Probing the Physical Mechanisms Responsible for Brown Dwarf and Giant Planet Formation

The disks that form around young stellar objects provide the essential material for their continued growth as well as the formation of planets, making them ideal laboratories to investigate the mechanisms and environments key for substellar and planetary formation. In this dissertation, I explore two main formation processes: the transportation of water necessary for giant planet formation, and the accretion and growth of young brown dwarfs. First, I study the water ice content in the circumstellar disk of AB Aurigae, a young Herbig Ae star. I detect and map icy grains on the disk surface using high contrast observations taken with LMIRCam on the LBTI. I find that within \u3c 200 au from the central star, far closer than expected, the surface is composed of grains that are approximately 5% ice by mass. This indicates there is either strong vertical mixing or the scattering surface is shielded from photodesorption, allowing these grains to survive. In the second part of this dissertation, I investigate the mechanisms responsible for accretion and formation of protoplanets and brown dwarfs. I detect, for the first time in a protoplanet, NIR emission from Delorme 1 (AB)b, and find its line emission appears to originate from shocks at the planet surface. To determine how accretion mechanisms vary between protoplanets, brown dwarfs, and low mass T Tauri stars, I compile the most complete, to date, database of published accretion rates of these objects (Comprehensive Archive of Substellar and Planetary Accretion Rates; CASPAR). From CASPAR, I show that the scatter in accretion rate at different stellar masses is best explained by a combination of the systematic (i.e. choice of model) scatter in surveys as well as the mass and age of the systems, with the rate of accretion steepening with age in the substellar regime. However, I also find that accretion rates derived from different tracers begin to diverge within the substellar mass regime when utilizing stellar empirical relationships between accretion and line luminosities. I then introduce initial work to investigate this divergence between continuum and line luminosity derived accretion rates through a survey of mass accretion in brown dwarfs using near infrared (NIR) accretion tracers. I create new empirical relationships between NIR hydrogen line luminosity and total accretion luminosity for brown dwarfs, and find that these revised relations resolve the offset in tracers shown in CASPAR. I show that material is infalling from accretion columns at low free fall velocities for BDs, indicating that, unlike stars, the emission could originate from the column or at the BD surface