A mini-survey of ultracool dwarfs at 4.9 GHz

Abstract

Context. A selection of ultracool dwarfs are known to be radio active, with both gyrosynchrotron emission and the electron cyclotron maser instability being given as likely emission mechanisms. Aims. We explore whether ultracool dwarfs previously undetected at 8.5 GHz may be detectable at a lower frequency. Methods. We select a sample of fast rotating ultracool dwarfs with no detectable radio activity at 8.5 GHz, observing each of them at 4.9 GHz. Results. From the 8 dwarfs in our sample, we detect emission from 2MASS J07464256+2000321, with a mean flux level of 286 ± 24 μJy. The light-curve of 2MASS J07464256+2000321, is dominated towards the end of the observation by a very bright, ≈100% left circularly polarized burst during which the flux reached 2.4 mJy. The burst was preceded by a raise in the level of activity, with the average flux being ≈160 μJy in the first hour of observation rising to ≈400 μJy in the 40 min before the burst. During both periods, there is significant variability. Conclusions. The detection of 100% circular polarization in the emission at 4.9 GHz points towards the electron cyclotron maser as the emission mechanism. However, the observations at 4.9 GHz and 8.5 GHz were not simultaneous, thus the actual fraction of dwarfs capable of producing radio emission, as well as the fraction of those that show periodic pulsations is still unclear, as indeed are the relative roles played by the electron cyclotron maser instability versus gyrosynchrotron emission, therefore we cannot assert if the previous non-detection at 8.5 GHz was due to a cut-off in emission between 4.9 and 8.4 GHz, or due to long term variability.

Spin-orbit alignment in the very low mass binary regime

“…The L dwarf was reported as an active radio source by Antonova et al (2008), who estimated magnetic field strengths of ∼1.7 kG based on the detection of a single highly polarized burst of emission. The dominant emission was quiescent, the nature of which is still debated.…”

Phase connecting multi-epoch radio data for the ultracool dwarf TVLM 513-46546

“…Recently, we have detected periodic pulses of 100% circularly polarized radio emission from four ultracool dwarfs Antonova et al 2008), with the periodicity for three of these dwarfs confirmed to correspond to the rotation period of the dwarf Hallinan et al 2008). Due to the high degree of polarization, the emission process for these pulses must be coherent.…”

Mapping radio emitting-region on low-mass stars and brown dwarfs

“…Berger et al (2001) [4] reported the first detection of quiescent and flaring radio emission from the brown dwarf LP944-20, with anomalous quiescent radio luminosity larger than predicted from an empirical relation between the X-ray and radio luminosities of active stars with spectral types from F to M [6] by at least four orders of magnitude. Up to now, about 11 radio active ultracool dwarfs, with spectral type from M8 to L3.5, including a binary system, have been found from various survey [1,3,7,8]. Three of these have been shown to have periodic radio emission [7,8].…”

“…Pritchett (1984) [11] pointed out that a shell instability (or an incomplete shell instability) caused by a magnetic-field-aligned electric field would generate intense cyclotron emissions, in which a ring shell distribution, i.e., model, the initial electron-distribution function would be deformed by the presence of a magnetic-fieldaligned electric field which may accelerate the down-going electrons to a relativistic state and the mirror effect of the magnetic field which may increase the pitch angle of the electrons, so that an incomplete ring-shell distribution namely the so-called horseshoe distribution would be formed. We are developing a theoretical model to simulate the ECM instability and apply this model to map the emitting-region on ultracool dwarfs 1 . In this paper, we describe the method and preliminary results.…”