Low-frequency GMRT observations of ultra-cool dwarfs

Abstract

Observations of radio emission in about 10 per cent of ultra-cool dwarfs (UCDs) indicate the presence of strong, persistent magnetic fields in these stars. These results are in contrast to early theoretical expectations on fully convective dynamos, and to other tracers of magnetic activity, such as H α and X-ray luminosity. Radio-frequency observations have been key to physically characterizing UCD magnetospheres, although explaining the diverse behaviour within them remains challenging. Most radio-frequency studies of UCDs have been conducted in the 4-8 GHz band, where traditional radio interferometers are typically most sensitive. Hence, the nature of UCD radio emission at low frequencies ({≲ } 1.4 GHz) remains relatively unexplored, but can probe optically thick emission, and regions of lower magnetic field strengths - regimes not accessible to higher-frequency observations. In this work, we present the results from Giant Metrewave Radio Telescope observations of nine UCDs taken at ∼610 and 1300 MHz. These are the first observations of UCDs in this frequency range to be published in the literature. Using these observations, we are able to constrain the coronal magnetic field strength and electron number density of one of the targets to 1 ≲ B ≲ 90 G and 4 ≲ log (N_e) ≲ 10, respectively. We do not detect the flaring emission observed at higher frequencies, to a limit of a few millijansky. These results show that some UCDs can produce low-frequency radio emission and highlight the need for simultaneous multiwavelength radio observations to tightly constrain the coronal and magnetospheric properties of these stars.