The M4 Transition: Toward a comprehensive understanding of the transition into the fully convective regime

Abstract

The difference in stellar structure above and below spectral type ~M4 is expected to be a very important one, connected directly or indirectly to a variety of observational phenomena in cool stars---such as rotation, activity, magnetic field generation and topology, timescales for evolution of these, and even the basic mass-radius relationship. In this Cool Stars XVI Splinter Session, we aimed to use the M4 transition as an opportunity for discussion about the interiors of low-mass stars and the mechanisms which determine their fundamental properties. By the conclusion of the session, several key points were elucidated. Although M dwarfs exhibit significant changes across the fully convective boundary, this "M4 transition" is not observationally sharp or discrete. Instead, the properties of M dwarfs (i.e. radius, effective temperature, rotation, activity lifetime, magnetic field strength and topology) show smooth changes across M3--M6 spectral types. In addition, a wide range of stellar masses share similar spectral types around the fully convective transition. There appears to be a second transition at M6--M8 spectral types, below which there exists a clear dichotomy of magnetic field topologies. Finally, we used the information and ideas presented in the session to construct a framework for how the structure of an M dwarf star, born with specific mass and chemical composition, responds to the presence of its magnetic field, itself driven by a feedback process that links the star's rotation, interior structure, and field topology.

Report from Cool Stars XVI Splinter Session, to appear in CS16 Proceedings. 12 pages, 10 figures. Fixed typos and problems with some figures

References in corpus (2)

• On the rotational evolution of solar- and late-type stars, its magnetic origins, and the possibility of stellar gyrochronology
• The Lower Main Sequence and the Orbital Period Distribution of Cataclysmic Variable Stars

Cited by in corpus (8)

• An Empirical Correction for Activity Effects on the Temperatures, Radii, and Estimated Masses of Low-Mass Stars and Brown Dwarfs
• White Dwarf/M Dwarf Binaries as Single Degenerate Progenitors of Type Ia Supernovae
• Rotation-activity relations and flares of M dwarfs with K2 long- and short-cadence data
• Recognition of M-type stars in the unclassified spectra of LAMOST DR5 using a hash learning method
• NGTS clusters survey -- III: A low-mass eclipsing binary in the Blanco 1 open cluster spanning the fully convective boundary
• Photometric Flaring Fraction of M dwarf Stars from the SkyMapper Southern Survey
• Flare stars in nearby Galactic open clusters based on {\it TESS} data
• Correcting for Activity Effects on the Temperatures, Radii, and Estimated Masses of Low-Mass Stars and Brown Dwarfs