LSPM J1314: An oversized magnetic star with constraints on the radio emission mechanism

LSPM J1314+1320 (=NLTT 33370) is a binary star system consisting of two nearly identical pre-main sequence stars of spectral type M7. The system is remarkable among ultracool dwarfs for being the most luminous radio emitter over the widest frequency range. Masses and luminosities are at first sight consistent with the system being coeval at age ~80 Myr according to standard (non-magnetic) evolutionary models. However, these models predict an average effective temperature, 2950 +/- 5 K, which is 180 K hotter than the empirical value. Thus, the empirical radii are oversized relative to the standard models by ~13%. We demonstrate that magnetic stellar models can account quantitatively for the oversizing. As a check on our models, we note that the radio emission limits the surface magnetic field strengths: the limits depend on identifying the radio emission mechanism. We find that the field strengths required by our magnetic models are too strong to be consistent with gyrosynchrotron emission, but are consistent with electron cyclotron maser emission.