Circumstellar disks are critical structures in the star and planet formation processes, as they provide a conduit to channel material onto the central object and supply a reservoir of dust and gas to form planets. This thesis focuses on the far-infrared, and sub-millimeter observations of circumstellar disks at two key evolutionary phases; pri- mordial proto-planetary disks, and evolved debris disks – remnants of a system that has undergone a degree of planet formation. Four individual studies of circumstellar disks are presented in this thesis. The results of a 97% complete census of far-infrared emission measured with the Herschel Space Observatory, targeting stars of spectral types M4 and later, in the Taurus molecular cloud are presented. This census is the first large-scale survey sensitive to emission across the stellar and sub-stellar boundary. Results from an initial test grid of model spectral energy distributions, generated with the radiative transfer code MCFOST, show that 73% of the observed Class II population are constrained by canonical disks that are viewed from face-on to edge-on inclinations. Sub-millimeter observations with the Caltech Sub-millimeter Observatory are presented for an association of young T-Tauri stars in the Aquila star-forming region. The results of disk frequency and disk mass of this complete census are investigated in this extremely low stellar density environment. A sub-millimeter investigation for two populations of candidate debris disk; warm and cold excess disks is presented. None of the candidate disks were detected in the sub-millimeter despite upper-limits below that expected, based on blackbody model fits to excesses at shorter wavelengths. Several scenarios are investigated in order to identify the null detection rate, such as stellar multiplicity and background-source contamination. Finally, a partially resolved sub-millimeter map of the debris disk around the HR 8799 multiple planetary system is presented. The planet formation history of the system is investigated through the witnessed morphology of the emission.

Using multi-wavelength and multi-epoch observations we examine the evolution of circumstellar disks around pre-main sequence stars from massive, optically thick flared disks to wispy debris disks. We examine a young cluster of nearby stars, identifying likely members and studying dust properties using 3-24μm photometry and accretion rates using optical spectroscopy. We find that 79% of the stars have disks and that almost all of the stars with disks are actively accreting. The stars that show evidence for evolution in their dust properties also exhibit a decrease in the accretion activity suggesting that the evolution of the dust and gas is closely connected. Focusing on a sub-sample of transition disks we study the source of recently discovered infrared variability and whether it can be used to further our understanding of disk structure. We are particularly interested in sources that show a ’seesaw’ behavior in their SED in which the short wavelength infrared flux increases while the long wavelength flux decreases causing the SED to pivot about one wavelength. We develop simple geometric models of disks with nonaxisymmetric structure and find that the precession of this structure is not able to reproduce the strength or the wavelength dependence of observed infrared variability while a model with an inner warp whose scale height rapidly varies is much more successful. We follow this up with detailed observations covering a wide range of wavelengths from optical to mid-infrared of six transition disks in order to better understand the physical source of the variability. We find that the variability is consistent with a variable scale height of the inner disk, finding direct evidence for this effect in two transition disks. Contemporaneous measures of the infrared flux and the accretion rate find in some cases a correlation between these two properties, although in none of our stars is it likely that the accretion rate variability is the source of the infrared variability. The most likely cause is either a companion embedded in the disk or a dynamic interface between the stellar magnetic field and the disk.

