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This dissertation is the sum of five studies of the structure and evolution of circumstellar disks, the birthplace of planets. These studies are all based on Infrared data from the Spitzer Space Telescope, and taken together trace the evolution of disks from the optically thick primordial stage to the optically thin debris disk stage. The five projects included in this dissertation are diverse but they are all interconnected and have a common underlying motivation: to impose observational constraints on different aspects of planet formation theories. In the first project, we study the near and mid-IR (1.2-24 [mu]m) emission of Classical T Tauri Star (CTTS), which are low-mass pre-main sequence (PMS) stars that show clear evidence for accretion. We discuss the implications of our results on the structure of their inner disks and their estimated ages. In the second project, we study the incidence as a function of age of disks around weak-line T Tauri stars (low-mass PMS stars that are mostly coeval with CTTS but that do not show clear evidence for accretion) and explore the structure of these disks. We estimate the dissipation timescale of the planet-forming region of primordial disks and discuss the implications for planet formation theories. The third and fourth projects deal with the evolution of angular momentum of PMS stars. We search for observational evidence for the connection between stellar rotation and the presence of a disk predicted by the current disk-braking paradigm, according to which the rotational evolution of PMS stars is regulated through magnetic interactions between the stellar magnetosphere and the inner disk. The last project deals with debris disks, which are second-generation disks where the dust is continuously replenished by collisions between planetesimals. We search for debris disks in the far-IR (24-160 [mu]m) around a sample of Hyades Cluster members. We discuss the implications of our results on the evolution of debris disks and on the Late Heavy Bombardment in the Solar System. / text

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Thanks to a range of discovery methods that are sensitive to different regions of parameter space, we now know of over 900 planets in over 700 planetary systems. This large population has allowed exoplanetary scientists to move away from a focus on simple discovery, and towards efforts to study the bigger pictures of planetary system formation and evolution. The interactions between planets and their host stars have proven to be varied in both mechanisms and scope. In particular, tidal interactions seem to affect both the physical and dynamical properties of planetary systems, but characterising the broader implications of this has proven challenging. In this thesis I present work that investigates different aspects of tidal interactions, in order to uncover the scope of their influence of planetary system evolution. I compare two different age calculation methods using a large sample of exoplanet and brown dwarf host stars, and find a tendency for stellar model fitting to supply older age estimates than gyrochronology, the evaluation of a star's age through its rotation (Barnes 2007). Investigating possible sources of this discrepancy suggests that angular momentum exchange through the action of tidal forces might be the cause. I then select two systems from my sample, and investigate the effect of tidal interactions on their planetary orbits and stellar spin using a forward integration scheme. By fitting the resulting evolutionary tracks to the observed eccentricity, semi-major axis and stellar rotation rate, and to the stellar age derived from isochronal fitting, I am able to place constraints on tidal dissipation in these systems. I find that the majority of evolutionary histories consistent with my results imply that the stars have been spun up through tidal interactions as the planets spiral towards their Roche limits. I also consider the influence of tidal interactions on the alignment between planetary orbits and stellar spin, presenting new measurements of the projected spin-orbit alignment angle, λ, for six hot Jupiters. I consider my results in the context of the full ensemble of measurements, and find that they support a previously identified trend in alignment angle with tidal timescale, implying that tidal realignment might be responsible for patterns observed in the λ distribution.

Stars form via the gravitational collapse of molecular clouds during which time the protostellar object contracts by over seven orders of magnitude. If all the angular momentum present in the natal cloud was conserved during collapse, stars would approach rotational velocities rapid enough to tear themselves apart within just a few Myr. In contrast to this, observations of pre-main sequence rotation rates are relatively slow (∼ 1 − 15 days) indicating that significant quantities of angular momentum must be removed from the star. I use observations of fully convective pre-main sequence stars in two well-studied, nearby regions of star formation (namely the Orion Nebula Cluster and Taurus-Auriga) to determine the removal rate of stellar angular momentum. I find the accretion disc-hosting stars to be rotating at a slower rate and contain less specific angular momentum than the disc-less stars. I interpret this as indicating a period of accretion disc-regulated angular momentum evolution followed by near-constant rotational evolution following disc dispersal. Furthermore, assuming that the age spread inferred from the Hertzsprung-Russell diagram constructed for the star forming region is real, I find that the removal rate of angular momentum during the accretion-disc hosting phase to be more rapid than that expected from simple disc-locking theory whereby contraction occurs at a fixed rotation period. This indicates a more efficient process of angular momentum removal must operate, most likely in the form of an accretion-driven stellar wind or outflow emanating from the star-disc interaction. The initial circumstellar envelope that surrounds a protostellar object during the earliest stages of star formation is rotationally flattened into a disc as the star contracts. An effective viscosity, present within the disc, enables the disc to evolve: mass accretes inwards through the disc and onto the star while momentum migrates outwards, forcing the outer regions of the disc to expand. I used spatially resolved submillimetre detections of the dust and gas components of protoplanetary discs, gathered from the literature, to measure the radial extent of discs around low-mass pre-main sequence stars of ∼ 1−10 Myr and probe their viscous evolution. I find no clear observational evidence for the radial expansion of the dust component. However, I find tentative evidence for the expansion ofthe gas component. This suggests that the evolution of the gas and dust components of protoplanetary discs are likely governed by different astrophysical processes. Observations of jets and outflows emanating from protostars and pre-main sequence stars highlight that it may also be possible to remove angular momentum from the circumstellar material. Using the sample of spatially resolved protoplanetary discs, I find no evidence for angular momentum removal during disc evolution. I also use the spatially resolved debris discs from the Submillimetre Common-User Bolometer Array-2 Observations of Nearby Stars survey to constrain the amount of angular momentum retained within planetary systems. This sample is compared to the protoplanetary disc angular momenta and to the angular momentum contained within pre-stellar cores. I find that significant quantities of angular momentum must be removed during disc formation and disc dispersal. This likely occurs via magnetic braking during the formation of the disc, via the launching of a disc or photo-evaporative wind, and/or via ejection of planetary material following dynamical interactions.