I have carried out three-dimensional numerical simulations of self-gravitating discs to determine under what circumstances they fragment to form bound clumps that may grow into giant planets. Through radiation hydrodynamical simulations using a Smoothed Particle Hydrodynamics code, I find that the disc opacity plays a vital role in determining whether a disc fragments. Specifically, opacities that are smaller than interstellar Rosseland mean values promote fragmentation (even at small radii, R < 25AU) since low opacities allow a disc to cool quickly. This may occur if a disc has a low metallicity or if grain growth has occurred. Given that the standard core accretion model is less likely to form planets in a low metallicity environment, I predict that gravitational instability is the dominant planet formation mechanism in a low metallicity environment. In addition, I find that the presence of stellar irradiation generally acts to inhibit fragmentation (since the discs can only cool to the temperature defined by stellar irradiation). However, fragmentation may occur if the irradiation is sufficiently weak that it allows the disc to attain a low Toomre stability parameter. With specific reference to the HR 8799 planetary system, I find that it is only possible for fragments to form in the radial range where the HR 8799 planets are located (approximately 24-68 AU) if the disc is massive. In such a high mass regime, mass transport occurs in the disc causing the surface mass density to alter. Therefore, fragmentation is not only affected by the disc temperature and cooling, but also by any restructuring due to the gravitational torques. The high mass discs also pose a problem for the formation of this system because the protoplanets accrete from the disc and end up with masses greater than those inferred from observation and thus, the growth of planets would need to be inhibited. In addition, I find that further subsequent fragmentation at small radii also takes place. By way of analytical arguments in combination with hydrodynamical simulations using a parameterised cooling method, I explore the fragmentation criteria which in the past, has placed emphasis on the cooling timescale in units of the orbital timescale, beta. I find that at a given radius the surface mass density (i.e. disc mass and profile) and star mass also play a crucial role in determining whether a disc fragments or not as well as where in the disc fragments form. I find that for shallow surface mass density profiles (p<2, where the surface mass density is proportional to R^{-p}), fragments form in the outer regions of the disc. However for steep surface mass density profiles (p is greater than or similar to 2), fragments form in the inner regions of a disc. In addition, I find that the critical value of the cooling timescale in units of the orbital timescale, beta_crit, found in previous simulations is only applicable to certain disc surface mass density profiles and for particular disc radii and is not a general rule for all discs. I obtain an empirical fragmentation criteria between the cooling timescale in units of the orbital timescale, beta, the surface mass density, the star mass and the radius. Finally, I carry out crucial resolution testing by performing the highest resolution disc simulations to date. My results cast some serious doubts on previous conclusions concerning fragmentation of self-gravitating discs.

My current work revels in a state of flux. I strive for the work to be electrically charged, conveying a feverish sense of immediacy and vitality that implies motion and frenetic energy. The work is an accretion of brightly colored biomorphic forms that extend out from the wall and onto the floor. Viscous parts ooze and drip while others are globular and bulbous. The hyper-organic forms suggest a paradoxical state of both ripening and rotting, becoming and unbecoming. The work is an attempt to traverse between seemingly divergent constructs, some of which include: growth and decay, the artificial and the natural, the body and the landscape, the infinite and the miniscule, and the real and the imagined. I intend for the works to be suggestive of mutation, of systems becoming cross-wired and melting into each other.

