Time-series Doppler images and surface differential rotation of the effectively single, rapidly rotating K-giant KU Pegasi

Abstract

Context. According to most stellar dynamo theories, differential rotation (DR) plays a crucial role in the generation of toroidal magnetic fields. Numerical models predict surface differential rotation to be anti-solar for rapidly rotating giant stars, i.e. their surface angular velocity could increase with stellar latitude. However, surface differential rotation has been derived only for a handful of individual giant stars to date. Aims. The spotted surface of the K-giant KU Pegasi is investigated in order to detect its time evolution and to quantify the surface differential rotation. Methods. We present 11 Doppler images from spectroscopic data collected with the robotic telescope STELLA between 2006 and 2011. All maps are obtained with the surface reconstruction code iMap. Differential rotation is extracted from these images by detecting systematic (latitude-dependent) spot displacements. We apply a cross-correlation technique to find the best differential rotation law. Results. The surface of KU Peg shows cool spots at all latitudes and one persistent warm spot at high latitude. A small cool polar spot exists for most but not all of the epochs. Re-identification of spots in at least two consecutive maps is mostly possible only at middle and high latitudes and thus restricts the differential-rotation determination mainly to these latitudes. Our cross-correlation analysis reveals solar-like differential rotation with a surface shear of α = + 0.040 ± 0.006, i.e., approximately five times weaker than on the Sun. We also derive a more accurate and consistent set of stellar parameters for KU Peg including a small Li abundance of ten times less than solar.