This work is a compilation and analysis of ultracool dwarfs (UCDs) and brown dwarfs within 25 parsecs. It supplements the work of Stauffer et al. [2010] who updated the reputable and widely relied upon Third Catalog of Nearby Stars [Gliese & Jahreiß 1991] with revised coordinates and cross-matched each object with the 2MASS point source catalog [Cutri et al. 2003]. I began by incorporating newly discovered (post 1991) cool companions to Gliese-Jahreiß stars that had been previously undetectable. I then expanded the compilation to include isolated UCDs and other nearby systems with at least one UCD component. Multiple systems are a panacea for astrophysical problems: by applying Kepler’s laws, the model-independent mass of brown dwarfs and low mass stars can be determined and hence serve to constrain theory. This work puts this data into context by exploring the history of brown dwarf theory and reviewing open questions concerning their nature.

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.

Brown dwarfs are objects with sub-stellar masses that are unable to sustain hydrogen burning, cooling down through out their lifetimes. This thesis presents two projects, the study of the IMF of the double cluster, h & χ Persei, and the determination of the dynamical masses of the brown dwarf binary, ε Indi Ba, Bb. The study of a cluster’s population distribution gives us the opportunity to study a statistically meaningful population of objects over a wide range of masses (from massive stars to brown dwarfs), with a similar age and chemical composition providing formation and dynamical evolution constraints. h & χ Persei is the largest double cluster known in our galaxy. Using optical and infrared photometric data we have produced the deepest mass function for the system. A study of the radial distribution shows evidence of mass segregation while the mass function shows that these clusters may be suffering from accelerated dynamical evolution due to their interaction, triggering the ejection of brown dwarfs. The physical parameterization of brown dwarfs is reliant on the use of interior and atmospheric models. The study of brown dwarf binaries can provide crucial model independent measurements, especially masses. ε Indi Ba, Bb (spectral types T1 and T6) is the closest known brown dwarf binary to Earth. The brown dwarf binary itself orbits a main sequence star allowing us to constrain the distance, metallicity and age of the system making it possible to break the sub-stellar mass-age-luminosity degeneracy. The relative motion of the brown dwarf binary has been studied with precision astrometry from infrared AO data, allowing the determination of the system mass, 121.16 ± 0.17 ± 1.08 MJup . The individual masses of the binary components were derived from the absolute movement of the binary to be MBa = 68.04±0.94 MJup and MBb = 53.12±0.32 MJup. We concluded that the isochronally-derived masses were underestimating the system mass by ∼ 60%, due to the likely underestimation of the age of the system. The evolutionary models are consistent with the parameters measured observationally if the system has an age ∼ 4 Gyr.

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.

Aims. Uncovering the ingredients and the architecture of planetary systems is a very active field of research that has fuelled many new theories on giant planet formation, migration, composition, and interaction with the circumstellar environment. We aim at discovering and studying new such systems, to further expand our knowledge of how low-mass companions form and evolve. Methods. We obtained high-contrast H-band images of the circumstellar environment of the F5V star HD 206893, known to host a debris disc never detected in scattered light. These observations are part of the SPHERE High Angular Resolution Debris Disc Survey (SHARDDS) using the InfraRed Dual-band Imager and Spectrograph (IRDIS) installed on VLT/SPHERE. Results. We report the detection of a source with a contrast of 3 : 6 x 10(-5) in the H-band, orbiting at a projected separation of 270 milliarcsec or 10 au, corresponding to a mass in the range 24 to 73 M-Jup for an age of the system in the range 0.2 to 2 Gyr. The detection was confirmed ten months later with VLT /NaCo, ruling out a background object with no proper motion. A faint extended emission compatible with the disc scattered light signal is also observed. Conclusions. The detection of a low-mass companion inside a massive debris disc makes this system an analog of other young planetary systems such as beta Pictoris, HR 8799 or HD 95086 and requires now further characterisation of both components to understand their interactions.

We present an outline of basic assumptions and governing structural equations describing atmospheres of sub-stellar mass objects, in particular the extrasolar giant planets and brown dwarfs. Although most of the presentation of the physical and numerical background is generic, details of the implementation pertain mostly to the code COOLTLUSTY. We also present a review of numerical approaches and computer codes devised to solve the structural equations, and make a critical evaluation of their efficiency and accuracy.

In this work, we report the discovery of a candidate planetary-mass object with a photoevaporating protoplanetary disk, Proplyd. 133-353, which is near the massive star theta(1) Ori C at the center of the Orion Nebula Cluster (ONC). The object was known to have extended emission pointing away from theta(1) Ori. C, indicating ongoing external photoevaporation. Our near-infrared spectroscopic data and the location on the H-R diagram suggest that the central source of Proplyd. 133-353 is substellar (similar to M9.5) and has a mass probably less than 13 Jupiter mass and an age younger than 0.5 Myr. Proplyd. 133-353 shows a similar ratio of X-ray luminosity to stellar luminosity to other young stars in the ONC with a similar stellar luminosity and has a similar proper motion to the mean one of confirmed ONC members. We propose that Proplyd. 133-353 formed in a very low-mass dusty cloud or an evaporating gas globule near theta(1) Ori C as a second generation of star formation, which can explain both its young age and the presence of its disk.

