We perform a spectroscopic survey of the foreground population in Orion. A with MMT/Hectospec. We use these data, along with archival spectroscopic data and photometric data, to derive spectral types, extinction values, and masses for 691 stars. Using the Spitzer Space Telescope data, we characterize the disk properties of these sources. We identify 37 new transition disk (TD) objects, 1 globally depleted disk candidate, and 7 probable young debris disks. We discover an object with a mass of. less than 0.018-0.030 M-circle dot, which harbors a flaring disk. Using the Ha emission line, we characterize the accretion activity of the sources with disks, and confirm that the. fraction of accreting TDs is lower than that of optically thick disks (46% +/- 7% versus 73% +/- 9%, respectively). Using kinematic data from the Sloan Digital Sky Survey and APOGEE INfrared Spectroscopy of the Young Nebulous Clusters program (IN-SYNC), we confirm that the foreground population shows similar kinematics to their local molecular clouds and other young stars in the same regions. Using the isochronal ages, we find that the foreground population has a median age of. around 1-2 Myr, which is similar to that of other young stars in Orion. A. Therefore, our results argue against the presence of a large and old foreground cluster in front of Orion. A.

We have obtained new images of the protoplanetary disk orbiting TW Hya in visible, total intensity light with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), using the newly commissioned BAR5 occulter. These HST/STIS observations achieved an inner working angle of similar to 0."2, or 11.7 au, probing the system at angular radii coincident with recent images of the disk obtained by ALMA and in polarized intensity near-infrared light. By comparing our new STIS images to those taken with STIS in 2000 and with NICMOS in 1998, 2004, and 2005, we demonstrate that TW Hya's azimuthal surface brightness asymmetry moves coherently in position angle. Between 50 au and 141 au we measure a constant angular velocity in the azimuthal brightness asymmetry of 22 degrees.7. 7 yr(-1) in a counterclockwise direction, equivalent to a period of 15.9. yr assuming circular motion. Both the (short) inferred period and lack of radial dependence of the moving shadow pattern are inconsistent with Keplerian rotation at these disk radii. We hypothesize that the asymmetry arises from the fact that the disk interior to 1 au is inclined and precessing owing to a planetary companion, thus partially shadowing the outer disk. Further monitoring of this and other shadows on protoplanetary disks potentially opens a new avenue for indirectly observing the sites of planet formation.

The disk mass is among the most important input parameter for every planet formation model to determine the number and masses of the planets that can form. We present an ALMA 887 mu m survey of the disk population around objects from similar to 2 to 0.03 M-circle dot in the nearby similar to 2 Myr old Chamaeleon I star-forming region. We detect thermal dust emission from 66 out of 93 disks, spatially resolve 34 of them, and identify two disks with large dust cavities of about 45 au in radius. Assuming isothermal and optically thin emission, we convert the 887 mu m flux densities into dust disk masses, hereafter M-dust. We find that the M-dust-M* relation is steeper than linear and of the form M-dust proportional to (M*)(1.3-1.9), where the range in the power-law index reflects two extremes of the possible relation between the average dust temperature and stellar luminosity. By reanalyzing all millimeter data available for nearby regions in a self-consistent way, we show that the 1-3 Myr old regions of Taurus, Lupus, and Chamaeleon. I share the same M-dust-M* relation, while the 10 Myr old Upper. Sco association has a steeper relation. Theoretical models of grain growth, drift, and fragmentation reproduce this trend and suggest that disks are in the fragmentation-limited regime. In this regime millimeter grains will be located closer in around lower-mass stars, a prediction that can be tested with deeper and higher spatial resolution ALMA observations.

