Radio Emission from a Nearby M dwarf Binary

Abstract

We present the detection of the binary system 2MASS J02132062+3648506 AB using the Karl G. Jansky Very Large Array (VLA) archive data observed at 4-8 GHz. The system is a triple consisting of a tight binary ($\sim 0.2"$) of two M dwarfs of spectral class M4.5 and M6.5 and a wide T3 brown dwarf companion ($\sim$16.4"). The binary displays coronal and chromospheric activity as traced by previously measured X-ray flux and H$\alpha$ emission. We detect the unresolved binary at a peak flux density of $\sim 356\mu {\mathrm{Jybeam}}^{-1}$ at a signal-to-noise ratio (SNR) of $\sim 36$ and determine a radio luminosity of $\log L_R/\log L_{\mathrm{bol}}\approx -7.76$. The radio emission is quiescent, polarised at a mean circular polarisation fraction $f_{\mathrm{c}}=45.20\pm 1.58$ % and exhibits a spectral index $\alpha =-0.44\pm 0.07$ . We probe the binary using the Enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN) with an angular resolution of $\sim 40$ mas at 5 GHz and detect a component at a peak flux density of $\sim 90\mu$Jy ${\mathrm{beam}}^{-1}$ at a SNR $\sim 5$ . We propose a gyrosynchrotron origin for the radio emission and estimate a magnetic field strength $B<174.86$ G, an emitting region of size $L<1.54$ times the radius of the M4.5 primary and a plasma number density $n_{\mathrm{e}}<2.91\times 10^5{\mathrm{cm}}^{-3}$. The brown dwarf companion is not detected. Additionally, we have analysed observations of 2MASS J04183483+213127, a chromospherically active L5 brown dwarf which is also not detected. Accordingly, we place $3\sigma$ flux density upper limits at $36.9\mu$Jy ${\mathrm{beam}}^{-1}$ and $42.3\mu$Jy ${\mathrm{beam}}^{-1}$ for Stokes I and V respectively.

10 pages, 5 figures

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