The Continuous Time Spectrum of Long-Term Activity of the Fast-Rotating K2 Dwarf V833 Tau

Abstract. The magnetic activity of the Sun and low-mass stars demonstrates often a cyclic behavior. Some stars exhibit solar-like activity cycles, while some show irregular cycles; there are several stars with double or multiple cycles which serve to challenge the underlying theoretical understanding. This is why a consistent method to separate ‘true’ cycles from stochastic variations is required. We suggest that a conservative method, based on the best practice of wavelet analysis previously applied to the study of solar activity, for studying and interpreting the one of the longest available stellar activity record – photometric monitoring of young, fast-rotating dwarf V833 Tau (K2.5 V) for 120 yr.   We find that the observed brightness variations of V833 Tau with time-scales of 2–50 yr should be comparable with the known quasi-periodic solar mid-term variations, whereas the true cycle of V833 Tau, if it exists, should be of about a century or even longer. We argue that this conclusion does not contradict the expectations from the stellar dynamo theory. Activity of this star is close to the saturated regime, and we expect, that the stellar dynamo in this mode enhances the quasi-stationary magnetic field with chaotic changes, but without a pronounced cycle. However, we cannot completely exclude the scenario that V833 Tau has essentially an unstable dynamo and its cycle length varies within the period of 10–40 yr.