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Since identifying the gap in the H-R Diagram (HRD) marking the transitionbetween partially and fully convective interiors, a unique type of slowlypulsating M dwarf has been proposed. These unstable M dwarfs provide newlaboratories in which to understand how changing interior structures canproduce potentially observable activity at the surface. In this work, we reportthe results of the largest high-resolution spectroscopic H$\alpha$ emissionsurvey to date spanning this transition region, including 480 M dwarfs observedusing the CHIRON spectrograph at CTIO/SMARTS 1.5-m. We find that M dwarfs withH$\alpha$ in emission are almost entirely found 0 to 0.5 magnitude above thetop edge of the gap in the HRD, whereas effectively no stars in and below thegap show emission. Thus, the top edge of the gap marks a relatively sharpactivity transition, and there is no anomalous H$\alpha$ activity for stars inthe gap. We also identify a new region at 10.3 $<M_{G}<$ 10.8 on the mainsequence where fewer M dwarfs exhibit H$\alpha$ emission compared to M dwarfsabove and below this magnitude range. Careful evaluation of literature resultsindicates that 1) rotation and H$\alpha$ activity distributions on the mainsequence are closely related, and 2) fewer stars in this absolute magnituderange rotate in less than $\sim$13 days than populations surrounding thisregion. This result suggests that the most massive fully convective stars losetheir angular momentum faster than both partially convective stars and lessmassive fully convective stars.

the astronomical journal

Mind the Gap I: H$\alpha$ Activity of M Dwarfs Near the Partially/Fully  Convective Boundary and a New H$\alpha$ Emission Deficiency Zone on the Main  Sequence