Dissertations, Theses, and Capstone Projects

Date of Degree

6-2017

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Emily Rice

Committee Members

Emily Rice

Kelle Cruz

Timothy Paglione

Justin Crepp

Charles Liu

Subject Categories

Astrophysics and Astronomy | Other Astrophysics and Astronomy | Stars, Interstellar Medium and the Galaxy

Abstract

Exoplanet direct detections are reaching the temperature regime of cool brown dwarfs, motivating further understanding of the coolest substellar atmospheres. These objects, T and Y dwarfs, are numerous and isolated in the field, thus making them easier to study in detail than objects in companion systems. Brown dwarf spectral types are derived from spectral morphology and generally appear to correspond with decreasing mass and effective temperature (Teff). However, spectral subclasses of the colder objects do not share this monotonic temperature correlation, indicating that secondary parameters (gravity, metallicity, dust) significantly influence spectral morphology. These secondary atmospheric parameters can provide insight into age and formation mechanisms. We seek to disentangle the fundamental parameters that underlie the spectral morphology of T dwarfs, the coolest fully populated spectral class of brown dwarfs, using comparisons to atmospheric models. We investigate the relationship between spectral type and Teff from the best fit model parameters for a sample of 151 T dwarfs with low resolution (R~75-100) near-infrared SpeX Prism spectra. We use synthetic spectra from three model grids (Morley+ 2012, Saumon+ 2012, and BT Settl 2013) and a Markov-Chain Monte Carlo (MCMC) analysis to determine robust best fit parameters with uncertainties. We perform our analysis on the full spectrum and on narrower wavelength ranges, for the BT-Settl 2013 model grid, where directly detected exoplanets are typically characterized. We provide foundational assessments of the factors that affect T dwarf spectral morphology to prescribe the best approach to interpreting spectra of cool substellar objects. Using T dwarfs as exoplanet analogs, we create spectral templates from observed spectra for comparison to cool companion spectra of high contrast imaged objects. Our analysis of these proof-of-concept cases provides the backbone for interpreting spectra for some of the benchmark companion objects found with today's exoplanet imagers. Our analysis is the most extensive T dwarf model comparison to date, thereby laying the foundation for interpretation of cool brown dwarf and exoplanet spectra.

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