Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005a%26a...444..531g&link_type=abstract
Astronomy and Astrophysics, Volume 444, Issue 2, December III 2005, pp.531-538
Astronomy and Astrophysics
Astrophysics
15
Stars: Activity, Stars: Coronae, Stars: Evolution, Stars: Late-Type, X-Rays: Stars, Stars: Magnetic Fields
Scientific paper
I study the relation between X-ray activity and rotation among intermediate-mass single G giants. The results show evidence that the quiescent coronal activity of these stars, as measured by their X-ray surface flux, increases linearly with the angular rotation velocity and with the inverse of the Rossby number. Even the most rapidly rotating G giants do not reach the canonical log(L_X/Lbol) ≈ -3 saturation level. The effect of rapid rotation on these stars could result mainly in an increased coverage of their surface with magnetic close loop structures. The empirical activity-rotation relationship accounts for the occurrence of a maximum of magnetic activity in the atmosphere of intermediate-mass stars as they evolve off the main-sequence near the bottom of the red giant branch. Remarkably, the relation between X-ray to bolometric luminosity ratio and the Rossby number or rotation period for G giants differs from the power law dependence with an index of about -2 that is observed for main-sequence stars. Possible implications for the dynamo generation of magnetic fields on giants are discussed.
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