La gravitation joue un rôle important dans de nombreux domaines de l'astrophysique : elle assure notamment la cohésion et la stabilité des planètes, des étoiles et des disques. Elle est aussi motrice dans le processus d'effondrement de structure et conduit, dès lors qu'un moment cinétique initial est significatif, à la formation d'un disque.Ma thèse est consacrée à l'étude des disques de gaz, et plus particulièrement à la description du potentiel et du champ de gravité qu'ils génèrent dans l'espace et sur eux-mêmes (l'auto-gravitation). Bien que la force de Newton soit connue depuis longtemps, la détermination des interactions auto-gravitantes reste difficile, en particulier lorsque l'on s'écarte significativement de la sphéricité. La principale difficulté tient dans la divergence hyperbolique du Noyau de Green 1/(r'-r) et nécessite un traitement propre. L'approche théorique est intéressante car elle fournit de nouveaux outils (techniques numériques, formules approchées, etc...) qui peuvent aider à produire des solutions de référence et à améliorer les simulations numériques.Dans une première partie, nous introduisons le sujet, les notions et les bases essentielles. Le chapitre $1$ est consacré à une présentation succinte du contexte scientifique et aux motivations de notre travail. Dans le chapitre $2$, nous reproduisons dans ces grandes lignes le cheminement conduisant au développement multipolaire, à partir de l'équation de Poisson et de la formule intégrale de Newton. Il s'agit de l'une des méthodes les plus classiques permettant d'obtenir le potentiel gravitationnel d'un corps. Les deux systèmes de coordonnées les plus utilisées sont mis en avant : sphériques et cylindriques. A travers quelques exemples, nous montrons les limites de cette approche, en particulier dans le cas de l'auto-gravité des disques.Dans une deuxième partie, nous abordons le vif du sujet. Le chapitre $3$ présente l'approche basée sur les intégrales elliptiques que nous retrouverons dans l'ensemble du manuscrit (cas général d'abord, puis cas axi-symétrique). Dans le chapitre $4$, nous établissons un premier résultat concernant le noyau de Green dans des systèmes axi-symétriques et verticalement homogènes : une forme alternative et régulière du noyau, quelque soit le point de l'espace. Nous avons exploité cette nouvelle formule pour déduire une bonne approximation du potentiel des disques géométriquement minces, des anneaux et des systèmes faiblement étendus en rayon. Ceci fait l'objet du chapitre $5$.Dans une troisième partie, nous étudions les effets de bords sur la composante verticale du champ de gravité, $g_z$, causés par un disque mince axi-symétrique. Le chapitre $6$ est dédié à l'approximation de Paczynski \citep{pacz78}, qui permet traditionnellement d'exprimer le champ comme une fonction linéaire de la densité de surface locale. Cette approximation n'est en fait strictement valide que dans le cas du modèle du "plan infini", loin d'un disque réaliste. Près du bord externe des disques où la gravité décroit, l'approximation de Paczynski s'avère assez imprécise (facteur $2$ typiquement), et ne donne pas de bons résultats et doit être corrigée. Toujours dans l'hypothèse d'une homogénéité verticale de la densité, nous avons construit une expression pour $g_z$ qui tient compte de ces effets de bords. Le chapitre $7$ est consacré à ce résultat.Dans une dernière partie, nous relâchons l'hypothèse de symétrie axiale (le disque est discrétisé en cellules cylindriques homogènes). Nous nous sommes inspirés du travail d'\cite{ansorg03} afin d'exprimer, via le théorème de Green, le potentiel d'une cellule cylindrique homogène par une intégrale de contour. Ce résultat s'applique directement aux simulations de disques, où ceux-ci sont découpés en cellules cylindriques, chacune ayant sa propre densité.Une conclusion et quelques perspectives sont données en fin de manuscrit. / Gravitation plays an important role in many fields in astrophysics: it appears in the cohesion and stability of bodies such as planets, stars, disks and galaxies. In the Universe, the formation of most astrophysical objects involves disk-like configurations by a main process: the gravitational collapse. The structure and the evolution of these disks (protoplanetary disks, circumplanetary disks...), are an important stage in the process of the formation of stars, planets or satellites. It is therefore fundamental to understand their physics and develop appropriate tools. I devoted my Ph.D. to the computation of the gravitational potential and field of astrophysical disks. Although Newton's force is known for long, the determination of self-gravitating interactions inside bodies remains a difficult task. Strong deviations to sphericity require more efforts. The main difficulty is to manage properly the hyperbolic divergence of the Green kernel $\frac{1}{|r-r'|}$. In this purpose, the theoretical approach is interesting as it can provide powerful formulae and new tools, which can also help to produce reference solutions. So, I have investigated new methods able to treat this question as rigorously as possible.In a first part, chapter $1$ is devoted to the scientifc context and motivations. In the chapter $2$ we derive the well known multipole expansion in spherical and cylindrical coordinates from the Poisson equation and Newton's equation. We show the limits of these two