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The relatively young field of exoplanets promises exciting new discoveries and insights into other planetary systems much like and unlike our own, thanks to the development of advanced instrumentation, observing techniques and analysis techniques. To date, thousands of nearby exoplanets have been discovered via various methods such as transit detection, radial velocity, astrometry, micro-lensing, and direct imaging. Among these methods, direct imaging is the only method to directly receive photons from the sub-stellar companions that are distinguishable from their host stars. As such, it offers unique characterizations to the surface properties of these companions and offer us rich information on the astrometry, mass, atmospheres and evolution of these objects, especially when combined with all other methods. Throughout my Astrophysics PhD thesis titled "Characterizing Exoplanets, Brown Dwarfs, and Proto-planetary Disks with High Contrast Imaging", I present an exploration into the development of advanced data analysis techniques for high contrast imaging data, alongside their application in characterizing celestial objects. The introductory chapter provides overviews for the subjects covered in each subsequent chapter. In Chapter 2, I contextualize the significance of direct imaging in characterizing these celestial objects, emphasizing the challenges posed by their extreme contrast and close angular separations from their host stars in ground based imaging. Leveraging sophisticated instrumentation and post-processing algorithms, I introduce the pyKLIP-CHARIS Post-Processing Pipeline within the pyKLIP framework, offering a Python-based data analysis tool for reducing and analyzing observation data taken by the CHARIS instrument on the Subaru Telescope in Hawaii. This pipeline incorporates state-of-the-art algorithms and observing techniques like KLIP, ADI, SDI, and forward modeling for PSF subtraction, facilitating the extraction of precise positions and calibrated planet spectra.In subsequent chapters, I demonstrate the application of various imaging analysis techniques to measure properties of sub-stellar companions and the scientific insights gained from them. In Chapter 3, I delve into the characterization of a binary brown dwarf system, , utilizing long-term relative orbit monitoring and absolute astrometry data from the VLT. This comprehensive analysis yields precise individual dynamical masses for the binary components, shedding light on their mass-luminosity relationship and providing invaluable benchmarks for substellar cooling models.In Chapter \ref{chap:disk}, my research extends to the investigation of a proto-planetary disk around HD 34700 A. Leveraging CHARIS' unique capability to obtain polarized integral field images, I gain access to the intricate structures of the disk at multiple wavelengths, and explore their implications through 3D Monte Carlo radiative transfer simulations. These simulations, combined with observed polarized intensity data, offer insights into the dust grain properties and scattering mechanisms within the disk, highlighting the complexities inherent in modeling such systems.Finally, in Chapter \ref{chap:gl504}, I conclude my research with an on-going investigation on the infra-red spectra of two ultra-cool T-dwarfs, Gl~758~B and Gl~504~b. These are two of the coldest brown dwarfs observed so far. They share very similar host star properties and provides invaluable insight to understanding brown dwarfs atmospheres at temperatures of 500-600 K. A special beam-steering mode on the \SCExAO and \CHARIS instrument was required to capture these widely separated companions and poses unique data analysis challenges. I emphasize these challenges and the solutions that employed to address them, along with preliminary results on the astrometry and the extracted spectra of these two brown dwarfs.

