Hα Activity Near mid-M dwarf Transition Boundary and a Newly identified Hα Emission Deficiency Zone on the Main Sequence

The discovery of main-sequence gap, where it marks the partially and fully convective boundary, reveals a unique type of M dwarfs with an interior structure that changes between one and three layers of energy transport. To make a connection between this unstable interior and atmospheric activity, we present results from the largest high resolution spectroscopic survey of 480 M dwarfs near the boundary. We find that M dwarfs with Hα in emission are almost entirely found 0.0 to 0.5 magnitude above the top edge of the gap, whereas effectively no stars in and below the gap show emission. Thus, the top edge of the gap marks a relatively sharp activity transition. We also identify a region on the main sequence where fewer M dwarfs exhibit Hα emission. This effort has been supported by the NASA’s ADAP program through grant 80NSSC21K2004, and NASA’s NN-EXPLORE Program using the CHIRON spectrograph on the SMARTS 1.5m at CTIO.