Doppler imaging of stellar surface structure

Abstract

We reconstruct a time series of 28 surface temperature maps (Doppler-images) of the spotted single K2-dwarf LQ Hya from 35 consecutive stellar rotations in Nov.–Dec. 1996. Two more maps are obtained from data in late April and early May 2000. All maps show spot activity preferably at low latitudes between -20° and +50°, with a concentration in a band centered at around +30°, and with only occasional evidence for a higher-latitude spot extension. No trace of a polar spot is found at any of the above epochs. Most of this morphology can be reproduced by our flux-tube emergence model, except for the equatorial activity where the strong Coriolis force due to the rapid rotation always deflects flux tubes to higher latitudes. We also present the detection of weak differential surface rotation from a number of cross-correlation maps of the time-series images in late 1996. A solar-type differential rotation law, i.e. the equator rotating faster than the poles, with rad/day (lap time of ≈280 days) is in agreement with the data. Using the available photoelectric observations from 21 years we refine the rotation period to days and find a remarkable phase coherence over the course of 21 years, supporting the recent finding of active longitudes by Berdyugina et al. Furthermore, our photometry shows a complex multi-cyclic long-term brightness variability with three periods of years, its harmonic and years, respectively. The 3.7-year period would be in good agreement with the fundamental-mode oscillation period predicted by Kitchatinov et al. from a distributed-dynamo model, but remains to be confirmed.