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

We present radio observations of a comprehensive sample of 90 dwarf stars and brown dwarfs ranging from spectral type M5 to T8. We detect three radio active sources in addition to the six objects previously detected in quiescence and outburst, leading to an overall detection rate of about 10% for objects later than M7. From the properties of the radio emission we infer magnetic field strengths of ~100 G in quiescence and nearly 1 kG during flares, while the majority of the non-detected objects have B<50 G. Depending on the configuration and size of the magnetic loops, the surface magnetic fields may approach 1 kG even in quiescence, at most a factor of few smaller than in early-M dwarfs. With the larger sample of sources we find continued evidence for (i) a sharp transition around spectral type M7 from a ratio of radio to X-ray luminosity of log(L_R/L_X) ~ -15.5 to >-12, (ii) increased radio activity with later spectral type, in contrast to H-alpha and X-ray observations, and (iii) an overall drop in the fraction of active sources from about 30% for M dwarfs to about 5% for L dwarfs, fully consistent with H-alpha 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 is likely accompanied by a change in the structure of the chromospheres and coronae, possibly due to the increasingly neutral atmospheres and/or a transition to a turbulent dynamo. A more extended radio survey currently holds the best promise for measuring the magnetic field properties of a large number of dwarf stars. [abridged]