Received 10 September 2004/ Accepted 20 October 2004 )

$\delta$ CrB is a single G giant whose distinctive characteristics include an X-ray luminosity exceptionally high for a slowly rotating star. $\delta$ CrB was observed in March 2003 by the $\it {XMM{-}Newton}$ observatory. The X-ray spectra of $\delta$ CrB are described by a MEKAL plasma model with two components at $6.5 \times10^{6}$ 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 $\delta$ 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 $\delta$ CrB suggests a corona configuration with little contribution from quiet regions similar to the Sun. On the contrary the temperature $T \approx 6.5 \times
10^{6}$  K of the "cool" plasma component is reminiscent of solar type active regions, while the hot ( $T \approx 10^{7}$ K) component may be caused by disruptions of magnetic fields associated with a permanent flaring activity. The analysis results of the $\it {XMM{-}Newton}$ observation of $\delta$ 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 $\delta$ CrB and slowly rotating giants could be related to the existence of magnetic fields induced by turbulent motion.

Key words: stars: individual: $\delta$ CrB -- stars: activity -- stars: coronae -- stars: evolution -- stars: late-type -- X-rays: stars

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© ESO 2005