On the Correlation between L Dwarf Optical and Infrared Variability and Radio Aurorae

TL;DR

This study investigates the link between optical/infrared variability and radio aurorae in brown dwarfs, finding that high-amplitude variability does not necessarily indicate magnetic activity, but auroral emissions are detectable in some cases.

Contribution

It provides the first radio detection of auroral emission in a brown dwarf with optical/infrared variability and analyzes the correlation between variability and radio activity.

Findings

Detected auroral radio emission in one brown dwarf, indicating a magnetic field of ≥2.9 kG.

Confirmed Hα emission as a reliable tracer of auroral activity.

High amplitude optical/IR variability does not reliably indicate radio magnetic activity.

Abstract

Photometric variability attributed to cloud phenomena is common in L/T transition brown dwarfs. Recent studies show that such variability may also trace aurorae, suggesting that localized magnetic heating may contribute to observed brown dwarf photometric variability. We assess this potential correlation with a survey of 17 photometrically variable brown dwarfs using the Karl G. Jansky Very Large Array (VLA) at 4 -- 8 GHz. We detect quiescent and highly circularly polarized flaring emission from one source, 2MASS J17502484-0016151, which we attribute to auroral electron cyclotron maser emission. The detected auroral emission extends throughout the frequency band at $\sim$5 -- 25$\sigma$, and we do not detect evidence of a cutoff. Our detection confirms that 2MASS J17502484-0016151 hosts a magnetic field strength of $\geq$2.9 kG, similar to those of other radio-bright ultracool dwarfs. We show that H$\alpha$ emission continues to be an accurate tracer of auroral activity in brown dwarfs. Supplementing our study with data from the literature, we calculate the occurrence rates of quiescent emission in L dwarfs with low- and high-amplitude variability and conclude that high amplitude O/IR variability does not trace radio magnetic activity in L dwarfs.