Rotation & Galactic kinematics of mid M dwarfs

We highlight the existence of older, slowly rotating stars without H{alpha} emission that nevertheless have strong photometric variability. We use parallaxes, proper motions, radial velocities, photometry, and near-infrared metallicity estimates to further characterize the population of rotators. The Galactic kinematics of our sample is consistent with the local population of G and K dwarfs, and rotators have metallicities characteristic of the solar neighborhood.

We use the W space velocities and established age-velocity relations to estimate that stars with P<10d have ages of on average <2Gyr, and that those with P>70d have ages of about 5Gyr. The period distribution is dependent on mass: as the mass decreases, the slowest rotators at a given mass have longer periods, and the fastest rotators have shorter periods. We find a lack of stars with intermediate rotation periods, and the gap between the fast and slow rotators is larger for lower masses.

Our data are consistent with a scenario in which these stars maintain rapid rotation for several gigayears, then spin down quickly, reaching periods of around 100d by a typical age of 5Gyr.