Constraining Substellar Magnetic Dynamos using Brown Dwarf Radio Aurorae

Citation

Kao, Melodie Minyu (2017) Constraining Substellar Magnetic Dynamos using Brown Dwarf Radio Aurorae. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9J964DK. https://resolver.caltech.edu/CaltechTHESIS:06052017-105748197

Abstract

Brown dwarfs share characteristics with both low-mass stars and gas giant planets, making them useful laboratories for studying physics occurring in objects throughout this low mass and temperature range. Of particular interest in this dissertation is the nature of the engine driving their magnetic fields. Fully convective magnetic dynamos can operate in low mass stars, brown dwarfs, gas giant planets, and even fluid metal cores in small rocky planets. Objects in this wide mass range are capable of hosting strong magnetic fields, which shape much of the evolution of planets and stars: strong fields can protect planetary atmospheres from evaporating, generate optical and infrared emission that masquerade as clouds in the atmospheres of other worlds, and affect planet formation mechanisms. Thus, implications from understanding convective dynamo mechanisms also extend to exoplanet habitability.

How the convective dynamos driving these fields operate remains an important open problem. While we have extensive data to inform models of magnetic dynamo mechanisms in higher mass stars like our Sun, the coolest and lowest-mass objects that probe the substellar-planetary boundary do not possess the internal structures necessary to drive solar-type dynamos. A number of models examining fully convective dynamo mechanisms have been proposed but they remain unconstrained by magnetic field measurements in the lowest end of the substellar mass and temperature space. Detections of highly circularly polarized pulsed radio emission provide our only window into magnetic field measurements for objects in the ultracool brown dwarf regime, but these detections are very rare; until this dissertation, only one attempt out of ~60 had been successful.

The work presented in this dissertation seeks to address this problem and examines radio emission from late L, T, and Y spectral type brown dwarfs spanning ~1-6 times the surface temperature of Earth and explores implications for fully convective magnetic dynamo models.

Thesis Files