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

TL;DR

This study presents the largest spectroscopic survey of M dwarfs near the convective boundary, revealing a sharp Hα activity transition at the gap's top edge and identifying a new emission deficiency zone linked to rotation and angular momentum loss.

Contribution

It provides new observational evidence of Hα activity transition across the convective boundary and identifies a novel emission deficiency zone related to stellar rotation and angular momentum.

Findings

Hα emission is almost absent below the gap in the HRD.

A new magnitude range shows fewer active M dwarfs, indicating a distinct stellar behavior.

Faster angular momentum loss in the most massive fully convective stars.

Abstract

Since identifying the gap in the H-R Diagram (HRD) marking the transition between partially and fully convective interiors, a unique type of slowly pulsating M dwarf has been proposed. These unstable M dwarfs provide new laboratories in which to understand how changing interior structures can produce potentially observable activity at the surface. In this work, we report the results of the largest high-resolution spectroscopic H$\alpha$ emission survey to date spanning this transition region, including 480 M dwarfs observed using the CHIRON spectrograph at CTIO/SMARTS 1.5-m. We find that M dwarfs with H$\alpha$ in emission are almost entirely found 0 to 0.5 magnitude above the top edge of the gap in the HRD, whereas effectively no stars in and below the gap show emission. Thus, the top edge of the gap marks a relatively sharp activity transition, and there is no anomalous H$\alpha$ activity for stars in the gap. We also identify a new region at 10.3 $<M_{G}<$ 10.8 on the main sequence where fewer M dwarfs exhibit H$\alpha$ emission compared to M dwarfs above and below this magnitude range. Careful evaluation of literature results indicates that 1) rotation and H$\alpha$ activity distributions on the main sequence are closely related, and 2) fewer stars in this absolute magnitude range rotate in less than $\sim$13 days than populations surrounding this region. This result suggests that the most massive fully convective stars lose their angular momentum faster than both partially convective stars and less massive fully convective stars.