L. Kriskovics

Antisolar differential rotation of the K1-giant σ Geminorum revisited

Context. Surface differential rotation and other global surface flows on magnetically active stars are among the observable manifestations 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 sigma 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 available 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 sigma 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 alpha 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, 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.

Investigating magnetic activity in very stable stellar magnetic fields - Long-term photometric and spectroscopic study of the fully convective M4 dwarf V374 Pegasi

The ultrafast-rotating (

Doppler imaging of stellar surface structure - XXIV. The lithium-rich single K-giants DP Canum Venaticorum and DI Piscium

P_\mathrm{rot}\approx0.44 d

Multiwavelength study of the low-luminosity outbursting young star HBC 722

) fully convective single M4 dwarf V374 Peg is a well-known laboratory for studying intense stellar activity in a stable magnetic topology. As an observable proxy for the stellar magnetic field, we study the stability of the light curve, and thus the spot configuration. We also measure the occurrence rate of flares and coronal mass ejections (CMEs). We analyse spectroscopic observations,

Magnitude-range brightness variations of overactive K giants

BV(RI)_C

Lithium enrichment on the single active K1-giant di Piscium: Possible joint origin of differential rotation and Li enrichment

photometry covering 5 years, and additional

Time-series Doppler images and surface differential rotation of the effectively single, rapidly rotating K-giant KU Pegasi

R_C

Antisolar differential rotation with surface lithium enrichment on the single K-giant V1192 Orionis

photometry that expands the temporal base over 16 years. The light curve suggests an almost rigid-body rotation, and a spot configuration that is stable over about 16 years, confirming the previous indications of a very stable magnetic field. We observed small changes on a nightly timescale, and frequent flaring, including a possible sympathetic flare. The strongest flares seem to be more concentrated around the phase where the light curve indicates a smaller active region. Spectral data suggest a complex CME with falling-back and re-ejected material, with a maximal projected velocity of

Rotation–differential rotation relationships for late-type single and binary stars from Doppler imaging

\approx

Measuring differential rotation of the K-giant

675km/s. We observed a CME rate much lower than expected from extrapolations of the solar flare-CME relation to active stars.