A Rotation-Activity Relation for Late-type M Dwarfs

A large fraction of field mid-to-late M dwarfs are found to be magnetically active, yet the exact mechanisms governing this activity are not well-understood. Simulations and observations indicate that rotation may play a role in magnetic field generation for these fully-convective stars, but the interpretation has been hampered by a lack of measured rotation rates for slowly-rotating stars as these are inaccessible to the vsini method. We present results from a study of the magnetic activity-rotation relation for a large sample of field M dwarfs for which we have measured rotation periods from MEarth and PTF programs (including many slow rotators; 50-150 days) and low-resolution spectroscopic observations from FLWO/FAST and SDSS. We quantify the magnetic activity of the stars using the chromospheric H-alpha emission line and find that there is a clear trend of decreasing activity with increased rotation periods for all M dwarf spectral types. In addition, a kinematic analysis of the three-dimensional space motions of the stars confirms that those with higher levels of magnetic activity are consistent with being younger than their less active (or inactive) counterparts.

We acknowledge MEarth funding from the Packard Fellowship for Science and Engineering and the NSF (AST-0807690).