The relation between X-ray activity and rotation in intermediate-mass G giants

European Space Agency, ESTEC – Postbus 299, 2200 AG Noordwijk, The Netherlands

Received: 3 June 2005
Accepted: 11 July 2005

Abstract

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() ≈ –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.