What do the Mt. Wilson stars tell us about solar activity?

Abstract

We relate the evolutionary status and mass of the Mt. Wilson project stars with the type and strength of stellar activity as established in decades of monitoring their chromospheric Ca II K line emission. We specifically derive their positions in the Hertzsprung-Russell-diagram (HRD) from Hipparcos parallaxes and SIMBAD B − V data, considering and correcting for the effects of different individual stellar metallicities, and place different activity groups of the Mt. Wilson stars on a common set of Z = 0.02 evolution tracks to obtain a quantitative picture of their relative evolutionary status and mass distribution. We find that, first, the downturn in stellar activity does not depend on absolute age but instead decreases with the relative age as stars advance on the main sequence and thus confirm theoretical expectations, while the most active of the irregularly variable stars are found to scatter around the zero-age main-sequence (ZAMS). Moderately active stars, both with clear cycles like the Sun and those without a dominant activity period, populate the 2nd quarter of main-sequence (MS) evolution. Almost inactive stars are mostly in their 3rd quarter of MS evolution and seem to represent stellar analogues of the solar Maunder minimum state. Totally inactive stars are all in the final quarter of their MS evolution and make up for over 70% of the Mt. Wilson stars that far evolved (the remainders being only weakly active). Most of these are more massive and younger than the Sun. Accordingly, less massive stars did not have enough time to significantly decrease their activity, since they generally evolve more slowly. We find, second, that the Sun is near an apparent upper mass limit for cyclic activity on the MS, because there are no cyclic MS stars much above one solar mass, at least not in the Mt. Wilson sample. Once put in proper perspective with the other Mt. Wilson stars, the Sun indeed ought to be approaching a gradual transition from moderate cyclic activity to a weak, Maunder-minimum-type state, as historic Maunder-type minima seem to indicate already. In addition, the apparent upper mass limit for MS stars to solar-like cyclic activity, not much above one solar mass, is providing dynamo theory with an interesting new challenge.