The rotation and Galactic kinematics of mid M dwarfs in the Solar Neighborhood

TL;DR

This study measures rotation periods of 387 nearby mid-to-late M dwarfs, revealing mass-dependent rotation behaviors, age estimates, and kinematic properties, providing insights into stellar dynamos and evolution.

Contribution

First large-scale measurement of rotation periods for mid-to-late M dwarfs in the Solar Neighborhood using MEarth photometry, with analysis of their kinematics and age implications.

Findings

Rotation periods range from 0.1 to 140 days.

Older, slowly-rotating stars can lack Hα emission.

Rotation period correlates with stellar mass and age.

Abstract

Rotation is a directly-observable stellar property, and drives magnetic field generation and activity through a magnetic dynamo. Main sequence stars with masses below approximately 0.35Msun (mid-to-late M dwarfs) are fully-convective, and are expected to have a different type of dynamo mechanism than solar-type stars. Measurements of their rotation rates provide insights into these mechanisms, but few rotation periods are available for these stars at field ages. Using photometry from the MEarth transit survey, we measure rotation periods for 387 nearby, mid-to-late M dwarfs in the Northern hemisphere, finding periods from 0.1 to 140 days. The typical detected rotator has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5 to 1%. We find no period-amplitude relation for stars below 0.25Msun and an anti-correlation between period and amplitude for higher-mass M dwarfs. We highlight the existence of older, slowly-rotating stars without H{\alpha} emission that nevertheless have strong photometric variability. 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<10 days are on average <2 Gyrs, and that those with P>70 days are about 5 Gyrs. The period distribution is mass dependent: 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. [Abridged]