The stellar rotation--activity relationship in fully convective M dwarfs

TL;DR

This study investigates the rotation-activity relationship in fully-convective M dwarfs using new X-ray data, revealing they follow similar magnetic activity patterns as partly-convective stars, which informs stellar dynamo theories.

Contribution

It provides the first measurement of the power-law slope of the rotation-activity relationship in fully-convective stars and confirms their behavior aligns with partly-convective stars.

Findings

Fully-convective stars follow the same rotation-activity relationship as partly-convective stars.

The power-law slope of the Rossby number and X-ray luminosity relationship is consistent across star types.

Empirical estimates of convective turnover time for fully-convective stars are improved.

Abstract

The coronal activity-rotation relationship is considered to be a proxy for the underlying stellar dynamo responsible for magnetic activity in solar and late-type stars. While this has been studied in considerable detail for partly-convective stars that are believed to operate an interface dynamo, it is poorly unconstrained in fully-convective stars that lack the necessary shear layer between radiative core and the convective envelope. We present new X-ray observations of 19 slowly-rotating fully-convective stars with rotation periods from the MEarth Project. We use these to calculate X-ray luminosities (or upper limits for undetected sources) and combine these with existing measurements from \citet{wrig16b}. We confirm the existence of fully-convective stars in the X-ray unsaturated regime and find that these objects follow the same rotation-activity relationship seen for partly-convective stars. We measure the power-law slope of the relationship between Rossby number (the ratio of the rotation period to the convective turnover time) and the fractional X-ray luminosity for X-ray unsaturated fully-convective stars for the first time, and find it to be consistent with that found for partly-convective stars. We discuss this implications of this result for our understanding of stellar magnetic dynamos in fully- and partly-convective stars. Finally, we also use this data to improve empirical estimates of the convective turnover time for fully-convective stars.