Ultracool dwarfs including the lowest mass stars and substellar dwarfs (or brown dwarfs) is a rapidly evolving and very active field. In this thesis I present the discovery and characterization of ultracool dwarfs and their binary systems with solar and subsolar abundances and try to answer a few scientific questions related to these ultracool objects. I use different techniques based on photometric and astrometric data of modern large scale surveys to identify ultracool dwarfs and their binaries. I identify around 1000 ultracool dwarfs from SDSS, 2MASS and UKIDSS surveys, including 82 L dwarfs and 129 L dwarf candidates (Chapter 2 and 4). This work largely increases the known number of ultracool dwarfs and aid the statistic study of these objects. Eighteen ultracool dwarfs in my sample are found to be in wide binary systems by common proper motion (Chapter 4 and 5). Wide binary systems are often used to test formation theories of low mass stars and brown dwarfs, which have different predictions of separations and binary fractions. One of these binary systems is the first L dwarf companion to a giant star eta Cancri. The eta Cancri B is clearly a useful benchmark object, with constrained distance, age, and metallicity. Further more, the L3.5 dwarf companion eta Cancri B is found to be a potential L4 + T4 binary. I focus on the studies of low mass stars and brown dwarfs with subsolar abundance referred as red and ultracool subdwarfs. They belong to the older Population II of the Galactic halo contain more information of the formation, early evolution and structure of the Milky Way. Using the most extensive optical survey, the Sloan Digital Sky Survey (SDSS), to select low mass stars with subsolar abundance, referred as red subdwarfs with spectral types of late K and M. I identify about 1800 M subdwarfs including 30 new >M6 subdwarfs and five M ultra subdwarfs with very high gravity as well as 14 carbon enhanced red subdwarfs. I also identify 45 red subdwarf binary systems from my red subdwarf sample. Thirty of them are in wide binary systems identified by common proper motion. Fifteen binaries are partially resolved in SDSS and UKIDSS. I estimate the M subdwarf binary fraction. I fit the relationships of spectral types and absolute magnitudes of optical and near infrared bands for M and L subdwarfs. I also measure $UVW$ space velocities of the my M subdwarf sample (Chapter 5). Our studies of the lowest mass stars and brown dwarfs of the Galactic halo are limited by the lack of known objects. There are only seven L subdwarfs published in the literature. I search for ultracool subdwarfs by a combine use of the most extensive optical and near infrared surveys, the SDSS and the UKIRT Infrared Deep Sky Survey. I identify three new L subdwarfs with spectral types of sdL3, sdL7 and esdL6. I re-examine the spectral types and metal classes of all known L subdwarfs and propose to use 2.3 um CO line as an indicator of L subdwarfs. Two of my new L subdwarfs are found to be candidates of halo brown dwarfs (or substellar subdwarfs). I find four of these known ten L subdwarfs could be halo brown dwarfs. I propose a new name "purple dwarf" for lowest-mass stars and brown dwarfs with subsolar abundance (Chapter 3). Finally I summarize and discuss the thesis project in Chapter 6 and describe future research plans in Chapter 7.

We present Spitzer/Infrared Array Camera Ch1 and Ch2 monitoring of six brown dwarfs during eight different epochs over the course of 20 months. For four brown dwarfs, we also obtained simulataneous Hubble Space Telescope (HST)/WFC3 G141 grism spectra during two epochs and derived light curves in five narrowband filters. Probing different pressure levels in the atmospheres, the multiwavelength light curves of our six targets all exhibit variations, and the shape of the light curves evolves over the timescale of a rotation period, ranging from 1.4 to 13 hr. We compare the shapes of the light curves and estimate the phase shifts between the light curves observed at different wavelengths by comparing the phase of the primary Fourier components. We use state-of-the-art atmosphere models to determine the flux contribution of different pressure layers to the observed flux in each filter. We find that the light curves that probe higher pressures are similar and in phase, but are offset and often different from the light curves that probe lower pressures. The phase differences between the two groups of light curves suggest that the modulations seen at lower and higher pressures may be introduced by different cloud layers.

