First measurement of the magnetic field on FK Com and its relation to the contemporaneous star-spot locations

Abstract

In this study, we present simultaneous low-resolution longitudinal magnetic field measurements and high-resolution spectroscopic observations of the cool single giant FK Com. The variation of the magnetic field over the rotational period of 2.4 d is compared with the star-spot location obtained using Doppler imaging techniques, V-band photometry and V − I colours. The chromospheric activity is studied simultaneously with the photospheric activity using high-resolution observations of the Hα ,H β and Hγ line profiles. Both the maximum (272 ± 24 G) and minimum (60 ± 17 G) in the mean longitudinal magnetic field, � Bz� , are detected close to the phases where cool spots appear on the stellar surface. A possible explanation for such a behaviour is that the active regions at the two longitudes separated by 0.2 in phase have opposite polarities.