The Ca II H&K Rotation-Activity Relation in 50 Mid-to-Late Type M-Dwarfs

In the canonical formulation of stellar magnetic dynamo theory, the tachocline in partially convective stars serves to arrange small scale randomly oriented fields, which are generated by stochastic movement of plasma, into a large scale coherent field. Mid-to-late M-dwarfs show more magnetic activity than classical magnetic dymano theory predicts. This leads to the open question: what mechanism generates and maintains these fields in fully convective stars? Mid-to-late M-dwarfs show tight correlations between rotation rates and magnetic activity, consistent with elements of classical dynamo theory. We use data from Magellan Inamori Kyocera Echelle (MIKE) Spectrograph to measure R'HK values for 50 spectroscopically identified M-dwarfs selected from the MEarth survey. These stars span spectral classes from M5.0 to M3.5 and have rotation periods ranging from hours to months. Here, we present the rotation–activity relationship as traced through these data. We find power law and saturated regimes consistent to within one sigma of previously published results, and find the saturated value to be mass-dependent.