Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999a%26a...352..217g&link_type=abstract
Astronomy and Astrophysics, v.352, p.217-227 (1999)
Astronomy and Astrophysics
Astrophysics
29
Stars: Activity, Stars: Evolution, Stars: Late-Type
Scientific paper
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 M_sun < M < 5 M_sun 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 >= 2.5 Msun 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.
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