H{alpha} emission in nearby M dwarfs

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 an era of rapid rotational evolution appears to cease. The well-defined active/inactive boundary indicates that H{alpha} activity is a useful diagnostic for stellar rotation period, e.g., for target selection for exoplanet surveys, and we present a mass-period relation for inactive M dwarfs.

We also find a significant, moderate correlation between L_H{alpha}_/L_bol_ and variability amplitude: more active stars display higher levels of photometric variability. Consistent with previous work, our data show that rapid rotators maintain a saturated value of L_H{alpha}_/L_bol_. Our data also show a clear power-law decay in L_H{alpha}_/L_bol_ with Rossby number for slow rotators, with an index of -1.7+/-0.1.