Wideband dynamic radio spectra of two ultra-cool dwarfs

Abstract

A number of radio-loud ultra-cool dwarf (UCD ) stars exhibit both continuous broadband and highly polarized pulsed radio emission. In order to determine the nature of the emission and the physical characteristics in the source region, we have made multi-epoch, wideband spectral observations of TVLM 0513-46 and 2M 0746+20. We combine these observations with archival radio data to fully characterize both the temporal and spectral properties of the radio emission. The continuum spectral energy distribution can be well modeled using gyrosynchrotron emission from mildly relativistic electrons in a dipolar field. The pulsed emission exhibits a variety of time-variable characteristics, including frequency drifts, frequency cutoffs, and multiple pulses per period. For 2M 0746+20 we determine a pulse period consistent with previously determined values. We modeled locations of pulsed emission using an oblique rotating magnetospheric model with beamed electron-cyclotron maser ( ECM) sources. The bestfit models have narrow ECM beaming angles aligned with the local source magnetic field direction, except for one isolated burst from 2M 0746+20. For TVLM 0513-46, the best-fit rotation axis inclination is nearly orthogonal to the line of sight. For 2M0746+20 we found a good fit using a fixed inclination i = 36°, determined from optical observations. For both stars the ECM sources are located near feet of magnetic loops with radial extents 1.2Rs-2.7Rsand surface fields 2.2-2.5 kG. These results support recent suggestions that radio over-luminous UCDs have a global "weak field" non-axisymmetric magnetic topologies.