The spectral resolution of instruments used to characterize substellar atmospheres has greatly increased over the past decade. As we more frequently observe exoplanet and brown dwarf atmospheres at higher spectral resolution, more work is needed to assess both what new information is contained in these improved observations as well as how our current modeling tools fall short in accurately reproducing these spectra. My dissertation has examined this question from multiple angles to ultimately prepare the field to better understand substellar atmospheres through the better quality spectra we will receive from the JWST and the upcoming ELTs. First, I present how high-resolution cross-correlation spectroscopy (R $\sim$ 25,000 to 100,000) will allow us to probe the regions in the atmospheres of sub-Neptune exoplanets above the clouds or hazes which obscure molecular features in observations at low spectral resolution. Using theoretical models of high-resolution observations for a typical hazy sub-Neptune, we calculate the signal-to-noise of these spectra required to robustly detect a host of molecules as a function of spectral resolution and wavelength coverage to aid in planning future observations and instruments. Next, I present two projects focused on adapting atmospheric retrieval methods for medium-resolution spectra of brown dwarfs. I first describe applying a GPU-version of the CHIMERA retrieval framework to a high signal-to-noise, medium-resolution (R$\sim$6000) FIRE spectrum of a T9 dwarf from 0.85-2.5 $\mu$m. At 60$\times$ higher spectral resolution than previous brown dwarf retrievals, a number of novel challenges arise, which I explore. I show that compared to retrieval results from a R$\sim$100 spectrum of the same object, constraints on atmospheric abundances improve by an order of magnitude or more with increased spectral resolution. Finally, I apply lessons learned from this project to JWST NIRSpec/G395H (R$\sim$2700, 2.87 - 5.14 $\mu$m) observations of a T8 dwarf, presenting the first retrieval analysis taking full advantage of the maximum spectral resolution available with NIRSpec. I obtain precise ($\sim$ 0.02 dex) abundance constraints for a number of species, which indicate shortcomings in our understanding of disequilibrium chemistry in brown dwarf atmospheres. I also present the measured $^{12}$CO/$^{13}$CO ratio for this brown dwarf, making it the fourth and coldest ($\sim$ 760 K) extrasolar object with such a measurement. Together, these projects illustrate the power of high-quality, medium-to-high resolution spectra to precisely constrain atmospheric properties, furthering our understanding of the formation and atmospheres of substellar objects.

Low mass star formation inside massive clusters is crucial to understand the effect of cluster environment on processes like circumstellar disk evolution, planet and brown dwarf formation. The young massive association of Cygnus OB2, with a strong feedback from massive stars, is an ideal target to study the effect of extreme environmental conditions on its extensive low-mass population. We aim to perform deep multi-wavelength studies to understand the role of stellar feedback on the IMF, brown dwarf fraction and circumstellar disk properties in the region. We introduce here, the deepest and widest optical photometry of 1.5$^\circ$ diameter region centred at Cygnus OB2 in r$_{2}$, i$_{2}$, z and Y-filters using Subaru Hyper Suprime-Cam (HSC). This work presents the data reduction, source catalog generation, data quality checks and preliminary results about the pre-main sequence sources. We obtain 713,529 sources in total, with detection down to $\sim$ 28 mag, 27 mag, 25.5 mag and 24.5 mag in r$_{2}$, i$_{2}$, z and Y-band respectively, which is $\sim$ 3 - 5 mag deeper than the existing Pan-STARRS and GTC/OSIRIS photometry. We confirm the presence of a distinct pre-main sequence branch by statistical field subtraction of the central 18$^\prime$ region. We find the median age of the region as $\sim$ 5 $\pm$ 2 Myrs with an average disk fraction of $\sim$ 9$\%$. At this age, combined with A$_V$ $\sim$ 6 - 8 mag, we detect sources down to a mass range $\sim$ 0.01 - 0.17 M$_\odot$. The deep HSC catalog will serve as the groundwork for further studies on this prominent active young cluster., Accepted for publication in Monthly Notices of Royal Astronomical Society (MNRAS), 20 pages, 21 figures, 4 Tables, typos corrected

[BHB2007]-1 is an accreting young stellar object which is surrounded by a protoplanetary disk with an enormous gap (~70 au wide). This cavity might have been carved by a very young brown dwarf candidate present at a distance of 0''.32. To date, the presence of this candidate has only been detected in the radio (VLA) and NACO's L' filter, limiting to constrain the mass accurately. We have GRAVITY observing time to extract the K-band spectrum of the companion candidate for the first time. We aim to constrain the dynamical mass of the companion, characterize it with atmospheric models and perform high-precision astrometry to constrain its position. A successful observation would confirm the presence of the brown dwarf companion candidate. Additionally, we also would be able to observe the circumplanetary environment with a much higher resolution compared to the previous observations.

