Measuring differential rotation of the K-giantζ Andromedae

Aims. We investigate the temporal spot evolution of the K-giant component in the RS CVn-type binary system ζ Andromedae to establish its surface differential rotation. Methods. Doppler imaging was used to study three slightly overlapping spectroscopic datasets, which were obtained independently at three different observing sites. Each dataset covers one full stellar rotation with good phase coverage, which results in a continuous coverage of almost three stellar rotations (P rot = 17.8 d). Therefore, these data are well-suited for reconstructing surface temperature maps and studying temporal evolution in spot configurations. Surface differential rotation was measured by the means of cross-correlation of all possible image pairs. Results. The individual Doppler reconstructions agree well in the revealed spot pattern, recovering numerous low-latitude spots with temperature contrasts of up to ≈ 1000 K with respect to the unspotted photosphere, and also an asymmetric polar cap that is diminishing with time. Our detailed cross-correlation study consistently indicates solar-type differential rotation with an average surface shear α ≈ 0.055, in agreement with former results.