In this thesis, we examine the role of accretion disks in the formation of stellar systems, focusing on young massive disks which regulate the flow of material from the parent molecular core down to the star. We study the evolution of disks with high infall rates that develop strong gravitational instabilities. We begin in chapter 1 with a review of the observations and theory which underpin models for the earliest phases of star formation and provide a brief review of basic accretion disk physics, and the numerical methods which we employ. In chapter 2 we outline the current models of binary and multiple star formation, and review their successes and shortcomings from a theoretical and observational perspective. In chapter 3 we begin with a relatively simple analytic model for disks around young, very massive stars, showing that instability in these disks may be responsible for the higher multiplicity fraction of massive stars, and perhaps the upper mass to which they grow. We extend these models in chapter 4 to explore the properties of disks and the formation of binary companions across a broad range of stellar masses. In particular, we model the role of global and local mechanisms for angular momentum transport in regulating the relative masses of disks and stars. We follow the evolution of these disks throughout the main accretion phase of the system, and predict the trajectory of disks through parameter space. We follow up on the predictions made in our analytic models with a series of high resolution, global numerical experiments in chapter 5. Here we propose and test a new parameterization for describing rapidly accreting, gravitationally unstable disks. We find that disk properties and system multiplicity can be mapped out well in this parameter space. Finally, in chapter 6, we address whether our studies of unstable disks are relevant to recently detected massive planets on wide orbits around their central stars.

abstract: Understanding the temperature structure of protoplanetary disks (PPDs) is paramount to modeling disk evolution and future planet formation. PPDs around T Tauri stars have two primary heating sources, protostellar irradiation, which depends on the flaring of the disk, and accretional heating as viscous coupling between annuli dissipate energy. I have written a "1.5-D" radiative transfer code to calculate disk temperatures assuming hydrostatic and radiative equilibrium. The model solves for the temperature at all locations simultaneously using Rybicki's method, converges rapidly at high optical depth, and retains full frequency dependence. The likely cause of accretional heating in PPDs is the magnetorotational instability (MRI), which acts where gas ionization is sufficiently high for gas to couple to the magnetic field. This will occur in surface layers of the disk, leaving the interior portions of the disk inactive ("dead zone"). I calculate temperatures in PPDs undergoing such "layered accretion." Since the accretional heating is concentrated far from the midplane, temperatures in the disk's interior are lower than in PPDs modeled with vertically uniform accretion. The method is used to study for the first time disks evolving via the magnetorotational instability, which operates primarily in surface layers. I find that temperatures in layered accretion disks do not significantly differ from those of "passive disks," where no accretional heating exists. Emergent spectra are insensitive to active layer thickness, making it difficult to observationally identify disks undergoing layered vs. uniform accretion. I also calculate the ionization chemistry in PPDs, using an ionization network including multiple charge states of dust grains. Combined with a criterion for the onset of the MRI, I calculate where the MRI can be initiated and the extent of dead zones in PPDs. After accounting for feedback between temperature and active layer thickness, I find the surface density of the actively accreting layers falls rapidly with distance from the protostar, leading to a net outward flow of mass from ~0.1 to 3 AU. The clearing out of the innermost zones is possibly consistent with the observed behavior of recently discovered "transition disks." / Dissertation/Thesis / Ph.D. Physics 2012

abstract: The lowest-mass stars, known as M-dwarfs, form target samples for upcoming exoplanet searches, and together with lower-mass substellar objects known as brown dwarfs, are among prime targets for detailed study with high-contrast adaptive optics (AO) imaging and sub-millimeter interferometry. In this thesis, I describe results from three studies investigating the companion properties and environments of low-mass systems: (1) The 245-star M-dwarfs in Multiples (MinMs) Survey, a volume-limited survey of field M-dwarf companions within 15 pc, (2) the Taurus Boundary of Stellar/Substellar (TBOSS) Survey, an ongoing study of disk properties for low-mass members within the Taurus star-forming region, and (3) spectroscopy of a brown dwarf companion using the Gemini Planet Imager (GPI). Direct imaging of M-dwarfs is a sensitive technique to identify low-mass companions over a wide range of orbital separation, and the high proper motion of nearby M-dwarfs eases confirmation of new multiple stars. Combining AO and wide-field imaging, the MinMs Survey provides new measurements of the companion star fraction (CSF), separation distribution, and mass ratio distribution for the nearest K7-M6 dwarfs. These results demonstrate the closer orbital separations (~6 AU) and lower frequency (~23% CSF) of M-dwarf binaries relative to higher-mass stars. From the TBOSS project, I report 885µm Atacama Large Millimeter/sub-millimeter Array continuum measurements for 24 Taurus members spanning the stellar/substellar boundary (M4-M7.75). Observations of submillimeter emission from dust grains around the lowest-mass hosts show decreasing disk dust mass for decreasing host star mass, consistent with low frequencies of giant planets around M-dwarfs. Compared to the older stellar association of Upper Scorpius, Taurus disks have a factor of four higher mass in submillimeter-sized grains. From the GPI Exoplanet Survey, I describe near-infrared spectroscopy of an unusually red companion orbiting inside the debris disk of an F5V star. As the second brown dwarf discovered within the innermost region of a debris disk, the properties of this system offer important dynamical constraints for companion-disk interaction and a useful benchmark for brown dwarf and giant planet atmospheric study. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2017

