Starspot activity of HD 199178

Doppler images from 1994–2017^⋆⋆⋆

¹ Department of Physics, PO Box 64 00014 University of Helsinki, Finland
e-mail: thomas.hackman@helsinki.fi
² Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁴ ReSoLVE Centre of Excellence, Department of Computer Science, Aalto University, PO Box 15400 00076 Aalto, Finland
⁵ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden

Received: 3 December 2018
Accepted: 11 March 2019

Abstract

Context. Studying the spots of late-type stars is crucial for distinguishing between the various proposed dynamo mechanisms believed to be the main cause of starspot activity. For this research it is important to collect observation time series that are long enough to unravel both long- and short-term spot evolution. Doppler imaging is a very efficient method for studying spots of stars that cannot be angularly resolved.

Aims. High-resolution spectral observations during 1994–2017 are analysed in order to reveal long- and short-term changes in the spot activity of the FK Comae-type subgiant HD 199178.

Methods. Most of the observations were collected with the Nordic Optical Telescope. The Doppler imaging temperature maps were calculated using an inversion technique based on Tikhonov regularisation and utilising multiple spectral lines.

Results. We present a unique series of 41 temperature maps spanning more than 23 years. All reliable images show a large cool spot region centred near the visible rotation pole. Some lower latitude cool features are also recovered, although the reliability of these is questionable. There is an expected anti-correlation between the mean surface temperature and the spot coverage. Using the Doppler images, we construct the equivalent of a solar butterfly diagram for HD 199178.

Conclusions. HD 199178 clearly has a long-term large and cool spot structure at the rotational pole. This spot structure dominated the spot activity during the years 1994–2017. The size and position of the structure has evolved with time, with a gradual increase during the last years. The lack of lower latitude features prevents the determination of a possible differential rotation.