Flaring up All over — Magnetic Activity in Rapidly-rotating Late-type M and L Dwarfs

We present Very Large Array observations of twelve bona-fide and candidate brown dwarfs in the Solar neighborhood. The observed sources were chosen to cover a wide range of physical characteristics — spectral type, rotation, age, binarity, and X-ray and Hα activity — to determine the role of these properties in the production of radio activity, and hence magnetic fields. Three of the twelve sources, TVLM 513-46546, 2MASS J0036159+182110, and BRI 0021−0214, were observed to flare and also exhibit persistent emission, indicating that magnetic activity is not quenched across the sub-stellar boundary. The radio emission extends to spectral type L3.5, and contrary to Hα activity, there is no apparent decrease in the ratios of flaring and persistent luminosities to bolometric luminosities between M8−L3.5; Hα activity drops significantly beyond M7. Similarly, the radio emission violates the phenomenological relations between the radio and X-ray luminosities of coronal active stars, indicating that radio and X-ray activity are also uncorrelated at the bottom of the main sequence. The radio active sources that have measured rotational velocities are all rapid rotators, vsini ≈ 30 − 60 km sec−1, pointing to a correlation between rotation and radio/magnetic activity. However, this correlation is puzzling given that the observed radio emission requires sustained magnetic fields of ∼ 10 − 103 G and densities of ∼ 1012 cm−3, indicating that the active sources ought to have slowed down considerably due to magnetic braking. Subject headings: stars: low mass,brown dwarf–stars: activity–stars: radio emission–stars: magnetic fields–radiation mechanisms: nonthermal