Polarised radio pulsations from a new T-dwarf binary
H. K. Vedantham, Trent J. Dupuy, E. L. Evans, Aniket Sanghi, J. R. Callingham, T. W. Shimwell, W. M. J. Best, Michael C. Liu, P. Zarka
Abstract
Brown dwarfs display Jupiter-like auroral phenomena such as magnetospheric H α emission and coherent radio emission. Coherent radio emission is a probe of magnetospheric acceleration mechanisms and it provides a direct measurement of the magnetic field strength at the emitter’s location, both of which are difficult to access by other means. Observations of the coldest brown dwarfs (spectral types T and Y) are particularly interesting as their magnetospheric phenomena may be very similar to those in gas-giant exoplanets. Here we present 144 MHz radio and infrared adaptive optics observations of the brown dwarf WISEP J101905.63+652954.2 made using the Low Frequency Array (LOFAR) and Keck telescopes, respectively. The radio data show pulsed, highly circularly polarised emission which yields a rotation rate of 0.32 ± 0.03 h −1 . The infrared imaging reveals the source to be a binary with a projected separation of 423.0 ± 1.6 mas between components of spectral type T5.5 ± 0.5 and T7.0 ± 0.5. With a simple ‘toy model’, we show that the radio emission can, in principle, be powered by the interaction between the two dwarfs with a mass-loss rate of at least 25 times the Jovian value. WISEP J101905.63+652954.2 is interesting because it is the first pulsed methane dwarf detected in a low radio-frequency search. Unlike previous gigahertz-frequency searches that were only sensitive to objects with kiloGauss fields, our low-frequency search is sensitive to surface magnetic fields of ≈50 G and above which might reveal the coldest radio-loud objects down to planetary mass scales.