This thesis presents a multiwavelength analysis, from the infrared to the microwave, of the young, forming stars in the Auriga-California Molecular Cloud and a first look at the disks they host and their potential for forming planetary systems. At the beginning of this thesis, Auriga-Cal had only recently been identified as one contiguous cloud with its distance placing it within the Gould Belt of nearby star-forming regions (Lada et al. 2009). This thesis presents the largest body of work to date on Auriga-Cal's star formation and disk population. Auriga-Cal is one of two nearby giant molecular clouds (GMCs) in the Gould Belt, the other being the Orion A molecular cloud. These two GMCs have similar mass (~10^5 Msolar), spatial scale (~80 pc), distance (~450 pc), and filamentary morphology, yet the two clouds present very different star formation qualities and quantities. Namely, Auriga-Cal is forming far fewer stars and does not exhibit the high-mass star formation seen in Orion A. In this thesis, I present a census of the star forming objects in the infrared with the Spitzer Space Telescope showing that Auriga-Cal contains at least 166 young stellar objects (YSOs), 15-20x fewer stars than Orion A, the majority of which are located in the cluster around LkHalpha 101, NGC 1529, and the filament extending from it. I find the submillimetre census with the James Clerk Maxwell Telescope, sensitive to the youngest objects, arrives at a similar result showing the disparity between the two clouds observed in the infrared continues to the submillimetre. Therefore the relative star formation rate between the two clouds has remained constant in recent times. The final chapter introduces the first study targeted at the disk population to measure the formation potential of planetary systems around the young stars in Auriga-Cal. The dust thermal emission at cm wavelengths is observed to measure the relative amounts of cm-sized grains, indicative of the grain growth processes that take place in disks and are necessary for planet formation. For a subsample of our targets, we are able to measure the spectral slope in the cm to confirm the thermal nature of the observed emission that we detect and characterize the signature of grain growth. The sensitivity of our observations probes masses greater than the minimum mass solar nebula (MMSN), the disk mass required to form the Solar System. We detect 19 disks, representing almost a third of our sample, comparable to the numbers of disks in other nearby star-forming regions with disks masses exceeding the MMSN, suggesting that the disk population in Auriga-Cal possesses similar planet formation potential as populations in other clouds. Confirmation of this result requires future observations with mm interferometry, the wavelength regime where the majority of statistics of disks has been measured. / Graduate

Be stars are rapidly rotating B-type stars that eject large amounts of material into a circumstellar disk. Evidence of the presence of a disk is found through hydrogen emission lines in their spectra, IR excess flux, and linear intrinsic polarization. In this dissertation, we report the first simultaneous interferometric and spectroscopic observations of circumstellar disks around 24 bright Be stars made using the techniques of long baseline interferometry and moderate resolution spectroscopy in the near infrared. The goal of the project is to characterize the fundamental geometrical and physical properties of the emitting regions that are responsible for the IR flux excesses detected in the K-band in our sample stars. This observational work has been conducted with both the Center for High Angular Resolution Astronomy (CHARA) Array at Mount Wilson Observatory, and the Mimir spectrograph at Lowell Observatory. The visibility measurements were interpreted with different geometrical and physical disk models in order to determine the spatial extension of the disk, the inclination angle, the position angle, and the density profile of the disk. We find that the spatial extension of the circumstellar disk in the K-band is only about a few stellar radii, and that the density structure of the disk is consistent with a radially decreasing function with a density exponent that ranges between 2.5 and 3.5. The resulting disk densities are in a good agreement with those derived from the Infrared Astronomical Satellite (IRAS) measurements, and the resulting disk geometries are consistent with previous polarimetric measurements. We find that the K-band sizes of the emitting regions in the disk are smaller by a factor of two than the Hα sizes, and we show that this is due to the lower opacity of the continuum in the disk. By combining recent measurements of the projected rotational velocities with the disk inclination angles derived from interferometry, we were able to estimate the actual equatorial linear rotational velocities of the Be stars in our sample. The obtained linear rotational velocities indicate that Be stars are rapid rotators with an equatorial velocity that is about 0.7 - 0.9 of their critical velocities.

Planets are thought to form in circumstellar disks, leaving behind planetesimals that collide to produce dusty debris disks. Characterizing the architectures of planetary systems, along with the structures and compositions of debris disks, can therefore help answer questions about how planets form. In this thesis, I present the results of five papers (three published, two in preparation) concerning the properties of extrasolar planetary systems and their circumstellar environments. Chapters 2 and 3 are studies of radial velocity (RV) exoplanetary systems. For years astronomers have been puzzled about the large number of RV-detected planets that have eccentric orbits (e>0.1). In Chapter 2 I show that this problem can partially be explained by showing that two circular-orbit planets can masquerade as a single planet on an eccentric orbit. I use this finding to predict that planets with mildly eccentric orbits are the most likely to have massive companions on wide orbits, potentially detectable by future direct imaging observations. Chapter 3 presents such a direct imaging study of the 14 Her planetary system. I significantly constrain the phase space of the putative candidate 14 Her c and demonstrate the power of direct imaging/RV overlap. Chapters 4 and 5 are high-contrast 2-4 μm imaging studies of the edge-on debris disks around HD 15115 and HD 32297. HD 15115's color is found to be gray, implying large grains 1-10 μm in size reside in stable orbits in the disk. HD 32297's disk color is red from 1-4 μm. Cometary material (carbon, silicates, and porous water ice) are a good match at 1-2 μm but not at L'. Tholins, organic material that is found in outer solar system bodies, or small silicates can explain the disk's red color but not the short wavelength data. Chapter 6 presents a dynamical study of dust grains in the presence of massive planets. I show that the width of a debris disk increases proportionally with the mass of its shepherding planet. I then make predictions for the masses and orbits of putative planets in five well-known disks. In Chapter 7, I summarize and discuss plans for future research in the exoplanet field.