Theoretical models and spectroscopic observations of newborn stars suggest that protoplantary disks have an inner "wall" at a distance set by the disk interaction with the star. Around T Tauri stars, the size of this disk hole is expected to be on a 0.1 au scale that is unresolved by current adaptive optics imaging, though some model-dependent constraints have been obtained by near-infrared interferometry. Here we report the first measurement of the inner disk wall around a solar-mass young stellar object, YLW 16B in the rho Ophiuchi star-forming region, by detecting the light-travel time of the variable radiation from the stellar surface to the disk. Consistent time lags were detected on two nights, when the time series in H (1.6 mu m) and K (2.2 mu m) bands were synchronized while the 4.5 mu m emission lagged by 74.5 +/- 3.2 s. Considering the nearly edge-on geometry of the disk, the inner rim should be 0.084 au from the protostar on average, with an error of order 0.01 au. This size is likely larger than the range of magnetospheric truncations and consistent with an optically and geometrically thick disk front at the dust sublimation radius at similar to 1500 K. The widths of the cross-correlation functions between the data in different wavebands place possible new constraints on the geometry of the disk.

Low mass, self-gravitating accretion disks admit quasi-steady, "gravito-turbulent" states in which cooling balances turbulent viscous heating. However, numerical simulations show that gravito-turbulence cannot be sustained beyond dynamical timescales when the cooling rate or corresponding turbulent viscosity is too large. The result is disk fragmentation. We motivate and quantify an interpretation of disk fragmentation as the inability to maintain gravito-turbulence due to formal secondary instabilities driven by: (1) cooling, which reduces pressure support; and/or (2) viscosity, which reduces rotational support. We analyze the axisymmetric gravitational stability of viscous, non-adiabatic accretion disks with internal heating, external irradiation, and cooling in the shearing box approximation. We consider parameterized cooling functions in 2D and 3D disks, as well as radiative diffusion in 3D. We show that generally there is no critical cooling rate/viscosity below which the disk is formally stable, although interesting limits appear for unstable modes with lengthscales on the order of the disk thickness. We apply this new linear theory to protoplanetary disks subject to gravito-turbulence modeled as an effective viscosity, and cooling regulated by dust opacity. We find that viscosity renders the disk beyond similar to 60 au dynamically unstable on radial lengthscales a few times the local disk thickness. This is coincident with the empirical condition for disk fragmentation based on a maximum sustainable stress. We suggest turbulent stresses can play an active role in realistic disk fragmentation by removing rotational stabilization against self-gravity, and that the observed transition in behavior from gravito-turbulent to fragmenting may reflect instability of the gravito-turbulent state itself.

We present the discovery of seven new proplyds (i.e., sources surrounded by cometary H alpha emission characteristic of offset ionization fronts (IFs)) in NGC 1977, located about 30' north of the Orion Nebula Cluster (ONC) at a distance of similar to 400 pc. Each of these proplyds is situated at projected distances 0.04-0.27 pc from the B1V star 42 Orionis (c Ori), which is the main source of UV photons in the region. In all cases the IFs of the proplyds are clearly pointing toward the common ionizing source, 42 Ori, and six of the seven proplyds clearly show tails pointing away from it. These are the first proplyds to be found around a B star, with previously known examples instead being located around O stars, including those in the ONC around theta(1) Ori C. The radii of the offset IFs in our proplyds are between similar to 200 and 550 au; two objects also contain clearly resolved central sources that we associate with disks of radii 50-70 au. The estimated strength of the FUV radiation field impinging on the proplyds is around 10-30 times less than that incident on the classic proplyds in the ONC. We show that the observed proplyd sizes are however consistent with recent models for FUV photoevaporation in relatively weak FUV radiation fields.

