Rotation and Spot Activity of Young Solar-Type Stars

We present results from a recent multidecade study of the spot activity of young solar-like stars, based on photometry gathered at the Fairborn observatory since 1987. Our period analysis of this photometry reveals systematic tendencies in the stellar rotation and activity behaviour with respect to stellar mass. Period variations seen in the light curves point to a flat rotation dependence of the absolute equator to pole differential rotation. Activity cycles are commonly seen in the light curve mean levels and amplitudes and their lengths fall on distinct branches when compared to the Rossby number and chromospheric activity levels. Our results indicate that there are systematic simultaneously excitable cycle modes present in the active stars in the form of a previously undescribed split into two parallel cycle branches. Several of the studied stars have superimposed cycles belonging to both of the parallel branches. Active longitudes are also frequently seen on the more active stars and range from short lived structures lasting for a few years into highly stable longitudinal confinement of the observed activity over decades. We find, however, that there is a sharp transition between weakly and strongly active stars so that only stars with a sufficiently high activity level have clearly developed active longitudes. When active longitudes do appear, they commonly have a markedly shorter rotation period than the photospheric mean rotation period, recovered from direct light curve period analysis. This may be a signature of radial differential rotation or that the active longitudes follow an azimuthal dynamo wave.