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Astron. Astrophys. 352, 217-227 (1999)

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Evolution of X-ray activity and rotation on G-K giants

P. Gondoin

Astrophysics Division, European Space Agency - Postbus 299, 2200 AG Noordwijk, The Netherlands (pgondoin@astro.estec.esa.nl)

Received 20 July 1999 / Accepted 20 September 1999

Abstract

The recent availability of stellar parallaxes provided by the Hipparcos star catalogue (ESA 1997) enables an accurate determination of the positions of single field giants in a theoretical H-R diagram and a reliable estimate of their masses. The present study combines these new astrometric data with previously published X-ray fluxes and rotational velocities. The results confirm the existence of a sharp decrease of X-ray emission at spectral type K1 for 2.5 [FORMULA] 5 [FORMULA] giants. The study shows that the rotational velocity of these stars reaches a minimum at the same location in the H-R diagram. However, no tight relationship between X-ray luminosities and projected equatorial velocities was found among the sample stars. I suggest that these results could reflect the importance of differential rotation in determining the level of coronal emission among [FORMULA] 2.5 [FORMULA] G and K giants. The restoration of rigid rotation at the bottom of the red giant branch could prevent the maintenance of large scale magnetic fields, thus explaining the sharp decrease of coronal X-ray emission at spectral type K1.

Key words: stars: activity – stars: evolution – stars: late-type

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© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999
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