We present Gemini Planet Imager polarized intensity imagery of HD 100453 in Y, J, and K1 bands that reveals an inner gap (9-18 au), an outer disk (18-39 au) with two prominent spiral arms, and two azimuthally localized dark features that are also present in Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) total intensity images. Spectral energy distribution fitting further suggests that the radial gap extends to 1 au. The narrow, wedge-like shape of the dark features appears similar to predictions of shadows cast by an inner disk that is misaligned with respect to the outer disk. Using the Monte Carlo radiative transfer code HOCHUNCK3D, we construct a model of the disk that allows us to determine its physical properties in more detail. From the angular separation of the features, we measure the difference in inclination between the disks (45 degrees) and their major axes, PA = 140 degrees east of north for the outer disk, and 100 degrees for the inner disk. We find an outer-disk inclination of 25 degrees +/- 10 degrees from face-on, in broad agreement with the Wagner et al. measurement of 34 degrees. SPHERE data in J and H bands indicate a reddish disk, which indicates that HD 100453 is evolving into a young debris disk.

La formation des planètes a lieu dans les disques protoplanétaires constitués de gaz et de poussières. Si ces dernières ne représentent que 1% de la masse totale du disque, elles jouent un rôle fondamental pour l’évolution chimique des disques en agissant comme catalyseurs pour la formation des molécules. Comprendre cette chimie est essentiel pour remonter aux conditions physiques initiales qui ont permis la naissance des planètes.Au cours de ma thèse, j’ai étudié la chimie à la surface des grains de poussières et son impact sur l’évolution chimique du nuage moléculaire, condition initale de la formation du disque, et du disque protoplanétaire. Grâce à des simulations numériques, à l’aide du code de chimie gaz-grain Nautilus, j’ai pu montrer l’importance des réactions de diffusion et des interactions gaz-grain pour les abondances des espèces en phase gazeuse. Les résultats du modèle couplés aux observations ont également mis en évidence les effets de la structure physique (température, densité, AV) sur la distribution des molécules dans les disques. / Planetary formation occurs in the protoplanetary disks of gas and dust. Although dust represents only 1% of the total disk mass, it plays a fundamental role in disk chemical evolution since it acts as a catalyst for the formation of molecules. Understanding this chemistry is therefore essential to determine the initial conditions from which planets form.During my thesis, I studied grain-surface chemistry and its impact on the chemical evolution of molecular cloud, initial condition for disk formation, and protoplanetary disk. Thanks to numerical simulations, using the gas-grain code Nautilus, I showed the importance of diffusion reactions and gas-grain interactions for the abundances of gas-phase species. Model results combined with observations also showed the effects of the physical structure (in temperature, density, AV) on the molecular distribution in disks.

This thesis presents a study of the sub-light year regions of Active Galactic Nuclei (AGN). These environments contain accretion discs that orbit a central super-massive black hole. The luminosity of the AGN inner regions varies over time across all wavelengths with variability at longer wavelengths lagging behind that at shorter wavelengths. Since the AGN themselves are too remote and too compact to resolve directly, I exploit these time lags to infer the physical characteristics of the accretion disc and surrounding gas clouds that emit broad emission lines. These characteristics include the inclination and temperature profile of the accretion disc, and the shape (or light curve) of the luminosity fluctuations that drive the accretion disc variability. This thesis details the work in the first author papers of Starkey et al. (2016, 2017), in which I detail the statistical code, CREAM (Continuum REverberting AGN Markov Chain Monte Carlo), that I developed to analyse AGN accretion disc variability. I apply the code to a set of AGN light curve observations of the Seyfert 1 galaxy NGC 5548 by the AGN STORM collaboration (De Rosa et al., 2015; Edelson et al., 2015; Fausnaugh et al., 2016a; Goad et al., 2016; Starkey et al., 2017). I also present work detailing my variability analysis of the Seyfert galaxies NGC 6814, NGC 2617, MCG 08-11-11 and NGC 4151. This work has contributed to the analysis presented in (Troyer et al. 2016, Fausnaugh et al. submitted). I also investigate the implications of a twin accretion disc structure (Nealon et al., 2015) on the disc time lag measurements across near UV and optical wavelengths. I finish by detailing a modification to CREAM that allows it to merge continuum light curves observed in a common filter, but taken by multiple telescopes with different calibration and instrumental effects to consider.

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