HD 100453AB is a 10 +/- 2 Myr old binary whose protoplanetary disk was recently revealed to host a global two-armed spiral structure. Given the relatively small projected separation of the binary (1.'' 05, or similar to 108 au), gravitational perturbations by the binary seemed to be a likely driving force behind the formation of the spiral arms. However, the orbit of these stars remained poorly understood, which prevented a proper treatment of the dynamical influence of the companion on the disk. We observed HD. 100453AB between 2015 and 2017, utilizing extreme adaptive optics systems on the Very Large Telescope and the Magellan Clay Telescope. We combined the astrometry from these observations with published data to constrain the parameters of the binary's orbit to a = 1.'' 06 +/- 0.'' 09, e = 0.17 +/- 0.07, and i = 32 degrees.5 +/- 6 degrees.5. We utilized publicly available ALMA (CO)-C-12 data to constrain the inclination of the disk, i(disk) similar to 28 degrees, which is relatively coplanar with the orbit of the companion and consistent with previous estimates from scattered light images. Finally, we input these constraints into hydrodynamic and radiative transfer simulations to model the structural evolution of the disk. We find that the spiral structure and truncation of the circumprimary disk in HD 100453 are consistent with a companion-driven origin. Furthermore, we find that the primary star's rotation, its outer disk, and the companion exhibit roughly the same direction of angular momentum, and thus the system likely formed from the same parent body of material.

We present Magellan adaptive optics Ha imaging of HH 508, which has the highest surface brightness among protostellar jets in the Orion Nebula. We find that HH 508 actually has a shorter component to the west, and a longer and knotty component to the east. The east component has a kink at 0.'' 3 from the jet-driving star theta(1) Ori B-2, so it may have been deflected by the wind/radiation from the nearby theta(1) Ori B1B5. The origin of both components is unclear, but if each of them is a separate jet, then theta(1) Ori B-2 may be a tight binary. Alternatively, HH 508 may be a slow-moving outflow, and each component represents an illuminated cavity wall. The ionization front surrounding theta(1) Ori B2B3 does not directly face theta(1) Ori B1B5, suggesting that the EUV radiation from theta(1) Ori C plays a dominant role in affecting the morphology of proplyds even in the vicinity of theta(1) Ori B1B5. Finally, we report an Ha blob that might be ejected by the binary proplyd LV 1.

In this paper, we investigate the relation between disk mass and mass accretion rate to constrain the mechanism of angular momentum transport in protoplanetary disks. We find a correlation between dust disk mass and mass accretion rate in Chamaeleon I with a slope that is close to linear, similar to the one recently identified in Lupus. We investigate the effect of stellar mass and find that the intrinsic scatter around the best-fit M-dust-M star and M-acc-M star relations is uncorrelated. We simulate synthetic observations of an ensemble of evolving disks using a Monte Carlo approach and find that disks with a constant alpha viscosity can fit the observed relations between dust mass, mass accretion rate, and stellar mass but overpredict the strength of the correlation between disk mass and mass accretion rate when using standard initial conditions. We find two possible solutions. In the first one, the observed scatter in M-dust and M-acc is not primordial, but arises from additional physical processes or uncertainties in estimating the disk gas mass. Most likely grain growth and radial drift affect the observable dust mass, while variability on large timescales affects the mass accretion rates. In the second scenario, the observed scatter is primordial, but disks have not evolved substantially at the age of Lupus and Chamaeleon I owing to a low viscosity or a large initial disk radius. More accurate estimates of the disk mass and gas disk sizes in a large sample of protoplanetary disks, through either direct observations of the gas or spatially resolved multiwavelength observations of the dust with ALMA, are needed to discriminate between both scenarios or to constrain alternative angular momentum transport mechanisms such as MHD disk winds.

