On Convective Turnover Times and Dynamos in Low-mass Stars

The relationship between magnetic activity and Rossby number is one way through which stellar dynamos can be understood. Using measured rotation rates and X-ray to bolometric luminosity ratios of an ensemble of stars, we derive empirical convective turnover times based on recent observations and reevaluate the X-ray activity–Rossby number relationship. In doing so, we find a sharp rise in the convective turnover time for stars in the mass range of 0.35−0.4 M⊙, associated with the onset of a fully convective internal stellar structure. Using MESA stellar evolution models, we infer the location of dynamo action implied by the empirical convective turnover time. The empirical convective turnover time is found to be indicative of dynamo action deep within the convective envelope in stars with masses 0.1–1.2 M⊙, crossing the fully convective boundary. Our results corroborate past works suggesting that partially and fully convective stars follow the same activity–Rossby relation, possibly owing to similar dynamo mechanisms. Our stellar models also give insight into the dynamo mechanism. We find that empirically determined convective turnover times correlate with properties of the deep stellar interior. These findings are in agreement with global dynamo models that see a reservoir of magnetic flux accumulates deep in the convection zone before buoyantly rising to the surface.