Measuring differential rotation of the K-giant $\zeta$\,And

TL;DR

This study uses Doppler imaging of the K-giant star $z$ Andromedae to measure its surface differential rotation, revealing solar-like shear and evolving spot patterns over nearly three stellar rotations.

Contribution

It provides the first detailed measurement of differential rotation on $z$ Andromedae using multi-site Doppler imaging over multiple rotations.

Findings

Surface spots with temperature contrasts up to 1000 K

Detection of a diminishing asymmetric polar cap

Solar-type differential rotation with shear $0.055$

Abstract

We investigate the temporal spot evolution of the K-giant component in the RS CVn-type binary system $\zeta$\,Andromedae to establish its surface differential rotation. Doppler imaging is used to study three slightly overlapping spectroscopic datasets, obtained independently at three different observing sites. Each dataset covers one full stellar rotation with good phase coverage, and in total, results in a continuous coverage of almost three stellar rotations ($P_{\rm 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 is measured by the means of cross-correlation of all the possible image pairs. The individual Doppler reconstructions well agree in the revealed spot pattern, recovering numerous low latitude spots with temperature contrasts of up to $\approx$1000\,K with respect to the unspotted photosphere, and also an asymmetric polar cap which is diminishing with time. Our detailed cross-correlation study consistently indicate solar-type differential rotation with an average surface shear $\alpha\approx0.055$, in agreement with former results.