Anti-solar differential rotation on the active K-giant  Geminorum

¹ Konkoly Observatory, 1525 Budapest, PO Box 67, Hungary e-mail: kovari@konkoly.hu
² Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: [jbartus;kstrassmeier]@aip.de
³ Eötvös University, Department of Astronomy, 1518 Budapest, PO Box 32, Hungary e-mail: vidakris@elte.hu
⁴ Astronomical Institute, Faculty of Mathematics and Physics, Charles University in Prague, V Holešovičkách 2, 18000 Prague 8, Czech Republic e-mail: svanda@asu.cas.cz
⁵ Astronomical Institute, Academy of Sciences of the Czech Republic, v. v. i., Fričova 298, 25165 Ondřejov, Czech Republic

Received: 8 June 2007
Accepted: 21 July 2007

Abstract

The active K1 giant σ Gem and its differential surface rotation is revisited. We refine our previous inconclusive result by recovering the spot migration pattern of this long-period RS CVn-type binary through application of the technique of “average cross-correlation of contiguous Doppler images” to a set of six Doppler images from 3.6 consecutive rotation cycles. We find an anti-solar differential rotation law with a shear of  -0.022±0.006. We also find evidence of a poleward migration trend of spots with an average velocity of ≈300 m s^-1.