Date of Award

Fall 11-18-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Todd J. Henry

Abstract

The aim of this dissertation is to address the topic of distinguishing very low mass stars from brown dwarfs through observational means. To that end, we seek to better characterize both populations and establish mechanisms that facilitate establishing an individual object's membership in either the very low mass star or the brown dwarf populations. The dissertation is composed of three separate observational studies. In the first study we report on our analysis of HST/NICMOS snapshot high resolution images of 255 stars in 201 systems within ~10 parsecs of the Sun. We establish magnitude and separation limits for which companions can be ruled out for each star in the sample, and then perform a comprehensive sensitivity and completeness analysis for the subsample of 138 M dwarfs in 126 systems. We calculate a multiplicity fraction of $0.0-0.0+3.5% for L companions to M dwarfs in the separation range of 5 to 70 AU, and $2.3-0.7+5.0% for L and T companions to M dwarfs in the separation range of 10 to 70 AU. Considering these results and results from several other studies, we argue that the so-called "brown dwarf desert" extends to binary systems with low mass primaries and is largely independent of primary mass, mass ratio, and separation.

In the second study we construct a Hertzsprung-Russell diagram for the stellar/substellar boundary based on a sample of 63 objects ranging in spectral type from M6V to L4. We report new VRI photometry for 63 objects and new trigonometric parallaxes for 37 objects. We employ a novel SED fitting algorithm to determine effective temperatures, bolometric luminosities, and radii. We find evidence for the local minimum in the radius-temperature and radius-luminosity trends that may indicate the end of the stellar main sequence and the start of the brown dwarf sequence at $Teff ~2075K, log(L/Lsun) ~ -3.9, and (R/Rsun) ~ 0.086.

The third study is a pilot study for future work and part of a long term search for astrometric binaries that have the potential to yield dynamical masses. We report the discovery of five new multiple systems and discuss their potential for determining dynamical masses: LHS 2071AB, GJ 1215 ABC, LTT 7434 AB, LHS 501 AC, and LHS 3738 AB.

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