Radio emission in a nearby, ultra-cool dwarf binary: A multifrequency study

Abstract

Context. The substellar triple system VHS J125601.92−125723.9 (hereafter VHS 1256−1257) is composed of an equal-mass M7.5 brown dwarf binary and an L7 low-mass substellar object. In Guirado et al. (2018, A&A, 610, A23) we published the detection of radio emission at 8.4 GHz coming from the central binary and making it an excellent target for further observations. Aims. We aim to identify the origin of the radio emission occurring in the central binary of VHS 1256−1257 while discussing the expected mechanisms involved in the radio emission of ultra-cool dwarfs. Methods. We observed this system with the Karl G. Jansky Very Large Array, the European very-long-baseline interferometry (VLBI) Network, the enhanced Multi-Element Remotely Linked Interferometer Network, the NOrthern Extended Millimeter Array, and the Atacama Large Millimetre Array at frequencies ranging from 5 GHz up to 345 GHz in several epochs during 2017, 2018, and 2019. Results. We found radio emission at 6 GHz and 33 GHz coincident with the expected position of the central binary of VHS 1256−1257. The Stokes I density fluxes detected were 73 ± 4 μJy and 83 ± 13 μJy, respectively, with no detectable circular polarisation or pulses. No emission is detected at higher frequencies (230 GHz and 345 GHz), nor at 5 GHz with VLBI arrays. The emission appears to be stable over almost three years at 6 GHz. To explain the constraints obtained both from the detections and non-detections, we considered multiple scenarios including thermal and nonthermal emission, and different contributions from each component of the binary. Conclusions. Our results can be well explained by nonthermal gyrosynchrotron emission originating at radiation belts with a low plasma density (ne = 300−700 cm−3), a moderate magnetic field strength (B ≈ 140 G), and an energy distribution of electrons following a power-law (dN/dE ∝ E−δ) with δ fixed at 1.36. These radiation belts would need to be present in both components and also be viewed equatorially.

Resolved imaging confirms a radiation belt around an ultracool dwarf

“…For plasma confined by the magnetic dipole of LSR J1835 + 3259, we estimate 15 MeV electron energies, consistent with Jupiter’s radiation belts 4 . Our results confirm recent predictions of radiation belts at both ends of the stellar mass sequence 8 , 16 – 19 and support broader re-examination of rotating magnetic dipoles in producing non-thermal quiescent radio emissions from brown dwarfs 7 , fully convective M dwarfs 20 and massive stars 18 , 21 .…”

Evidence for a radiation belt around a brown dwarf

“…However, the details of the emission mechanism may vary between UCDs and planets ( 5 , 6 ). The quiescent component is usually interpreted as being caused by radio emission from mildly or ultrarelativistic electrons spiraling in magnetic fields [gyrosynchrotron and synchrotron mechanisms, respectively ( 7 , 8 )], which could originate in the UCD’s corona and/or radiation belts ( 9 – 11 ). Studies of the radio emission from UCDs have analyzed their light curves (brightness as a function of time), spectral energy distributions, and dynamic spectra.…”