Debris disks are a fundamental component of exoplanetary systems. Understanding their relationship with host stars and neighboring planets can help contextualize the evolution of exoplanetary systems. In order to further that goal, this thesis addresses some extreme outlier examples of debris disk systems. First, the highly asymmetric debris disk around HD 111520 is resolved and analyzed at multiple wavelengths to create a self-consistent model of the disk thermal emission and scattered light. The best-fit model is proposed to be an asymmetric disk from a recent collision of large, icy bodies on one side of the disk. In contrast, most debris disks are thought to be in a steady collisional cascade and this disk model could represent a relatively rare event in the creation of debris disks. Secondly, an optical spectroscopic survey of stars is conducted on stars where far-infrared observations exist to detect the presence of debris disks. Specifically, AF-type stars are targeted in order to provide context regarding the Lambda Boo phenomenon, where stars are found to be specifically refractory metal-poor. One mechanism for this was hypothesized to be from planetary scattering of debris disks, causing the accretion of volatiles from comets. The findings were that over the entire unbiased sample, stars which were refractory metal poor tended to be the stars with brightest debris disks. This supports a planet-disk hypothesis underlying the accretion of volatile gases, since debris disks undergoing active planetary stirring are brighter. This would mean about 13\% of stars with debris disk are undergoing strong planetary scattering based on the occurrence rate of Lambda Boo stars relative to debris disk stars. These two tacks in our observational understanding of these extreme examples of debris disks provide constraints on the volatility at work. / Graduate

Estrelas Be são reconhecidas pela sua rápida rotação e pulsação não radial. São as únicas estrelas da Sequência Principal que apresentam discos circunstelares, os quais são formados por meio de processos ainda não completamente compreendidos. A modelagem das forças que atuam neste sistema conduz a previsões teóricas sobre a estrutura do disco que podem ser comparadas com dados observacionais. Podemos estudar as propriedades físicas dos discos de estrelas Be pelo efeito que a luz estelar sofre ao passar por eles, por exemplo, modelando a transferência radiativa. Neste ponto, a polarização surge como uma ferramenta muito útil para a investigação destes discos, permitindo a determinação de quantidades físicas importantes do sistema, como a densidade numérica de partículas e o ângulo de inclinação. Uma variabilidade intrigante observada em estrelas Be é a transição aperiódica entre uma fase B normal (sem disco) e uma fase Be (com disco). Estudos de monitoramento recentes encontraram, a partir da análise da polarização intrínseca decorrente da transição entre estas fases, uma relação significante entre a mudança da polarização através do salto de Balmer versus a polarização na banda V, fazendo surgir uma estrutura em loop como função do tempo, no assim denominado Diagrama Cor-Polarização. Neste trabalho, apresentamos uma análise do Diagrama Cor-Polarização por meio de modelos diversos. Fazemos uso do Disco de Decréscimo Viscoso que é o paradigma atual para explicar a formação e evolução dos discos de estrelas Be. Com isso, visamos determinar como a polarimetria pode contribuir para a compreensão dos mecanismos fundamentais envolvidos no processo de formação e dissipação do disco. / Be stars are recognized by their rapid rotation and non-radial pulsation. They are the only stars in the Main Sequence that have circumstellar disks that are formed by processes not yet fully understood. The modeling of the forces acting on this system leads to theoretical predictions about the structure of the disk that can be compared to observational data. We can study physical the properties of Be disks by modeling how stellar light is reprocessed by them. This requires solving the detailed radiative transfer problem involved. In this point, the study of polarization arise as a useful tool to investigate these disks, allowing for the determination of important physical quantities of the system, such as the particle number density and inclination angle. An intriguing variability observed in Be stars is the aperiodic transition between a B normal phase (without disk) to a Be phase (with disk). Recent monitoring studies found, from the analysis of the intrinsic polarization arising of the transition between these phases, a significant relation between the polarization change through the Balmer jump versus the V-Band polarization, giving rise to a loop structure as a function of time, in the so-called Color-Polarization Diagram. This work presents an analysis of the Color-Polarization Diagram by several models. We make use of the Viscous Decretion Disk Model, which assumes the existence of some injection mechanism of material at keplerian velocities in the disk base, where the turbulent viscosity acts carrying angular momentum from de inner parts to the outer regions. With this, we aimed to extend our knowledge about the fundamental mechanisms involved in the formation and dissipation processes of the disk.

