Radio emission in ultracool dwarfs: The nearby substellar triple system VHS 1256–1257

¹ Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, València, Spain
e-mail: guirado@uv.es
² Observatori Astronòmic, Universitat de València, Parc Científic, C. Catedrático José Beltrán 2, 46980 Paterna, València, Spain
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁶ Departamento de Astronomía, Universidad de Chile, Camino el Observatorio 1515, Casilla 36-D, Las Condes, Santiago, Chile
⁷ Instituto de Astrofísica de Andalucía (IAA, CSIC), Glorieta de la Astronomía, s/n, 18008 Granada, Spain
⁸ Departamento de Física Teórica, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain
⁹ Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain
¹⁰ Centro de Astrobiología (CSIC-INTA), Crta. Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain

Received: 19 October 2017
Accepted: 1 December 2017

Abstract

Aim. With the purpose of investigating the radio emission of new ultracool objects, we carried out a targeted search in the recently discovered system VHS J125601.92–125723.9 (hereafter VHS 1256–1257); this system is composed by an equal-mass M7.5 binary and a L7 low-mass substellar object located at only 15.8 pc.

Methods. We observed in phase-reference mode the system VHS 1256–1257 with the Karl G. Jansky Very Large Array at X band and L band and with the European VLBI Network at L band in several epochs during 2015 and 2016.

Results. We discovered radio emission at X band spatially coincident with the equal-mass M7.5 binary with a flux density of 60 μJy. We determined a spectral index α = −1.1 ± 0.3 between 8 and 12 GHz, suggesting that non-thermal, optically thin, synchrotron, or gyrosynchrotron radiation is responsible for the observed radio emission. Interestingly, no signal is seen at L band where we set a 3σ upper limit of 20 μJy. This might be explained by strong variability of the binary or self-absorption at this frequency. By adopting the latter scenario and gyrosynchrotron radiation, we constrain the turnover frequency to be in the interval 5–8.5 GHz, from which we infer the presence of kG-intense magnetic fields in the M7.5 binary. Our data impose a 3σ upper bound to the radio flux density of the L7 object of 9 μJy at 10 GHz.