Radio Observations of the Brown Dwarf- Exoplanet Boundary

Open Access
Author:
Route, Matthew Philip
Graduate Program:
Astronomy and Astrophysics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 12, 2013
Committee Members:
  • Alexander Wolszczan, Dissertation Advisor
  • Alexander Wolszczan, Committee Chair
  • Lyle Norman Long, Committee Member
  • Steinn Sigurdsson, Special Member
  • Kevin Luhman, Committee Member
  • John David Mathews, Committee Member
  • Richard Alan Wade, Committee Member
Keywords:
  • Brown Dwarfs
  • Exoplanets
  • Radio Emission
  • Planetary Magnetic Fields
  • Stellar Magnetic Fields
  • Stellar Activity
Abstract:
Although exoplanets and brown dwarfs have been hypothesized to exist for many years, it was only in the last two decades that their existence has been directly verified. Since then, a large number of both types of substellar objects have been discovered; they have been studied, characterized, and classified. Yet knowledge of their magnetic properties remains difficult to obtain. Only radio emission provides a plausible means to study the magnetism of these cool objects. At the initiation of this research project, not a single exoplanet had been detected in the radio, and only a handful of radio emitting brown dwarfs were known. This project was launched to attempt to detect emission from brown dwarfs cooler than spectral type L3.5, the coolest brown dwarf detected prior to this project, and also to attempt to discover radio emission from nearby exoplanets. These objects are known to emit radio waves via the gyrosynchrotron and electron cyclotron maser instability mechanisms. By analyzing flaring radio emission from these objects, we would therefore gain insight into their magnetic field properties and the characteristics of the surrounding plasma environment. This dissertation presents the results from surveys of 33 brown dwarfs, 18 exoplanetary systems, and one additional M dwarf for flaring radio emission, conducted with the 305-m Arecibo radio telescope at a center frequency of 4.75 GHz using the broadband, fast-sampled Mock spectrometer. During the course of these surveys, we failed to detect flaring radio emission from any exoplanets, including the young exoplanetary system HR 8799, which theoretical work indicated may have strong magnetic fields capable of generating radio emission at gigahertz frequencies due to their relatively hot temperatures and high masses. Such a detection would provide an exciting alternative to the previous unsuccessful low radio frequency searches for the emission from exoplanets orbiting middle-aged, solar type stars. Among the brown dwarfs we examined, many were not observed to emit bursts of radio emission, with a successful detection rate of 6%. However, we have detected flaring radio emission from four ultracool dwarfs, two of which are new: the L1 dwarf J1439284+192915 and the T6.5 dwarf J10475385+2124234. Among the two known sources that we have detected, J07464256+2000321, an L dwarf binary system, and TVLM 513-46546, a periodically emitting M9 dwarf, we have conducted a lengthy observing campaign of the latter. These observations allow for an unprecedented examination of the burst morphology of the source in time and frequency domains over several years. Our investigation of this source has also resulted in the tantalizing possibility that the temporal properties of the radio bursts go through cycles over the course of months. The discovery of J1047+21 dramatically extends the temperature range over which brown dwarfs appear to be at least sporadic radio-emitters, from ~1,900 K (L3.5) down to ~900 K (T6.5). Follow up observations of this object indicate that while it has detectable quiescent emission, its flaring behavior appears to lack any periodic component. The detection of radio emission from J1439+19, while tentative, is potentially significant due to its relatively slow rotation, which may have implications for dynamo generation theory.