A VOLUME-LIMITED SAMPLE OF 63 M7-M9.5 DWARFS. II. ACTIVITY, MAGNETISM, AND THE FADE OF THE ROTATION-DOMINATED DYNAMO

In a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this second paper, we present projected rotation velocities, average magnetic field strengths, and chromospheric emission from the H alpha line. We confirm earlier results that the mean level of normalized H alpha luminosity decreases with lower temperature, and we find that the scatter among H alpha luminosities is larger at lower temperature. We measure average magnetic fields between 0 and 4 kG with no indication for a dependence on temperature between M7 and M9.5. For a given temperature, H alpha luminosity is related to magnetic field strength, consistent with results in earlier stars. A few very slowly rotating stars show very weak magnetic fields and H alpha emission, and all stars rotating faster than our detection limit show magnetic fields of at least a few hundred Gauss. In contrast to earlier-type stars, we observe magnetic fields weaker than 1 kG in stars rotating faster than similar to 3 km s(-1), but we find no correlation between rotation and magnetic flux generation among them. We interpret this as a fundamental change in the dynamo mechanism; in ultracool dwarfs, magnetic field generation is predominantly achieved by a turbulent dynamo, while other mechanisms can operate more efficiently at earlier spectral types.