The rotation-activity correlation among G and K giants in binary systems

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

Received: 15 November 2006
Accepted: 13 December 2006

Abstract

Aims.The present study aims (i) to test the existence of a correlation between magnetic activity and rotation among G and K giants in binary systems and (ii) to test whether parameters other than rotation play a role in determining the X-ray emission level of intermediate-mass giants.

Methods.The method consists in testing the existence of correlations between measured stellar parameters including the X-ray surface flux, rotation period, Rossby number and surface gravity of a sample of G and K giants with masses included between 1.5  and 3.8 .

Results.I found evidence that the X-ray surface flux F_X of intermediate-mass G and K giants is correlated with their rotation period P as previously observed on single G giants. Confidence in the degree of correlation is not higher when the Rossby number is used in place of the rotation period, but it significantly improves when stellar gravity g is taken into account. The empirical relation given by     differs from the power-law dependence with an index of about -2 between X-ray to bolometric luminosity ratio and the rotation period that is observed for main-sequence stars. The X-ray surface flux of single G giants and of intermediate-mass G and K giants in close binary systems, such as RS CVn systems, also depends on the stellar gravity. This dependence could result from the effect of gravity on the electron density and emission measure of the X-ray emitting plasmas, as well as on the characteristic sizes of coronal magnetic loops. The measured X-ray surface-flux dependence on gravity is, however, not as steep as the one predicted by simple models of hydrostatic loops that assume a fixed ratio between the coronal energy losses by thermal conduction and by radiation.

Conclusions.I conclude that (i) a relation exists between the rotation and X-ray activity level in giants, (ii) that this relation is not directly dependent on the presence of a companion and applies to all intermediate-mass giants with either G or K spectral type, and (iii) that gravity is an important stellar parameter in determining the X-ray surface flux of intermediate-mass giants.