Main-oval auroral emission from a T6 brown dwarf: observations, modeling, and astrometry

TL;DR

This study reports the detection of auroral radio emissions from a T6 brown dwarf, models their origin, and refines its astrometric parameters, revealing similarities to planetary auroras and constraining potential companions.

Contribution

It provides the first detailed observation and modeling of auroral emissions in a T6 brown dwarf, and offers high-precision astrometry linking it to a wide companion and constraining nearby objects.

Findings

Detection of highly-polarized radio emission consistent with electron cyclotron maser emission.

Identification of auroral rings similar to Jupiter's main-oval auroras.

Astrometric measurements confirm the brown dwarf's association with a wide companion and rule out large-mass companions.

Abstract

From a series of 5 GHz VLBA radio observations taken over 1-yr span, we present the detection of compact highly-polarized radio emission from the T6 brown dwarf WISE J112254.72+255022.2 compatible with electron cyclotron maser emission. Both the total and polarized lightcurves show variability in correspondence with a rotation period of 1.95+/-0.03 hr. Comparison with models indicates that the quasi-steady radio emission of this brown dwarf is produced in circumpolar auroral rings, with remarkable similarity to the main-oval auroras in Jupiter. We have detected a large 100% polarized flare in one of the VLBA epochs (2022.82) which may imply the existence of active longitudes in the auroral rings with a non-axisymmetric beaming cone radiation pattern, similar to the dusk/dawn asymmetries seen in the Jovian radio emissions. We also present a high-precision astrometric analysis of the sky motion of WISE J112254.72+255022.2, resulting in revised values of proper motion and parallax with an improvement in precision of one order of magnitude. The common kinematics of WISE J112254.72+255022.2 with its wide companion, the M-dwarf LHS 302, is confirmed with submilliarcsecond precision, suggesting that this brown dwarf may have formed by gravitational fragmentation of the outer part of a protostellar disc around LHS 302. The astrometric analysis imposes very tight bounds to the presence of low-mass companions around WISE J112254.72+255022.2, ruling out objects more massive than Saturn. Our results strengthen the analogy between radio emitting brown dwarfs and the magnetized planets of our solar system.