developments in the context of astrophysical disks. In chapter $3$, we discuss the formalism based on elliptic integrals, its advantages and drawbacks, and we describe two methods which use this approach in the special case of axisymmetrical disks.In the second part, chapter $4$ is about the discovery of an alternate formula for the Green kernel, which involves regilar function. To obtain this result, we assume that the disk is vertically homogeneous (i.e., the density varies only with the radius), and that it is axially symetric. In chapter $5$, by using this new expression, we build an approximation for the potential in the special case of geometrically thin disks and rings, and another one for systems which are radially confined.In the third part, chapter $6$ is devoted to the study of edge effects on the vertical component of the gravitational field caused by a thin disk. According to Paczynski's approximation, the field is a linear function of the surface density \cite{pacz78}. This approximation is strictly valid only in the infinite slab model, while we are interested in a realistic disk. Close to the outer edges, where gravity decreases, Paczynski's approximation fails and must be corrected. By assuming again a density varying with the radius only, we have derived a new expression for the vertical component of gravitational field, which properly accounts of the presence of the edge of the disk. This is the main subject of the chapter $7$. In the last part (chapter $8$), we generalize the work by \cite{ansorg03}, valid under axial symmetry only. Using a similar approach, we built an expression for the self-gravitating potential of cylindrical cells, which is not known in closed form yet. This expression is made of a single integral over the boundary of the cell. This result can be applied in hydrodynamical simulations, where disks are usually discretised into homogeneous cylindrical cells, each cell having its own density.A conclusion and a few perspectives end the thesis.

Photometry of eclipsing dwarf-nova EX Draconis was performed at the observatory of Astronomical Institute of the Academy of Sciences of the Czech Republic in Ondřejov and at the Astronomical Observatory on Kolonické sedlo during 63 nights. I calculated times of minimum light by two methods - the mirror method and the derivative method. The mirror method is more precise for these measurements with longer exposure time and smaller coverage of the light curve of eclipse. 53 new times of minima calculated by the mirror method and times of minima obtained from the older articles about EX Dra were included in the O-C diagram and fitted by the sine function and theoretical curve of LITE caused by an unseen third body. Period of cyclic changes for the sine function is 25 years, instead of 4 or 5 years period given in older papers.Period of the third body orbit is aproximately 17 years and its minimum mass is 53.5 Jupiter's mass. The sum of squared deviations is 5 times smaller for the LITE, which means that the LITE ilustrates the O-C diagram better. I determined the average outburst period for three observational seasons and I drew the phase curve of outburst. Finally, I calculated the short period of the light changes outside eclipse.

All massive nearby galaxies, including our own, host supermassive black holes. Active galactic nuclei (AGN) are seen when such black holes accrete, and when they produce powerful jets of synchrotron-emitting plasma, they are termed radio-loud AGN. The close correlation between black hole mass and galaxy bulge mass in elliptical galaxies indicates that AGN feedback may be the key to the regulation of galaxy formation. It is thus necessary to fully understand the structure of AGN, the way that they are fuelled, and their duty cycle, in order to study the feedback processes and get a clear picture of galaxy formation. In this thesis, independent methods are developed to constrain the accretion disk and radio jet angles to the line of sight. H IX emission from a sub-sample of high-redshift quasars is measured from near-infrared spectroscopy and modelled as sums of different components, including the characteristic double-peaked profile which results from a thin, rotating accretion disk. Comparing the models using Bayesian evidence, almost all quasars were found to have infrared spectra consistent with the presence of a disk. The jet inclination angles of the same set of quasars were constrained by fitting a model, including the effect of Doppler boosting and the receding torus model for dust obscuration, to the radio \ spectral energy distribution. The fitted disk and jet angles correlate strongly, and are consistent with a model in which the radio jets are launched orthogonally to the plane of the accretion disk, as expected if the jet is powered by energy drawn from the spin of the black hole. Both disk and jet angles correlate with the observed linear source size, which is a projection effect; when deprojected using the fitted angles, the distribution of source sizes agrees with a scenario in which the sources expand into the surrounding medium at a constant rate up to ~ 1 Mpc and then shut off, probably as the nuclei become quiescent. The accretion disk angle was found to correlate weakly with the low-frequency radio luminosity, which provides direct, albeit tenuous, evidence for the receding torus model.

