The Pennsylvania State University The Graduate School Department of Astronomy and Astrophysics RADIO OBSERVATIONS OF THE BROWN DWARF- EXOPLANET BOUNDARY A Dissertation in Astronomy and Astrophysics by Matthew Philip Route 2013 Matthew Philip Route Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2013 The dissertation of Matthew Philip Route was reviewed and approved* by the following: Alexander Wolszczan Evan Pugh Professor of Astronomy and Astrophysics Dissertation Advisor Chair of Committee Lyle Long Distinguished Professor of Aerospace Engineering, and Mathematics Director of the Computational Science Graduate Minor Program Kevin Luhman Associate Professor of Astronomy and Astrophysics John Mathews Professor of Electrical Engineering Steinn Sigurdsson Professor of Astronomy and Astrophysics Chair of the Graduate Program for the Department of Astronomy and Astrophysics Special Signatory Richard Wade Associate Professor of Astronomy and Astrophysics Donald Schneider Distinguished Professor of Astronomy and Astrophysics Head of the Department of Astronomy and Astrophysics *Signatures are on file in the Graduate School iii 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. iv TABLE OF CONTENTS List of Figures .......................................................................................................................... vi List of Tables ........................................................................................................................... ix Acknowledgements .................................................................................................................. x Chapter 1 Classification of Substellar Objects ........................................................................ 1 1.1. Definitions ................................................................................................................ 1 1.2. Theory of Substellar Objects .................................................................................... 3 1.2.1. Stellar Interior Physics ................................................................................... 3 1.2.2. Interior Physics of Substellar Objects ............................................................ 4 1.2.3. Structural Properties of Substellar Objects ..................................................... 6 1.3. Exoplanets ................................................................................................................ 8 1.3.1. A Brief History of Exoplanet Discoveries ...................................................... 8 1.3.2. Exoplanet Characterization ............................................................................ 9 1.3.3. Exoplanet Magnetic Fields ............................................................................. 13 1.4. Brown Dwarfs .......................................................................................................... 22 1.4.1. A Brief History of Brown Dwarf Discoveries ................................................ 22 1.4.2. Brown Dwarf Characterization ...................................................................... 23 1.4.3. Characterization of Activity and Magnetic Properties of Brown Dwarfs ...... 27 1.5. Dissertation Outline.................................................................................................. 33 Chapter 2 Radio Emission Mechanisms .................................................................................. 35 2.1. Overview .................................................................................................................. 35 2.1.1. Solar System Planetary Radio Emission ........................................................ 35 2.1.2. Solar Radio Emission ..................................................................................... 37 2.1.3. Stellar Radio Emission ................................................................................... 39 2.1.4. Ultracool Dwarf Radio Emission ................................................................... 39 2.2. Gyrosynchrotron Emission ....................................................................................... 45 2.3. The Electron Cyclotron Maser Instability ................................................................ 46 2.3.1. History ............................................................................................................ 46 2.3.2. Physics of the Electron Cyclotron Maser ....................................................... 46 Chapter 3 Instrumentation and Computation ........................................................................... 55 3.1. Observing Instrumentation ....................................................................................... 55 3.1.1. Arecibo Radio Telescope ............................................................................... 55 3.1.2. C-band Receiver ............................................................................................. 56 3.1.3. Wideband Arecibo Pulsar Processor .............................................................. 58 3.1.4. Mock Spectrometer ........................................................................................ 58 3.2. Signal Processing and Computation ......................................................................... 59 3.2.1. Calculation of Stokes Parameters ................................................................... 59 3.2.2. Description of Data Products ......................................................................... 60 v 3.2.3. Data Processing Pipelines .............................................................................. 60 3.2.4. Computational Results ................................................................................... 63 Chapter 4 The Arecibo Survey for Radio Emission from Exoplanets ..................................... 65 4.1. Motivation ................................................................................................................ 65 4.2. Target Selection and Observations ............................................................................ 66 4.3. Results .....................................................................................................................