Angular differential imaging provides a novel way of probing high contrast regions of our universe. Until now, its applications have been primarily localized to searching for exoplanets around nearby stars. This work presents a suite of applications of angular differential imaging from the theoretical underpinning of data reduction, to its use characterizing substellar objects, to a new application looking for the host galaxies of damped Lyman α systems which are usually lost in the glare of ultra-bright quasars along the line of sight. The search for exoplanets utilizes angular differential imaging and relies on complex algorithms to remove residual speckles and artifacts in the images. One such algorithm, the Template Locally Optimized Combination of Images (TLOCI), uses a least-squares method to maximize the signal-to-noise ratio and can be used with variable parameters, such as an input spectral template, matrix inversion method, aggressivity and unsharp mask size. Given the large volume of image sequences that need to be processed in any exoplanet survey, it is important to find a small set of parameters that can maximize detections for any conditions. Rigorous testing of these parameters were done with on-sky images and simulated inserted planets to find the optimal combination of parameters. Overall, a standard matrix inversion, along with two to three input templates, a modest aggressivity of 0.7 and the smallest unsharp mask was found to be the best choice to balance optimal detection. Beyond optimizations, TLOCI has been used in conjunction with angular differential imaging to characterize substellar objects in our local solar neighbourhood. In particular, the star HD 984 was imaged as a part of the Gemini Planet Imager Exoplanet Survey. Although previously known to have a substellar companion, new imaging presented here in the H and J bands help further characterize this object. Comparisons with a library of brown dwarf spectral types found a best match to HD 984 B of a type M7±2. Orbital fitting suggests an 18 AU (70 year) orbit, with a 68% confidence interval between 12 and 27 AU. Object magnitude was used to find the luminosity, mass and temperature using DUSTY models. Although angular differential imaging has proven its value in high contrast imaging, it has largely remained in the field of substellar object detection, despite other high contrast regimes in which it could be applied. One potential application is outside the local solar neighbourhood with studies of damped Lyman α systems, which have struggled to identify host galaxies thought to be caused by systems seen in the spectra of bright quasars. Work herein presents the first application of angular differential imaging to finding the host galaxies to damped Lyman α systems. Using ADI we identified three potential systems within 30kpc of the sightline of the quasar and demonstrate the potential for future imaging of galaxies at close separations. In summary, this thesis presents a comprehensive look at multiple aspects of high contrast angular differential imaging. It explores optimizations with a data reduction algorithm, implementations characterizing substellar objects, and new applications imaging galaxies. / Graduate

Upon its discovery in 2006, the young L7.5 companion to the solar analog HD 203030 was found to be approximate to 200 K cooler than older late-L dwarfs, which is quite unusual. HD. 203030B offered the first clear indication that the effective temperature at the L-to-T spectral type transition depends on surface gravity: now a well-known characteristic of low-gravity ultra-cool dwarfs. An initial age analysis of the G8V primary star indicated that the system was 130-400 Myr old, and so the companion would be between 12 and 31 M-Jup. Using moderate-resolution near-infrared spectra of HD. 203030B, we now find features of very low gravity comparable to those of 10-150 Myr old L7-L8 dwarfs. We also obtained more accurate near-infrared and Spitzer/IRAC photometry, and we find a (J - K) MKO color of 2.56 +/- 0.13 mag-comparable to those observed in other young planetary-mass objects-and a luminosity of log (L-bol/L-circle dot) = -4.75 +/- 0.04 dex. We further re-assess the evidence for the young age of the host star, HD 203030, with a more comprehensive analysis of the photometry and updated stellar activity measurements and age calibrations. Summarizing the age diagnostics for both components of the binary, we adopt an age of 100 Myr for HD 203030B and an age range of 30-150 Myr. Using cloudy evolutionary models, the new companion age range and luminosity result in a mass of 11 M-Jup with a range of 8-15 M-Jup, and an effective temperature of 1040 +/- 50 K.

Aims. We numerically studied close encounters between a young stellar system hosting a massive, gravitationally fragmenting disk and an intruder diskless star with the aim of determining the evolution of fragments that have formed in the disk prior to the encounter. Methods. Numerical hydrodynamics simulations in the non-inertial frame of reference of the host star were employed to simulate the prograde and retrograde co-planar encounters. The initial configuration of the target system (star plus disk) was obtained via a separate numerical simulation featuring the gravitational collapse of a solar-mass pre-stellar core. Results. We found that close encounters can lead to the ejection of fragments that have formed in the disk of the target prior to collision. In particular, prograde encounters are more efficient in ejecting the fragments than the retrograde encounters. The masses of ejected fragments are in the brown-dwarf mass regime. They also carry away an appreciable amount of gas in their gravitational radius of influence, implying that these objects may possess extended disks or envelopes, as also previously suggested. Close encounters can also lead to the ejection of entire spiral arms, followed by fragmentation and formation of freely-floating objects straddling the planetary mass limit. However, numerical simulations with a higher resolution are needed to confirm this finding.