AURORAL RADIO EMISSION FROM LATE L AND T DWARFS: A NEW CONSTRAINT ON DYNAMO THEORY IN THE SUBSTELLAR REGIME

We have observed six late. L and T dwarfs with the Karl G. Jansky Very Large Array (VLA) to investigate the presence of highly circularly polarized radio emission, associated with large-scale auroral currents. Previous surveys encompassing similar to 60 L6 or later targets have yielded only one detection. Our sample includes the previously detected T6.5 dwarf 2MASS. 10475385+2124234, as well as five new targets selected for the presence of H alpha emission and/or optical infrared photometric variability, which are possible manifestations of auroral activity. We detect 2MASS. 10475385+2124234, as well as four of the five. targets in our biased sample, including the strong IR-variable. source SIMP. J01365662+0933473 and bright H alpha emitter 2MASS. 12373919+6526148, reinforcing the possibility that activity at these disparate wavelengths is related. The radio emission frequency corresponds to a precise determination of the lower-bound magnetic field strength near the surface of each dwarf, and this new sample provides robust constraints on dynamo theory in the low-mass brown dwarf regime. Magnetic fields. greater than or similar to 2.5 kG are confirmed for five of six targets. Our results provide tentative evidence that the dynamo operating in this mass regime may be inconsistent with predicted values from a recently proposed model. Further observations at higher radio frequencies are essential for verifying this assertion.