We have completed a search for low luminosity companions, including high mass brown dwarfs, to all M dwarfs known within eight parsecs of the sun, and north of -25°. We found six new companions orbiting the survey stars. The masses of the six new secondaries fall between 0.39 and 0.05 M(⊙). Three of the new companions, G208-44B, GL 623B and LHS 1047B, and one previously known secondary in the survey, Ross 614B, are brown dwarf candidates with masses ∼80 Jupiters (0.08 M(⊙)), the dividing line between stars and brown dwarfs. In addition, we provide infrared photometry at J, H and K for all 99 survey members, and spectral types on standard system for half. Analysis of the entire sample indicates that 50% of the stars in the more distant half of the survey volume remain undetected, as is supported by the steadily growing M dwarf census over the last 45 years. The binary fraction of M dwarfs, 30-40%, is lower than that of earlier type main sequence stars, and there are more companions to M dwarfs found between 1 and 10 AU than in any other decade interval. We find that the luminosity function of the lowest mass stars is flat or rising to the end of the main sequence, and that the mass function undoubtedly rises to the stellar/substellar break. We illustrate that the resolution of close binaries is crucial if accurate luminosity and mass functions are to be determined. Finally, we estimate 0.02 M(⊙)/pc³ to be the amount of mass contributed by M dwarfs to the galactic mass. Based upon new mass-luminosity relations developed at infrared wavelengths using a sample of stars with well-determined masses between 1.2 and 0.08 M(⊙), we are able to define empirically the end of the main sequence. We present absolute magnitudes, colors and spectral types for objects at the theoretical lowest stellar mass. Using these relations, we conclude that a few brown dwarfs may have already been discovered. (Abstract shortened with permission of author.)

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Supervisor: Daniel T. Jaffe. Vita. Includes bibliographical references.

Thesis (M.S.)--University of Delaware, 2006. / Principal faculty advisor: John E. Gizis, Dept. of Physics & Astronomy. Includes bibliographical references.

Not available / text

Typescript. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1990. / Includes bibliographical references. / Microfiche. / xii, 178 leaves, bound ill. 29 cm

Thesis (Ph. D.)--Georgia State University, 2009. / Title from file title page. Harold A. McAlister, committee chair; Russel J. White, Brian D. Mason, Douglas R. Gies, David W. Latham, A.G. Unil Perera, Todd J. Henry, committee members. Description based on contents viewed Aug. 24, 2009. Includes bibliographical references (p. 391-410).

Thesis (Ph. D.)--University of Hawaii at Manoa, 2004. / On title page "[mu]" appears as Greek symbol. Includes bibliographical references (leaves 167-180).

Thesis (Ph.D.)--University of Delaware, 2008. / Principal faculty advisor: John E. Gizis, Dept. of Physics & Astronomy. Includes bibliographical references.

Rotational phase mapping of brown dwarfs allows exploration of different cloud and photospheric properties within the same atmospheres, allowing a separation of these parameters from global parameters, such as composition, surface gravity, and age. This work presents an in-depth characterization of high SNR light curves from the Spitzer Space Telescope with up to 13 hours of continuous monitoring of four dwarfs spanning the L3 to L8 spectral type. An exhaustive exploration of currently available state-of-the-art models explains the observed color changes for two of these dwarfs with a linear combination of two model cloudy surfaces differing in effective temperature, cloud opacity and vertical mixing. Using state-of-the-art purely radiative convective atmospheric models, we calculate basic radiative timescales for temperature perturbations in the atmosphere, and consider the effects of dynamics on these timescales. Along with dynamical atmospheric advection timescales, we discuss the relationships between model timescales and the observed light curve evolution.

abstract: The field of exoplanet science has matured over the past two decades with over 3500 confirmed exoplanets. However, many fundamental questions regarding the composition, and formation mechanism remain unanswered. Atmospheres are a window into the properties of a planet, and spectroscopic studies can help resolve many of these questions. For the first part of my dissertation, I participated in two studies of the atmospheres of brown dwarfs to search for weather variations. To understand the evolution of weather on brown dwarfs we conducted a multi-epoch study monitoring four cool brown dwarfs to search for photometric variability. These cool brown dwarfs are predicted to have salt and sulfide clouds condensing in their upper atmosphere and we detected one high amplitude variable. Combining observations for all T5 and later brown dwarfs we note a possible correlation between variability and cloud opacity. For the second half of my thesis, I focused on characterizing the atmospheres of directly imaged exoplanets. In the first study Hubble Space Telescope data on HR8799, in wavelengths unobservable from the ground, provide constraints on the presence of clouds in the outer planets. Next, I present research done in collaboration with the Gemini Planet Imager Exoplanet Survey (GPIES) team including an exploration of the instrument contrast against environmental parameters, and an examination of the environment of the planet in the HD 106906 system. By analyzing archival HST data and examining the near-infrared colors of HD 106906b, we conclude that the companion shows weak evidence of a circumplanetary dust disk or cloud. Finally, I measure the properties of the low mass directly imaged planet 51 Eridani b. We combined published J, H spectra with updated LP photometry, new K1, K2 spectra, and MS photometry. The new data confirms that the planet has redder than similar spectral type objects, which might be due to the planet still transitioning from to L-to-T. Model atmospheres indicate a cooler effective temperature best fit by a patchy cloud atmosphere making 51 Eri b an excellent candidate for future variability studies with the James Webb Space Telescope. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2017