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

• Main Authors: Jao, Wei-Chun, Henry, Todd J, White, Russel J, Nisak, Azmain H, Hubbard-James, Hodari-Sadiki, Paredes, Leonardo A
• Format: Journal Article
• Language: English
• Published: 27-04-2023
• Subjects: Physics - Earth and Planetary Astrophysics; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.2304.14452

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.