I. Tuominen

Linear polarization and activity in late-type stars

Five-colour (UBVRI) polarimetric observations of a sample of solar-type stars are reported. The observations show a clear wavelength dependence of polarization for a few stars, and a statistical dependence on activity (CaII(H+K)-emission) in the short wavelengths (U and B). Recent observations on successive nights show also rotational variations of linear polarization in U or B band for some stars. These observations indicate large spotty areas and/or scattering patches on these stars.

Polarimetry of stellar active regions and flares

Observations of regular and irregular polarimetric variability in late-type stars are reviewed, and the related physical and geometrical effects are discussed. There are indications that the irregular part of the variability could be caused by transient events, possibly associated with flares. Polarimetric observations during flares are reviewed, and preliminary results of new observations of a well-known flare star, YY Geminorum, are presented. The results show that the small flare in YY Gem did not cause any significant variations in linear polarization, while the binary eclipse evidently causes an enhancement in the polarization. The reasons for the difficulties in stellar flare polarimetry are discussed. Finally, future prospects for the observations of flaring stars and for the utilization of linear polarimetry as a complementary method to other techniques of surface imaging of stellar activity and flares are presented.

Broadband linear polarization and chromospheric CA II H and K emission in late-type main sequence stars

We discuss correlations between broadband linear polarization, intrinsic stellar parameters, and Ca 11 H+K emission in G and K dwarfs.