Flaring up All Over—Radio Activity in Rapidly Rotating Late M and L Dwarfs

Abstract

We present Very Large Array observations of 12 late M and L 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 emission and hence magnetic fields. Three of the 12 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 at the bottom of the main sequence. The radio emission extends to spectral type L3.5, and there is no apparent decrease in the ratio of flaring luminosities to bolometric luminosities between M8 and L3.5. Moreover, contrary to the significant drop in persistent Hα activity beyond spectral type M7, the persistent radio activity appears to steadily increase between M3 and L3.5. Similarly, the radio emission from BRI 0021-0214 violates the phenomenological relations between the radio and X-ray luminosities of coronally active stars, hinting that radio and X-ray activity are also uncorrelated at the bottom of the main sequence; an even stronger violation was found for the brown dwarf LP 944-20. The radio-active sources that have measured rotational velocities are rapid rotators, v sin i > 30 km s -1 , while the upper limits on radio activity in slowly rotating late M dwarfs ( v sin i < 10 km s -1 ) from this survey and from the literature are lower than these detections. These observations provide tantalizing evidence that rapidly rotating late M and L dwarfs are more likely to be radio active. This possible correlation is puzzling given that the observed radio emission requires sustained magnetic fields of ~10-10 3 G and densities of ~10 12 cm -3 , indicating that the active sources should have slowed down considerably as a result of magnetic braking.