The evolution of protoplanetary disks is believed to be driven largely by angular momentum transport resulting from magnetized disk winds and turbulent viscosity. The ionization of the disk that is essential for these processes has been thought to be due to host star coronal X-rays but could also arise from energetic particles produced by coronal flares, or traveling shock waves, and advected by the stellar wind. We have performed test-particle numerical simulations of energetic protons propagating into a realistic T. Tauri stellar wind, including a superposed small-scale magnetostatic turbulence. The isotropic (Kolmogorov power spectrum) turbulent component is synthesized along the individual particle trajectories. We have investigated the energy range [0.1-10] GeV, consistent with expectations from Chandra X-ray observations of large flares on T. Tauri stars and recent indications by the Herschel Space Observatory of a significant contribution of energetic particles to the disk ionization of young stars. In contrast with a previous theoretical study finding a dominance of energetic particles over X-rays in the ionization throughout the disk, we find that the disk ionization is likely dominated by X-rays over much of its area, except within narrow regions where particles are channeled onto the disk by the strongly tangled and turbulent magnetic field. The radial thickness of such regions is 5 stellar radii close to the star and broadens with increasing radial distance. This likely continues out to large distances from the star (10 au or greater), where particles can be copiously advected and diffused by the turbulent wind.

We present observations of the Herbig Ae star HD 169142 with the VLT/SPHERE instruments InfraRed Dual-band Imager and Spectrograph (IRDIS) (K1K2 and H2H3 bands) and the Integral Field Spectrograph (IFS) (Y, J and H bands). We detect several bright blobs at similar to 180 mas separation from the star, and a faint arc-like structure in the IFS data. Our reference differential imaging (RDI) data analysis also finds a bright ring at the same separation. We show, using a simulation based on polarized light data, that these blobs are actually part of the ring at 180 mas. These results demonstrate that the earlier detections of blobs in the H and K-S bands at these separations in Biller et al. as potential planet/substellar companions are actually tracing a bright ring with a Keplerian motion. Moreover, we detect in the images an additional bright structure at similar to 93 mas separation and position angle of 355 degrees, at a location very close to previous detections. It appears point-like in the YJ and K bands but is more extended in the H band. We also marginally detect an inner ring in the RDI data at similar to 100 mas. Follow-up observations are necessary to confirm the detection and the nature of this source and structure.

Depuis la découverte de la première planète orbitant une étoile de la séquence principale autre que le Soleil en 1995, ce champ de recherche a connu une croissance vertigineuse, tant au niveau des observations, que des modèles théoriques développés en parallèle. Même si la formation et l’évolution des systèmes planétaires restent encore mal comprises dans leur globalité, Il est à peu près certain que les planètes se forment dans des disques protoplanétaires et interagissent avec ces derniers durant la phase primordiale de leur évolution. Cette thèse s’attache à décrire certains aspects de ces interactions. Parmi les problèmes soulevés par les nombreuses observations d’exoplanètes, on peut citer l’existence des Jupiter chaudes, géantes gazeuses dont la révolution autour de leur étoile s’effectue en quelques jours à peine. Il est communément admis qu’elles se sont formées dans les parties externes du disque, pour ensuite migrer vers l’intérieur. Cependant , les processus de migration restent encore débattus. On pourra aussi noter qu’un nombre important de planètes détectées, notamment par la méthode des vitesses radiales, présentent de fortes excentricités. Cette observation contraste avec celle de notre propre Système Solaire, où les planètes géantes ont des orbites quasi-circulaires. Cette distribution d’excentricités témoigne probablement d’une certaine richesse dans les interactions dynamiques entre les planètes d’un même système. Un autre résultat majeur des quelques dernières années est l’observation de planètes à faible période orbitale dont l’orbite n’est pas alignée avec l’axe de rotation de leur étoile. Cette observation pourrait potentiellement remettre en question l’idée selon laquelle ces planètes acquièrent leur faible période par le biais de la migration au sein du disque. Par conséquent, il est important de pouvoir différencier quelles sont les caractéristiques observationelles des exoplanètes qui sont le fruit de leurs interactions mutuelles, et celles qui peuvent être expliquées lors de la phase d’interaction avec le disque protoplanétaire. D’une part, cela permet d’imposer des contraintes sur la physique des disques protoplanétaires. D’autre part, il est intéressant de savoir à quoi ressemble le système de planètes une fois que le disque se dissipe, et à quelles conditions intiales peut-on s’attendre lorsque les planètes commencent à interagir entre elles sans la présence du disque. Par exemple, est-il possible pour une ou des planètes d’acquérir de l’excentricité et de l’inclinaison au sein du disque, et de les maintenir par la suite. De plus, il est certain que le disque domine l’évolution des planètes au stage primordial de leur vie, mais jusqu’à quel point cela limite-t-il les interactions entre les planètes ? / Since the discovery of the first planet orbiting a main-sequence star outside the solar system in 1995, the field of exoplanet studies has grown rapidly, both from the observational and theoretical sides. Despite the fact that we are still lacking a global picture for the formation and evolution of planetary systems, it is now commonly accepted that planets form in protoplanetary discs and interact with them in the early stages of their evolution. This thesis aims at studying some of these interactions. The observations of extrasolar planets have brought several puzzling results to the attention of the community. One of them is the existence of hot Jupiters, giant gaseous planets which orbit their parent star with a period of a few days only. The commonly accepted scenario is that they formed in the outer parts of the disc and migrated inward. Furthermore, a significant number of planets detected so far, especially by the method of radial velocities, have high eccentricities. This is in contrast with our own solar system where giant planets have quasi-circular orbits. Such a distribution of eccentricities may be the signature of strong dynamical interactions between the different components of a same planetary system. Finally, there are short-period planets whose orbits is misaligned with the axis of rotation of their host star, which could possibly argue against the smooth migration of planets in their disc. Therefore, it is important to disentangle between the orbital characteristics that planets acquired through mutual dynamical interactions, and the ones they acquired when they interacted with the disc. Firstly, it gives constraints on the physical parameters of protoplanetary discs. Secondly, it is interesting to know the properties of the system of planets after the disc has dissipated, and what sort of initial conditions one can expect when planets start to interact freely one with each other. For instance, one can ask if it is possible for planets to reach large eccentricities and inclinations when the disc was still present, and whether they could maintain them or not.

