Constraints on magnetospheric radio emission from Y dwarfs

Abstract

As a pilot study of magnetism in Y dwarfs, we have observed the three known infrared variable Y dwarfs WISE J085510.83−071442.5, WISE J140518.40+553421.4, and WISEP J173835.53+273258.9 with the NSF’s Karl G. Jansky Very Large Array in the 4–8 GHz frequency range. The aim was to investigate the presence of non-bursting quiescent radio emission as a proxy for highly circularly polarized radio emission associated with large-scale auroral currents. Measurements of magnetic fields on Y dwarfs may be possible by observing auroral radio emission, and such measurements are essential for constraining fully convective magnetic dynamo models. We do not detect any pulsed or quiescent radio emission, down to rms noise levels of 7.2 µJy for WISE J085510.83−071442.5, 2.2 µJy for WISE J140518.40+553421.4, and 3.2 µJy for WISEP J173835.53+273258.9. The fractional detection rate of radio emission from T dwarfs is ∼10 per cent suggesting that a much larger sample of deep observations of Y dwarfs is needed to rule out radio emission in the Y dwarf population. We discuss a framework that uses an empirical relationship between the auroral tracer Hα emission and quiescent radio emission to identify brown-dwarf auroral candidates. Finally, we discuss the implications that Y dwarf radio detections and non-detections can have for developing a picture of brown dwarf magnetism and auroral activity.