First measurement of the magnetic field on FK Com and its relation to the contemporaneous starspot locations

TL;DR

This paper reports the first measurement of the magnetic field on FK Com, correlating magnetic variations with starspot locations and chromospheric activity, revealing opposite polarities at active regions.

Contribution

It provides the first simultaneous magnetic field measurements and starspot mapping on FK Com, linking magnetic field variations with surface activity.

Findings

Maximum magnetic field of 272 G detected.

Magnetic field varies with starspot phases.

Active regions may have opposite polarities.

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 days is compared with the starspot 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 Halpha, Hbeta and Hgamma 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.