Dissertation: Investigating Brown Dwarf Atmospheres: Gravity, Dust Content, Cloud Structure and Metallicity

(Cassiopeia – Autumn/l’automne 2017)

by Kendra Kellogg
Thesis defended on July 13, 2017
Department of Physics and Astronomy, Western University
Thesis advisor: Dr. Stanimir Metchev

Brown dwarfs are the lowest mass products of star formation. Their low masses don’t allow them to sustain, or sometimes even begin, the thermonuclear processes that provide stars with internal energy and the thermal pressure necessary to maintain hydrostatic equilibrium. Thus, their radii and effective temperatures decrease as they age, continually changing their spectral classification. However, it is now a well-known fact that the spectral appearance of ultra-cool dwarfs is governed by more than just temperature. Factors such as gravity, metallicity and cloud distribution play an important role in the structure and composition of ultra-cool dwarf atmospheres and ultimately their spectra.

Pinning down the effects of some of the contributing factors to the structure and evolution of brown dwarf atmospheres has been the goal of my thesis research. Through a joint positional and colour cross-match of optical, near-infrared and mid-infrared all-sky surveys, I have identified 20 new brown dwarfs with “peculiar” photometric colours which are candidates for having unusual atmospheric properties. I have determined that a number of these objects have atypical surface gravities and/or atmospheric dust content using near-infrared spectroscopic observations. I have also determined the timescales for the various peculiarities by comparing these objects to the population of “normal” objects. In addition, I have studied in detail a few of the most peculiar objects in order to understand how conditions on individual objects affect their atmospheric structure and composition.

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