This thesis is a computational study of circumstellar gas disks, with a special focus on modeling techniques and on numerical methods not only as scientific tools but also as a target of study. In particular, in-depth discussions are included on the main numerical strategy used, namely the moving-mesh method for astrophysical hydrodynamics. In this work, the moving-mesh approach is used to simulate circumstellar disks for the first time. / Astronomy

What processes are responsible for the dispersal of protoplanetary disks? In this dissertation, beginning with a brief Introduction to planet detection, disk dispersal and high-contrast imaging in Chapter 1, I will describe how ground-based adaptive optics (AO) imaging can help to inform these processes. Chapter 2 presents Polarized Differential Imaging (PDI) of the transitional disk SR21 at H-band taken as part of the Strategic Exploration of Exoplanets and Disks with Subaru (SEEDS). These observations were the first to show that transition disk cavities can appear markedly different at different wavelengths. The observation that the sub-mm cavity is absent in NIR scattered light is consistent with grain filtration at a planet-induced gap edge. Chapter 3 presents SEEDS data of the transition disk Oph IRS 48. This highly asymmetrical disk is also most consistent with a planet-induced clearing mechanism. In particular, the images reveal both the disk cavity and a spiral arm/divot that had not been imaged previously. This study demonstrates the power of multiwavelength PDI imaging to verify disk structure and to probe azimuthal variation in grain properties. Chapter 4 presents Magellan visible light adaptive optics imaging of the silhouette disk Orion 218-354. In addition to its technical merits, these observations reveal the surprising fact that this very young disk is optically thin at H-alpha. The simplest explanation for this observation is that significant grain growth has occurred in this disk, which may be responsible for the pre-transitional nature of its SED. Chapter 5 presents brief descriptions of several other works-in-progress that build on my previous work. These include the MagAO Giant Accreting Protoplanet Survey (GAPlanetS), which will probe the inner regions of transition disks at unprecedented resolution in search of young planets in the process of formation. Chapters 6-8 represent my educational research in quantitative literacy, beginning with an introduction to the literature and study motivation in Chapter 6. Chapter 7 describes the development and validation of the Quantitative Reasoning for College Science (QuaRCS) Assessment instrument. Chapter 8 briefly describes the next steps for Phase II of the QuaRCS study.

We present Herschel observations of the debris disk around γ Doradus (HD 27290, HIP 19893) from the Herschel Key Programme DEBRIS (Disc Emission via Bias-free Reconnaissance in the Infrared/Submillimetre). The disk is well-resolved with PACS at 70, 100 and 160 micron and detected with SPIRE at 250 and 350 micron. The 250 micron image is only resolved along the disk's long axis. The SPIRE 500 micron 3 σ detection includes a nearby background source. γ Dor's spectral energy distribution (SED) is sampled in the submillimetre for the first time and modelled with multiple modified-blackbody functions to account for its broad shape. Two approaches are used, both of which reproduce the SED in the same way: a model of two narrow dust rings and a model of an extended, wide dust belt. The former implies the dust rings have temperatures of ~90 and ~40 K, corresponding to blackbody radii of 25 and 135 AU, respectively. The latter model suggests the dust lies in a wide belt extending from 15 to 230 AU. The resolved images, however, show dust extending beyond ~350 AU. This is consistent with other debris disks whose actual radii are observed to be a factor of 2 - 3 times larger than the blackbody radii. Although it is impossible to determine a preferred model from the SED alone, the resolved images suggest that the dust is located in a smooth continuous belt rather than discrete narrow rings. Both models estimate that the dust mass is 6.7 x 10^{-3} Earth masses and that fractional luminosity is 2.5 x 10^{-5}. This amount of dust is within the levels expected from steady state evolution given the age of γ Dor and therefore a transient event is not needed to explain the dust mass. No asymmetries that would hint at a planetary body are evident in the disk at Herschel's resolution. However, the constraints placed on the dust's location suggest that the most likely region to find planets is within 20 AU of the star. / Graduate