The Evolution of Rotation and Magnetism in Small Stars Near the Sun

Despite the prevalence of fully-convective stars, very few members of the field population have measured rotation periods. The lack of observational constraints at field ages has hampered studies of rotational evolution. We present rotation periods for 387 nearby mid-to-late M dwarfs in the Northern hemisphere, including detections from 0.1 to 140 days. 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 dearth of stars with intermediate rotation periods, which suggests that fully-convective stars undergo rapid angular momentum evolution. The typical detected rotator has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5 to 1%. We find no correlation between period and amplitude for stars below 0.25 M_sun. We use Galactic kinematics and established age-velocity relations to estimate the M dwarf spin-down timescale. We find that stars with P<10 days are on average <2 Gyrs, and that those with P>70 days are about 5 Gyrs. We observe a threshold in the mass–period plane that separates active and inactive M dwarfs. The threshold coincides with the fast-period edge of the slowly rotating population, at approximately the rotation period at which the era of rapid rotational evolution appears to cease. Our data show that rapid rotators maintain a saturated value of LHα/Lbol, and that for slow rotators there is a power-law decay in LHα/Lbol with Rossby number.