Ultra-Cool and Extra-Vigorous: Rotation and Activity in M and L dwarfs

TL;DR

This paper reviews recent progress in understanding rotation, magnetic activity, and braking in low-mass M and L dwarfs, highlighting how magnetic field topology and braking efficiency change at the fully convective threshold.

Contribution

It synthesizes recent observational results on activity, rotation, and magnetic fields in M and L dwarfs, emphasizing the change in rotational braking at the fully convective boundary.

Findings

Magnetic activity generation remains consistent across the convective boundary.

Rotational braking weakens significantly in fully convective stars.

Field topology likely shifts from dipolar to small-scale structures in fully convective objects.

Abstract

The study of rotation and activity in low-mass stars or brown dwarfs of spectral classes M and L has seen enormous progress during the last years. I summarize the results from different works that measured activity, rotation, and sometimes magnetic fields. The generation of magnetic activity seems to be unchanged at the threshold to completely convective stars, i.e. no change in the efficiency of the magnetic dynamos is observed. On the other hand, a sudden change in the strength of rotational braking appears at the threshold mass to full convection, and strong evidence exists for rotational braking weakening with lower mass. A probable explanation is that the field topology changes from dipolar to small scale structure as the objects become fully convective.