325MHz very large array observations of ultracool dwarfs TVLM 513-46546 and 2MASS J0036+1821104

Abstract

We present 325MHz (90cm wavelength) radio observations of ultracool dwarfs TVLM 513-46546 and 2MASS J0036+1821104 using the Very Large Array (VLA) in 2007 June. Ultracool dwarfs are expected to be undetectable at radio frequencies, yet observations at 8.5GHz (3.5cm) and 4.9GHz (6cm) have revealed sources with >100 μJy quiescent radio flux and >1mJy 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 325MHz 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 1.4GHz observation of 2MASS J0036+1821104 from 2005 January also yields a non-detection at the level of <130 μJy. The combined radio observation history (0.3GHz to 8.5GHz) for these sources suggests a continuum emission spectrum for ultracool dwarfs that is either flat or inverted below 2-3GHz. 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 two stellar radii and driven by electron beams with energies less than 5 keV. © 2011 The American Astronomical Society. All rights reserved.