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

Abstract

Aims. With the purpose to investigate 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-and Lband 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 3-σ 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 3-σ upper bound to the radio flux density of the L7 object of 9 µJy at 10 GHz.

Constraints on magnetospheric radio emission from Y dwarfs

“…In fact, Zeeman broadening measurements confirm mean surface field strengths in M7-M9 dwarfs that are strong enough to drive ECM emission at several GHz (Morin et al 2010;Reiners & Basri 2010;Berdyugina et al 2017;Shulyak et al 2017), yet not all of these strongly magnetized ultracool dwarfs have been detected in radio (e.g. Antonova et al 2013;Route & Wolszczan 2013;Williams et al 2014;Lynch et al 2016;Route & Wolszczan 2016b;Guirado et al 2018, and references therein). While possible mechanisms for generating aurorae in brown dwarfs remain unconfirmed, if the primary driver for ECM emission in isolated brown dwarfs is co-rotation breakdown of a plasma sheet in the magnetosphere (and references therein Cowley & Bunce 2001;Hill 2001;Bagenal et al 2014;Badman et al 2015), slower rotation may prevent such co-rotation breakdown from occurring (Nichols et al 2012).…”

The milliarcsecond-scale radio structure of AB Doradus A

“…The study of samples of ultracool dwarfs (McLean et al 2012;Route & Wolszczan 2013) has shown that these kind of objects are not typically expected to exhibit radio emission. Nonetheless, some authors have detected radio emission from late M, L and T objects (Matthews 2013;Hallinan et al 2015;Guirado et al 2018) which has been associated with an electron cyclotron maser emission mechanism. They might have benefited from the strong correlation between auroral radio emission and the presence of a H α line (Kao et al 2016).…”

Uncovering the radio emission of low-mass systems

“…Follow-up observations of UCDs at GHz frequencies have shown that ∼10% exhibit radio emission (Route & Wolszczan 2016) that can be classified as quiescent (stable in time with low to moderate degree of circular polarization) or pulses (highly polarized, rapidly varying coherent bursts of radio emission). The former are usually attributed to gyrosynchrotron radiation (Berger 2002, Guirado et al 2018) whereas the latter to the electron cyclotron maser instability (Hallinan et al 2007(Hallinan et al , 2008; Kao et al 2016; ECMI) linked to auroral emission.…”

“…New observations are needed to distinguish among the different proposed mechanisms for the origin of the strong magnetic fields and radio emission detected in these objects. As such, the system VHS J125601.92-125723.9 (hereafter VHS 1256-1257; Gauza et al 2015) represents an excellent opportunity since its radio emission has been previously confirmed (Guirado et al 2018) and new observations can provide further constraints on its origin. This nearby system (22.2 Stone et al 2016).…”