Wideband Dynamic Radio Spectra of Two Ultra-cool dwarfs

Abstract

A number of radio-loud ultra cool dwarf stars (UCD) 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 best-fit 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 2M 0746+20 we found a good fit using a fixed inclination i=36 deg, determined from optical observations. For both stars the ECM sources are located near feet of magnetic loops with radial extents 1.2Rs-2.7 Rs and 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.

References in corpus (13)

• Large-scale magnetic topologies of late M dwarfs
• Simulations of dynamo action in fully convective stars
• The First Direct Measurements of Magnetic Fields on Very Low-Mass Stars
• Radio Observations of a Large Sample of Late-M, L, and T Dwarfs: The Distribution of Magentic Field Strengths
• Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf
• Rotational Modulation of the Radio Emission from the M9 Dwarf TVLM 513-46546: Broadband Coherent Emission at the Substellar Boundary?
• Radiative Hydrodynamic Models of Optical and Ultraviolet Emission from M Dwarf Flares
• The Arecibo Detection of the Coolest Radio-flaring Brown Dwarf
• Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM513-46546
• Discovery of Radio Emission from the Tight M8 Binary: LP 349-25
• A Mini-survey of Ultracool Dwarfs at 4.9 GHz
• Timing Analysis of the Periodic Radio and Optical Brightness Variations of the Ultracool Dwarf, TVLM 513-46546
• Radio Constraints on Activity in Young Brown Dwarfs

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• First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
• The Second Arecibo Search for 5 GHz Radio Flares from Ultracool Dwarfs
• A search for circularly polarised emission from young exoplanets
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• Modelling the environment around five ultracool dwarfs via the radio domain
• Multi-Epoch VLBI of L Dwarf Binary 2MASS J0746+2000AB: Precise Mass Measurements and Confirmation of Radio Emission from Both Components