Date of Degree

9-2020

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Jacqueline K. Faherty

Committee Members

Benjamin G. Burningham

Kelle L. Cruz

Emily L. Rice

K.E. Saavik Ford

Subject Categories

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

Keywords

Brown dwarfs, low-mass stars, fundamental parameters, atmospheres, retrievals, subdwarfs

Abstract

Complexities in the atmospheres of sources that straddle the boundary between low-mass stars and planets drive the spectral features we observe and the fundamental parameters that we derive. In an effort to understand the underlying cause of these spectral features, this work examines low-mass stars and brown dwarfs using two approaches: distance-calibrated spectral energy distributions (SEDs) and atmospheric retrievals. SEDs allow for the determination of semi-empirical fundamental parameters, while atmospheric retrievals allow us to derive the atmospheric structure and composition, as well as extrapolated fundamental parameters for a source. Low-metallicity and high surface gravity sources, known as subdwarfs, are the primary vehicles used for this work. They are particularly useful in probing how metallicity affects the atmospheric features we observe.

The first half of this work focuses on using SEDs to determine semi-empirical fundamental parameters and to draw insights from comparisons between sources of similar effective temperatures (Teff) or bolometric luminosity (Lbol). By examining one of the bluest known subdwarfs, SDSS J125637.13-022452.4, I explore how overall SED shape and individual spectral features vary with age. Reanalyzing the age of TRAPPIST-1, a late-M dwarf that hosts seven rocky Earth-sized planets and has two contradicting age indicators, provides insight on a population of brown dwarfs with peculiar gravity-related features.

The second half of this work focuses on using atmospheric retrievals to dig into the thermal structure and atmospheric composition of subdwarfs and field-aged brown dwarfs. Currently, subdwarfs are thought to have cloudless atmospheres due to reduced condensate opacities from their low-metallicities, causing their very blue near-infrared colors. Using observational and theoretical approaches I address the following questions with a sample of 15 subdwarfs and comparative brown dwarfs: (1) Are subdwarfs cloudless? and (2) How does their thermal profile compare to objects of similar Teff or spectral type? Using the retrieval framework known as Brewster, which is specifically targeted at studying the cloudy atmospheres of brown dwarfs, I aim to compare gas abundances across the sample and explore what may be causing the differences seen in the SEDs of these sources.

Approaching characterization at the stellar/substellar boundary from observational and theory-driven methods will reveal the atmospheric complexities of these sources. The results from this work will provide unprecedented information on subdwarf atmospheres. SEDs provide semi-empirical fundamental parameters while retrievals will dig into their atmospheres, which has never been done before for these ultracool low-metallicity objects.

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