||The M4 Transition: Toward a Comprehensive Understanding of the
Transition into the Fully Convective Regime
||448, 16th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun
||Stassun, K. G.; Hebb, L.; Covey, K.; West, A. A.; Irwin, J.; Jackson, R.; Jardine, M.; Morin, J.; Mullan, D.; Reid, I. N.
||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 (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.