Radio Emission in Ultracool Dwarfs

Cool dwarf stars are the most common planetary hosts. They are also very active in radio flaring, and hence provide the best means to study star–planet magnetospheric interactions, and possibly their influence on the development of life.

 To investigate the radio emission of ultracool objects, Guirado et al. carried out a targeted search in the recently discovered system VHS J125601.92–125723.9 (hereafter simply VHS 1256–1257). this system is composed of an equal-mass M7.5 binary and an L7 low-mass substellar object located at only 15.8 parsecs. The research team observed VHS 1256–1257 system with the Karl G. Jansky Very Large Array in phase-reference mode at X- and L-band, and with the European Very Long Baseline Interferometry Network at L-band, in several epochs during 2015 and 2016.

Radio emission was discovered at X-band that is spatially coincident with the equal-mass M7.5 binary with a flux density of 60 μJy. The measured spectral index was 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 to 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, the authors constrain the turnover frequency to be in the interval 5–8.5 GHz, from which they infer the presence of kiloGauss-intense magnetic fields in the M7.5 binary. These data impose a 3σ upper bound to the radio flux density of the L7 object of 9 μJy at 10 GHz.

Image: [Left] VLA image of the VHS 1256–1257 field at X-band. The detected source is readily assigned to the M7.5 binary. The (undetected) L7-object b location is at the solid white box. The 3σ threshold detection is 9 μJy. At 15.8 parsecs, the separation between components AB and b corresponds to 128.4 AU. [Right] VLA image of the VHS 1256–1257 field at L-band. A solid box, with size that of the X-band image, is centered at the position of the X-band detection. None of the VHS 1256–1257 components is detected. The 3σ threshold detection is 20 μJy. The two bright knots seen in the map at the NW correspond to known extragalactic radio sources.

Publication: J.C. Guirado (Universitat de València) et al., Radio Emission in Ultracool Dwarfs: The Nearby Substellar Triple System, 2018, Astronomy & Astrophysics, 610, A23.