325 MHz VLA Observations of Ultracool Dwarfs TVLM 513-46546 and 2MASS J0036+1821104

Abstract

We present 325 MHz (90 cm wavelength) radio observations of ultracool dwarfs TVLM 513-46546 and 2MASS J0036+1821104 using the Very Large Array (VLA) in June 2007. Ultracool dwarfs are expected to be undetectable at radio frequencies, yet observations at 8.5 GHz (3.5 cm) and 4.9 GHz (6 cm) of have revealed sources with > 100 μJy quiescent radio flux and > 1 mJy pulses coincident with stellar rotation. The anomalous emission is likely a combination of gyrosynchrotron and cyclotron maser processes in a long-duration, large-scale magnetic field. Since the characteristic frequency for each process scales directly with the magnetic field magnitude, emission at lower frequencies may be detectable from regions with weaker field strength. We detect no significant radio emission at 325 MHz from TVLM 513-46546 or 2MASS J0036+1821104 over multiple stellar rotations, establishing 2.5σ total flux limits of 795 μJy and 942 μJy respectively. Analysis of an archival VLA 1.4 GHz observation of 2MASS J0036+1821104 from January 2005 also yields a non-detection at the level of < 130 μJy . The combined radio observation history (0.3 GHz to 8.5 GHz) for these sources suggests a continuum emission spectrum for ultracool dwarfs which is either flat or inverted below 2-3 GHz. Further, if the cyclotron maser instability is responsible for the pulsed radio emission observed on some ultracool dwarfs, our low-frequency non-detections suggest that the active region responsible for the high-frequency bursts is confined within 2 stellar radii and driven by electron beams with energies less than 5 keV.

11 pages, 5 figures, submitted to AJ

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Cited by in corpus (11)

• Limits on Fast Radio Bursts and other transient sources at 182 MHz using the Murchison Widefield Array
• A search for long-timescale, low-frequency radio transients
• The First Millimeter Detection of a Non-Accreting Ultracool Dwarf
• Rotational modulation of the linear polarimetric variability of the cool dwarf TVLM 513$-$46546
• Electron beam induced radio emission from ultracool dwarfs
• Probing the magnetosphere of the M8.5 dwarf TVLM513-46546 by modelling its auroral radio emission. Hint of star exoplanet interaction?
• A LOFAR mini-survey for low-frequency radio emission from the nearest brown dwarfs
• Low-frequency GMRT observations of ultra-cool dwarfs
• Radio Emission from Ultra-Cool Dwarfs
• Kinetic simulation of the electron-cyclotron maser instability: effect of a finite source size
• The Potential of Detecting Radio-flaring Ultracool Dwarfs at L band in the FAST Drift-scan Survey