Activity trends in young solar-type stars

We apply the Continuous Period Search (CPS) time series analysis method on Johnson B and V band photometry of 21 young and active solar-type, collected over 16 to 27 years and characterize the behaviour of their activity. Using the CPS method, differential rotation could be estimated from the observed variations of the photometric rotation period. Active longitudes were retrieved by applying a non-parametric period search on the light curve minimum epochs, and activity cycles by applying a secondary period search on the modelled light curve mean and amplitude values. We supplemented the time series results by calculating new $\log{R'_{\rm HK}}$ emission indices for the stars from high resolution spectroscopy. The measurements of the photometric rotation period variations point to a trend of increasing differential rotation coefficients towards longer rotation periods but do not reveal any dependence from the effective temperature of the stars. The secondary period searches revealed activity cycles in 18 of the stars and temporary or persistent active longitudes in 11 of them. The activity cycles fall into specific activity branches when $P_{\rm rot}/P_{\rm cyc}$ is examined against ${\rm Ro}^{-1}$ and $\log{R'_{\rm HK}}$. We find a new split into subbranches, indicating multiple simultaneously present cycle modes. Active longitudes appear to be present only on the more active stars. There is a sharp break separating the less active stars with no active longitudes from the more active ones with active longitudes. In seven stars with the estimated active longitude periods are significantly shorter than the mean photometric rotation periods. This systematic trend can be interpreted either as a sign of the active longitudes being sustained from a deeper level in the stellar interior or as azimuthal dynamo waves exhibiting prograde propagation.