Nesta dissertação discutimos o processo da acreção de materia sobre objetos compactos em suas diferentes abordagens. Iniciando com o caso clássico, estudamos sua contraparte relativística e, por fim, investigamos a acreção de fluidos cosmológicos (energia escura e matéria escura) em buracos negros. Devido a simetria esférica adotada, a formação dos chamados discos de acréscimo é proibida (tanto no caso clássico quanto no relativístico) e, portanto, os problemas relacionados com a física dos discos (sua formação e evolução) não foram investigados. No contexto clássico, analisamos inicialmente a chamada acreçao de Bondi, onde o fluido acretado obedece a uma equação de estado politropica e o processo de acreção é descrito pela hidrodinâmica euleriana. A existância de 6 tipos possíveis de solucões para o campo de velocidades é identicada e suas consequências fsicas são discutidas em detalhe. Apenas uma dessas soluções descreve de forma fisicamente consistente o processo de acreção. A taxa de materia acretada é constante, um resultado esperado devido a hipotese de regime estacionário. O estudo do caso relativstico é completamente baseado na Teoria da Relatividade Geral, com o campo gravitacional do corpo central sendo descrito pela metrica de Schwarzschild. O processo relativstico também ocorre sob condições estacionárias e, portanto, a taxa de acreção resultante também é constante. Uma atenção especial foi dedicada para a acreção de fluidos cosmologicos satisfazendo uma equação de estado linear e tambem para o chamado gás de Chaplygin. Estudamos separadamente o comportamento espacial do fluido na região dominada pela acreção e também a influência da evolução cosmologica nas regiões mais distantes. Mostramos que a massa do buraco negro central pode apresentar uma evolução no tempo em escala cosmológica. Os resultados de Babichev (caso linear) e o gás de Chaplygin foram unicadamente descritos através de uma equacão de estado generalizada. Por fim, determinamos também sob que condições a acreção de matéria pode provocar mudancas significativas na massa do buraco negro. / In this dissertation the matter accretion process upon compact objects is discussed in its diferent approaches. Starting with the classical case, the relativistic type was studied and, in the end, the accretion of cosmological fluids (dark energy and dark matter) onto a black hole is investigated. Due to spherical symmetry adopted, the formation of accretion disks is forbidden (both in the classical and relativistic case) and, thus, the problems related to disk physics (the formation and evolution) were not investigated. On the classical approach, the so called Bondi accretion is examined, in which the matter flux occurs according to a polytropic equation of state and the accretion itself is described by the Eulerian hydrodynamic. The existence of 6 possible families of solutions for the velocity field is identied and its physical consequences are thoroughly discussed. Only one of these solutions describes the accretion process in a physically consistent manner. The mass accretion rate is found to be constant, as expected duo to the steady-state hypothesis. The relativistic approach is completely based on the General Relativity Theory. In this case, the gravitational field of the central body is described by the Schwarzschild metric. The relativistic process also occurs in steady-state conditions and, therefore, the accretion flux also is constant. A particular interest is given to the accretion of cosmological fluids with a linear equation of state and of Chaplygin gas. Both the spacial behaviour of the fluids in the accretion dominated region and their cosmological evolution in farther regions are looked into individually. The mass of the central black hole\'s evolution is shown to occur in cosmological times. The Babichev (linear equation of state) and Chaplygin results were unied through a generalised equation of state. At last, it is also determined under which conditions the accretion of cosmological fluids can have astonishing effects on the black hole\'s mass.