Super-Jupiters, brown dwarfs, and stars can form from the collapse of self-gravitating discs. Such discs are turbulent, with flocculent spiral arms accelerating gas to transonic speeds horizontally and vertically. Objects that fragment from gravito-turbulent discs should spin with a wide range of directions, reflecting the random orientations of their parent eddies. We show by direct numerical simulation that obliquities of newly collapsed fragments can range up to 45$^\circ$. Subsequent collisions between fragments can further alter the obliquity distribution, up to 90$^\circ$ or down to near-zero. The large obliquities of newly discovered super-Jupiters on wide orbits around young stars may be gravito-turbulent in origin. Obliquely spinning fragments are born on orbits that may be inclined relative to their parent discs by up to 20$^\circ$, and gravitationally stir leftover material to many times the pre-fragmentation disc thickness., 9 pages, 7 figures. Accepted to MNRAS

In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broadband radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner-magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broadband radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere's co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultra-cool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons., Comment: Accepted to MNRAS; 26 pages, 13 figures

We report the discovery of a rare close binary system, SMSS J160639.78-100010.7, comprised of a magnetic white dwarf with a field of about 30 MG and a brown dwarf. We measured an orbital period of 92 min which is consistent with the photometric period. Minimum and maximum light occur at the orbital quadratures Phi=0.25 and 0.75, respectively, and cannot be caused by reflection on the brown dwarf, but, instead, by a spot on the synchronously rotating magnetic white dwarf. The brown dwarf does not fill its Roche lobe and the system may be in a low-accretion state or, more likely, in a detached state following episodes of mass transfer. SMSS J160639.78-100010.7 is the nearest known magnetic white dwarf plus brown dwarf system., Comment: Accepted for publication in MNRAS Letters

We present the first short-duration candidate microlensing events from the Kepler K2 mission. From late April to early July 2016, Campaign 9 of K2 obtained high temporal cadence observations over a 3.7 square degree region of the Galactic bulge. Its primary objectives were to look for evidence of a free-floating planet (FFP) population using microlensing, and demonstrate the feasibility of space-based planetary microlensing surveys. Though Kepler K2 is far from optimal for microlensing, the recently developed MCPM photometric pipeline enables us to identify and model microlensing events. We describe our blind event-selection pipeline in detail and use it to recover 22 short-duration events with effective timescales of less than 10 days previously announced by the OGLE and KMTNet ground-based surveys. We also announce five new candidate events. One of these is a caustic-crossing binary event, consistent with a bound planet and modelled as such in a companion study. The other four have very short durations (effective timescales less than 0.1 days) typical of an Earth-mass FFP population. Whilst Kepler was not designed for crowded-field photometry, the K2C9 dataset clearly demonstrates the feasibility of conducting blind space-based microlensing surveys towards the Galactic bulge., Comment: 25 pages including appendices, published MNRAS

In order to improve the physical interpretation about innermost dusty regions in protoplanetary discs around brown dwarf (BD), and even very low mass star (VLMS), we present a grid of models taking into account two different sets: (i) The set called standard model, that simulates an axisymmetric dusty disc with an inner curved wall. (ii) The perturbed one called non-standard where the axisymmetry of the inner edge has been broken. We have achieved a fitting for the disc structure able to explain the spectral energy distribution (SED). As the main condition, we assume that the changes of the inner wall geometry in the tongue-like shape depend on the Rayleigh–Taylor instability (R-TIns) generated in the inner disc edge. For each object, we parametrize the shape of the inner wall to find a time-dependent model that enables us to explain the photometric near-Infrared variability and connect the changes on the inner disc structure with the amplitude of such variability. We re-analysed photometric measurements from near to mid-infrared wavelengths of a sample of 6 BDs and one VLMS in different cloud associations which were previously studied by other authors. We also show that the flux change calculated between the non-standard and the standard configurations models the observed variability in LRLL 1679. The magnitude changes due to these fluctuations slightly depend on the wavelength and they can present differences of up to 0.9 mag. We suggest that if the R-TIns persist enough time, the features in the protoplanetary inner disc, e.g. inner holes or gaps evolve.