Antisolar differential rotation of the K1-giant sigma Geminorum revisited

Kővári, Zsolt and Kriskovics, Levente and Künstler, A. and Carroll, T. A. and Strassmeier, K. G. and Vida, Krisztián and Oláh, Katalin Ilona and Bartus, János (2015) Antisolar differential rotation of the K1-giant sigma Geminorum revisited. ASTRONOMY & ASTROPHYSICS, 573. ISSN 0004-6361

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Abstract

Context. Surface differential rotation and other global surface flows on magnetically active stars are among the observable manifes- tations of the stellar dynamo working underneath. Therefore, such observations are important for stellar dynamo theory and useful constraints for solar dynamo studies as well. Aims. The active K1-giant component of the long-period RS CVn-type binary system σ Gem and its global surface flow pattern is revisited. 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 avail- able cross-correlation function maps from the formerly used 4 to 17 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 dataset 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 stronger 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.

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