In low-mass (≲1.2 M⊙) main-sequence stars, the combination of differential rotation and turbulent flows in the outer convective region generates strong magnetic fields. It has been observed that in these stars, the rotation rate and the strength of the magnetic field decrease over time. This is thought to result from a feedback loop in which magnetized winds carry angular momentum away from the star, braking its rotation and weakening the magnetic dynamo. A well-calibrated age-rotation-activity relation (ARAR) would be particularly valuable for low-mass stars. If we knew the dependence of rotation or magnetic activity on age, a measurement of one of these quantities could be used to determine an accurate age for any isolated field star. Empirical calibrations of the ARAR rely on observations of the co-eval populations of stars in open clusters. In this work, I characterize rotation and magnetic activity, using light curves for the former and X-ray and Hα emission for the latter, in two open clusters of different ages (Alpha Persei, ≈60 Myr, and Messier 37, ≈500 Myr) to analyze the relation between rotation and activity across the low-mass stellar range. I also compare coronal (X-rays) and chromospheric (Hα) activity to understand how magnetic heating varies across stellar atmospheric layers. My results inform models of angular momentum evolution in low-mass stars.

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We present the first spectral search for neutron stars (NSs) in low-mass X-ray binaries (LMXBs) between outbursts in the Small Magellanic Cloud (SMC). We identify and discuss candidate LMXBs in quiescence in the SMC using deep Chandra X-ray observations of two portions of the SMC. We produce X-ray color-magnitude-diagrams of XRSs of these two fields and identify 10 candidates for quiescent NS LMXBs. Spectral fitting and searches for optical counterparts rule out five, leaving five candidate quiescent NS LMXBs. We estimate that we are sensitive to ~10% of quiescent NS LMXBs in our fields. Our fields include 4.410^7 M of stellar mass, giving an upper limit of 10^{6} LMXBs per M in the SMC. We place a lower limit on the average duty cycle of NS LMXBs as ~0.003.

Since the first successful measurements of stellar trigonometric parallax in the 1830s, the study of nearby stars has focused on the highest proper motion stars (mu > 0.18"/yr). Those high proper motion stars have formed the backbone of the last 150 years of study of the Solar Neighborhood and the composition of the Galaxy. Statistically speaking, though, there is a population of stars that will have low proper motions when their space motions have been projected onto the sky. At the same time, over the last twenty years, populations of relatively young stars (less than ~100 Myr), most of them with low proper motions, have been revealed near (<100 >pc) the Sun. This dissertation is the result of two related projects: A photometric search for nearby (<25 >pc) southern-hemisphere M dwarf stars with low proper motions (mu < 0.18"/yr), and a search for nearby (

A wide range of observed luminosities of young forming stars conflicts with predictions of the standard star formation model, which features a constant accretion rate. To resolve this discrepancy, an episodic accretion model has been suggested. The focus of this dissertation is to test this model in low mass star formation. I present new observations of the CB130 region. The observed photometric data from Spitzer and ground-based telescopes are used to determine the luminosity, and radiative transfer modeling of dust and gas are used to characterize the envelope and disk. I compare molecular line observations to models to constrain the chemical characteristics and abundance variations. Based on the chemical model result and molecular line observations, the low luminosity of the embedded protostar is explained better as a quiescent stage between episodic accretion bursts rather than as the first hydrostatic core stage. I present CO₂ ice observations toward 19 low luminosity embedded protostars. About half of the sources have evidence for pure CO₂ ice, and six have significant double-peaked features, which are strong evidence of pure CO₂ ice. The presence of detectable amounts of pure CO₂ ice signify a higher past luminosity, consistent with the past high accretion. Using chemical evolution modeling, the episodic accretion scenario, in which mixed CO-CO₂ ice is converted to pure CO₂ ice during each high luminosity phase, explains the presence of pure CO₂ ice, the total amount of CO₂ ice, and the observed residual C18O gas. I used CARMA to observe a sample of embedded protostars that spans the full range of protostellar luminosities, especially lower luminosity sources. The standard model predicts the disk mass increases steadily while the episodic accretion model predicts no clear relationship between disk mass and bolometric temperature. Masses of six detected disks spread out regardless of bolometric temperature. With the pure CO₂ ice detection, I can explain disk masses of the source in the context of episodic mass accretion. I conclude that episodic mass accretion provides a good explanation of the low luminosity of protostars, molecular line strength, pure CO₂ ice detection, total CO₂ ice amount and spread of disk masses. / text

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