La zone morte, région laminaire confinée au coeur des disques protoplanétaires dont la turbulence de l'écoulement à petite échelle explique l'accrétion de matière sur l'étoile en formation, semble être un lieu propice à la formation planétaire. En effet, au bord interne de la zone morte la différence d'accrétion entraîne le développement d'une sur-densité capable de piéger les grains de poussière qui dérivent vers l'étoile. L'écoulement à cet endroit est de plus potentiellement instable. Le cas échéant, il s'organise en structures tourbillonnaires appelées ''vortex'' qui collectent efficacement la poussière. La position du bord interne est toutefois très incertaine et dépend en particulier de la thermodynamique du modèle de disque considéré. Récemment, le déplacement du bord interne a été envisagé pour expliquer la variabilité de l'accrétion des étoiles jeunes. Cette thèse aborde le problème posé par l'influence de la thermodynamique sur la dynamique du bord interne de la zone morte. Des simulations MHD qui incluent le couplage entre les processus thermodynamiques avec la dynamique de l'écoulement ont tout d'abord permis de confirmer le comportement dynamique du bord interne ainsi que de réaliser la mesure inédite de sa vitesse typique de déplacement. La comparaison de ces résultats avec les prédictions données par un modèle de champ moyen a révélé le rôle du transport d'énergie par des ondes excitées au bord interne de la zone morte. Ces simulations présentent de plus un phénomène nouveau: les vortex formés à l'interface suivent un cycle de formation-migration-destruction. Cette découverte est susceptible de modifier notre vision du scénario de formation planétaire. En résumé, cette thèse met en évidence le fait que les processus thermodynamiques sont au coeur du fonctionnement de la région du bord interne de la zone morte dans les disques protoplanétaires. / The dead zone, a quiescent region enclosed in the turbulent flow of a protoplanetary disk, seems to be a promising site for planet formation. Indeed, the development of a density maximum at the dead zone inner edge, that has the property to trap the infalling dust, is a natural outcome of the accretion mismatch at this interface. Moreover, the flow here may be unstable and organize itself into vortical structures that efficiently collect dust grains. The inner edge location is however loosely constrained. In particular, it depends on the thermodynamical prescriptions of the disk model that is considered. It has been recently proposed that the inner edge is not static and that the variations of young stars accretion luminosity are the signature of this interface displacements. This thesis address the question of the impact of the gas thermodynamics onto its dynamics around the dead zone inner edge. MHD simulations including the complex interplay between thermodynamical processes and the dynamics confirmed the dynamical behaviour of the inner edge. A first measure of the interface velocity has been realised. This result has been compared to the predictions of a mean field model. It revealed the crucial role of the energy transport by density waves excited at the interface. These simulations also exhibit a new intriguing phenomenon: vortices forming at the interface follow a cycle of formation-migration-destruction. This vortex cycle may compromise the formation of planetesimals at the inner edge. This thesis claims that thermodynamical processes are at the heart of how the region around the dead zone inner edge in protoplanetary disks works.