Effect of chromospheric activity on the mean colours of late-type stars

Abstract

We confirm the existence of an excess in the mean dereddened ( U B)0 colour of active single-lined spectroscopic giants over quiescent stars of the same spectral type and find evidence for this excess in ( B V)0, although at a lower scale, when plotted against (R I)0. This result suggests that the use of ( B V)0 calibrations to determine fundamental parameters or spectral types for late-type active stars should be taken with caution. Since magnetic activity is definitely involved, these and similar calibrations using (U B)0 and/or (B V)0 should not be used for the study of late-type young clusters' members or late-type pre-main sequence stars without some consideration. Instead, near-infrared colours like ( R I)0 should be better temperature indicators for these stars.