On Convective Turnover Times and Dynamos In Low-Mass Stars

TL;DR

This study derives empirical convective turnover times for low-mass stars, revealing a sharp increase at the fully convective boundary and supporting the idea of similar dynamo mechanisms across different stellar structures.

Contribution

It provides new empirical convective turnover times based on recent observations and links these to stellar interior properties and dynamo mechanisms.

Findings

Convective turnover time sharply increases for stars around 0.35-0.4 solar masses.

Empirical turnover times indicate dynamo action occurs deep within the convection zone.

Partially and fully convective stars follow a similar activity-Rossby relation.

Abstract

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 re-evaluate 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 $0.35-0.4\ \rm M_{\odot}$, associated with the onset of a fully convective internal stellar structure. Using $\texttt{MESA}$ 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 $0.1-1.2\ \rm M_{\odot}$, 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 accumulate deep in the convection zone before buoyantly rising to the surface.