UC San Diego

Ultracool dwarfs (UCDs) are the lowest-mass stars and brown dwarfs, with mass $<$ 0.1~M$_{\odot}$ effective temperatures $\leq 3,000$~K. Since they are abundant and long-lived, they can be tracers of the Milky Way formation and evolution history. In the past two decades, tens of thousands of UCDs have been discovered with all-sky surveys including the Two Micron All Sky Survey (2MASS), Wide-field Infrared Survey Explorer (\textit{WISE}), and more recently the \textit{Gaia} satellite.It is now possible to statistically and critically assess these objects as a population, by examining local UCD kinematics and angular momentum evolution, probing their formation history and evolution, and testing brown dwarf evolutionary models. However, only a few hundred precise (3~km~s$^{-1}$) radial (RV) and rotation velocities ($v\sin{i}$) are presently available due to these sources' faintness. I have developed a Markov Chain Monte Carlo forward-modeling technique to extract precise RVs and $v\sin{i}$s in the UCD regime and applied it to new and existing high-resolution near-infrared spectroscopic data obtained with the Near-Infrared Spectrometer (NIRSPEC) on the Keck II Telescope and the Apache Point Observatory Galactic Evolution Experiment (APOGEE) from the Sloan Digital Sky Survey (SDSS). These spectrographs cover the near-infrared waveband, which allow me to increase the number of precise RV and $v\sin{i}$ measurements for low-temperature UCDs, even for relatively low signal-to-noise ratios. I have modeled 219 unique UCDs with measurements taken over 773 epochs. Such a statistically large sample enables assessments of local UCD kinematics and angular momentum evolution, and identifications of substellar binaries. From my analysis, I have resolved a decade-long mystery of local L dwarfs having old kinematics, inconsistent with population simulations incorporating brown dwarf evolutionary models; this can be explained by a high rate of contamination of thick disk L dwarfs in the local sample. I also identified a kinematic break around L4--L6 subtypes, which aligns with the terminus of the stellar Main Sequence. I have identified and confirmed 21 UCD binaries using the RV method, including the first two T dwarf binary systems and one of the shortest period UCD binaries identified to date. UCDs statistically rotate faster from late-M to T types, indicating that their angular momentum is not efficiently lost through magnetized winds as it is for normal stars, consistent with previous studies. For a subset of M and L dwarfs with APOGEE data and measured variability periods, I found that the projected radii generally decline as a function of age, and that the inclination distribution of the $\sim$10~Myr Upper Scorpius cluster and field objects are both consistent with random orientation distribution. I provide a comprehensive sample of RVs, $v\sin{i}$s, effective temperatures ($T_\mathrm{eff}$) and surface gravities ($\log{g}$) for 349 late-M, L, and T dwarfs based on observations made in this work.