Antisolar differential rotation of the K1-giant σ Geminorum revisited

¹ Konkoly Observatory of the Hungarian Academy of Sciences, Konkoly Thege út 15-17, 1121 Budapest, Hungary
e-mail: kovari@konkoly.hu
² Leibniz-Institute for Astrophysics (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

Received: 5 May 2014
Accepted: 3 November 2014

Abstract

Context. Surface differential rotation and other global surface flows on magnetically active stars are among the observable manifestations of the underlying stellar dynamo. Therefore, these types of observations are important for stellar dynamo theory and useful constraints for solar dynamo studies as well.

Aims. We revisit the active K1-giant component of the long-period RS CVn-type binary system σ Gem and its global surface flow pattern.

Methods. We refine the differential rotation law from recovering the spot migration pattern. We apply a detailed cross-correlation technique to a unique set of 34 time-series Doppler images recovered using data from 1996−97. By increasing the number of the available cross-correlation function maps, we expect a more robust determination of the differential surface rotation law. In addition, we present a new time-series Doppler imaging study of σ Gem using our advanced surface reconstruction code iMap for a data set collected in 2006−07.

Results. Results from the reprocessed cross-correlation study confirm that the star performs antisolar-type differential rotation with a surface shear α of − 0.04 ± 0.01, i.e., almost a factor of two larger compared to the previously claimed value. We also confirm the evidence of a global poleward spot migration, with an average velocity of 0.21 ± 0.03 km s^-1, in accordance with theoretical predictions. From the new observations, we obtain three subsequent Doppler images. The time evolution of these images confirms the antisolar-type differential rotation of the same amount.