Chih-Chun Hsu (UCSD): "Kinematics, Rotation, and Multiplicity of Ultracool Dwarfs with High-Resolution Near-Infrared Spectroscopy "


Ultracool dwarfs (UCDs) are the lowest-mass stars and brown dwarfs, with effective temperatures less than 3,000 K. They are abundant and long-lived, which can be a useful probe for understanding the formation and evolution of the Milky Way Galaxy. Over the last two decades, thousands of UCDs have been discovered thanks to large all-sky surveys such as DENIS, 2MASS, SDSS, UKIDSS, WISE, and Gaia. These surveys provide precise 5-D kinematics through astrometry and tangential motions. High-resolution near-infrared spectroscopy provides the radial velocities (RV) and projected rotational velocities (vsini) for UCDs. In my thesis research, I have compiled the largest high-resolution near-infrared spectroscopic sample of UCDs to date, using Keck/NIRSPEC and SDSS/APOGEE spectrometers. I employed a Markov chain Monte Carlo technique to extract RV and vsini for these sources. In the first paper of my thesis, I presented an analysis of 172 late-M, L, and T dwarfs in the local 20 pc sample. I found that the local L dwarfs are more kinematically dispersed than late-M and T dwarfs, which can be explained by a higher fraction of Galactic thick disk sources in the local L dwarf population. The local L dwarfs also show a kinematic break around L4–L6 subtypes, indicative of the terminus of the stellar Main Sequence. These kinematic features have been reconfirmed using both archival and new NIRSPEC observations of local late-M and L dwarfs. Our vsini analysis finds that the average vsini is higher from late-M to T dwarfs, consistent with previous studies and indicative of a progressive weakening in angular momentum loss mechanisms for lower-temperature/lower-mass objects. Combing measurements of rotation periods, we found that the projected radii (Rsini) decrease as a function of age from young clusters to field populations. We present new population simulations of this kinematic sample that allow us to constrain the local UCD population parameters, including the star formation rate and mass function, as well as potential modifications to evolutionary models. I will show examples of binaries identified in the NIRSPEC and APOGEE samples. Finally, I will summarize the future high-resolution spectroscopic instruments and surveys that can provide a more complete and precise RV and vsini sample and extension to abundance measurements that enables chemodynamics to study our Galaxy using UCDs.