Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths

I present radio observations of 90 dwarf stars and brown dwarfs of spectral type M5-T8. Three sources exhibit radio activity, in addition to the six objects previously detected in quiescence and outburst, leading to an overall detection rate of ~10% for objects later than M7. The inferred magnetic field strengths are ~102 G in quiescence and nearly 1 kG during flares, while the majority of the nondetected objects have B ≲ 50 G. Depending on the configuration and size of the magnetic loops, the surface fields may approach 1 kG even in quiescence, at most a factor of a few smaller than in early M dwarfs. With the larger sample of sources I find continued evidence for (1) a sharp transition around spectral type of M7 from a ratio of radio to X-ray luminosity of log(LR/LX) ~ -15.5 to ≳-12, (2) increased radio activity (LR/Lbol) with later spectral type, in contrast to Hα and X-ray observations, and (3) an overall drop in the fraction of active sources from ~30% for M dwarfs to ~5% for L dwarfs, consistent with Hα and X-ray observations. Taken together, these trends suggest that some late M and L dwarfs are capable of generating 0.1-1 kG magnetic fields, but the overall drop in the fraction of such objects likely reflects changes in the structure of the chromospheres and coronae, possibly due to increasingly neutral atmospheres and/or a transition to a turbulent dynamo. These possibilities can best be tested through simultaneous observations, which can trace the effect of magnetic dissipation in a direct, rather than a statistical, manner. Still, a more extended radio survey currently holds the best promise for measuring the magnetic field properties of a large number of dwarf stars.