Document Type

Dissertation

Date of Degree

Summer 2014

Degree Name

PhD (Doctor of Philosophy)

Degree In

Physics

First Advisor

Robert L. Mutel

Abstract

Magnetic fields are prevalent in a wide variety of low mass stellar systems and play an important role in their evolution. Yet the process through which these fields are generated is not well understood. To understand how such systems can generate strong field structures characterization of these fields is required. Radio emission traces the fields directly and the properties of this emission can be modeled leading to constraints on the field geometry and magnetic parameters. The new Karl Jansky Very Large Array (VLA) provides highly sensitive radio observations. My thesis involves combining VLA observations with the development of magnetospheric emission models in order to characterize the magnetic fields in two fully convective cool star systems: (1) Young Stellar Objects (YSOs); (2) Ultracool dwarf stars.

I conducted multi epoch observations of DG Tau, a YSO with a highly active, collimated outflow. The radio emission observed from this source was found to be optically thick thermal emission with no indication of the magnetic activity observed in X-rays. I determined that the outflow is highly collimated very close to the central source, in agreement with jet launching models. Additionally, I constrained the mass loss of the ionized component of the jet and found that close to the central source the majority of mass is lost through this component. Using lower angular resolution observations, I detected shock formations in the extended jet of DG Tau and modeled their evolution with time.

Taking full advantage of the upgraded bandwidth on the VLA, I made wideband observations of two UCDs, TVLM513-46 and 2M 0746+20. Combining these observations with previously published and archival VLA observations I was able to fully characterize the spectral and temporal properties of the radio emission. I found that the emission is dominated by a mildly polarized, non-thermal quiescent component with periodic strongly polarized flare emission. The spectral energy distribution and polarization of the quiescent emission is well modeled using gyrosynchrotron emission with a mean field B~100 G, mildly relativistic power-law electrons with a density ne~105-6 cm-3, and source size of R~2R*. We were able to model the pulsed emission by coherent electron cyclotron radiation from a small number of isolated loops of high magnetic field (2-3 kG) with scale heights~1.2-2.7 stellar radii. The loops are well-separated in magnetic longitude, and are not part of a single dipolar magnetosphere. The overall magnetic configuration of both stars appears to confirm recent suggestions that radio over-luminous UCD's have `weak field' non-axisymmetric topologies, but with isolated regions of high magnetic field.

Keywords

Magnetic Fields, Radio, Ultracool Dwarf, Very Large Array, Young Stellar Objects

Pages

xiii, 231 pages

Bibliography

Includes bibliographical references (pages 220-231).

Comments

This thesis has been optimized for improved web viewing. If you require the original version, contact the University Archives at the University of Iowa: http://www.lib.uiowa.edu/sc/contact/.

Copyright

Copyright 2014 Christene Rene Lynch

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