In this dissertation, I use seismic imaging and waveform modeling methods to investigate melt migration processes and the structure of the magma plumbing system beneath the East Pacific Rise (EPR) and Endeavour segment of the Juan de Fuca Ridge, respectively. This work begins by studying shallow mantle reflections beneath the EPR. I find the amplitude versus offset and waveform characteristics of the reflections to be consistent with a sub-horizontal dunite channels located up to 20 km off-axis. The depth of the dunite channels correlate with patterns of mantle melt delivery and the predicted base of the thermal lithosphere, suggesting the channels are thermally controlled and may have formed in situ via dissolution by focused flow at the base of the lithosphere. This interpretation is consistent with field observations in ophiolites and numerical modeling of melt-focusing channels. The three-dimensional velocity structure of the Endeavour segment is then investigated to identify how patterns of mantle melt delivery influence the segment-scale distribution of crustal melt and crustal accretion. The results from this study indicate that the mantle magmatic system is skewed relative to the ridge-tracking crustal magmatic system and that this skew exerts primary control on magmatic, tectonic, and hydrothermal activity at the Endeavour segment. In regions where mantle melt delivery is axis-centered, mantle-derived melts are efficiently transported from the mantle to the crust, resulting in frequent crustal melt replenishment, associated seismogenic cracking, and enhanced crustal melt content that drives vigorous hydrothermal activity. Conversely, sites of off-axis melt delivery are characterized by less efficient vertical melt transport, resulting in infrequent crustal melt injection and hence, reduced crustal melt content and hydrothermal activity. Next, I focus on how along-axis variations in magma replenishment modulate crustal permeability and the intensity of hydrothermal circulation. Using full-waveform inversion, I show that sites of localized magma replenishment to the axial magma lens, along with induced seismogenic cracking, coincide with enhanced permeability. I conclude that the frequency of magma injection governs hydrothermal circulation patterns and heat flux at mid-ocean ridges. This dissertation includes previously published and unpublished coauthored material.

Protoplanetary discs, composed of gas and dust, usually surround young stellar objects and serve two main purposes: they determine the accretion of matter onto the central object and also represent sites of planet formation. The accretion proceeds through the transport of angular momentum outwards allowing the disc matter to fall towards the centre. A mechanism responsible for the transport can be turbulence, waves or other coherent structures originating from various instabilities in discs that could, in addition, play a role in the planet formation process. For an understanding of these instabilities, it is necessary to study perturbation dynamics in differentially rotating, or sheared media. Thus, this thesis focuses on new aspects in the perturbation dynamics in non-magnetised protoplanetary discs that arise due to their self-gravity and velocity shear associated with the disc’s differential rotation. The analysis is carried out in the framework of the widely employed local shearing box approximation. We start with 2D discs and then move on to 3D ones. In 2D discs, there are two basic perturbation types/modes – spiral density waves and vortices – that are responsible for angular momentum transport and that can also contribute to accelerating planet formation. First, in the linear regime, we demonstrate that the vortical mode undergoes large growth due to self-gravity and in this process generates density waves via shear-induced linear mode coupling phenomenon. This is noteworthy, because commonly only density waves are considered in self-gravitating discs. Then we investigate vortex dynamics in the non-linear regime under the influence of self-gravity by means of numerical simulations. It is shown that vortices are no longer well-organised and long-lived structures, unlike those occurring in non-self-gravitating discs. They undergo recurring phases (lasting for a few disc rotation periods) of formation, growth and eventual destruction. We also discuss the dust trapping capability of such transient vortices. Perturbation dynamics in 3D vertically stratified discs is richer, as there are more mode types. We first consider non-axisymmetric modes in non-self-gravitating discs and then only axisymmetric modes in the more complicated case when self-gravity is present. Specifically, in non-self-gravitating discs with superadiabatic vertical stratification, motivated by the recent results on the transport properties of incompressible convection, we show that when compressibility is taken into account, the non-axisymmetric convective mode excites density waves via the same shear-induced linear mode coupling mechanism mentioned above. These generated density waves transport angular momentum outwards in the trailing phase, and we suggest that they may aid and enhance the transport due solely to convection in the non-linear regime, where the latter becomes outward. In the final part of the thesis, we carry out a linear analysis of axisymmetric vertical normal modes in stratified self-gravitating discs. Although axisymmetric modes do not display shear-induced couplings, their analysis provides insight into how gravitational instabilities develop in the 3D case and their onset criterion. We examine how the structure of dispersion curves and eigenfunctions of 3D modes are influenced by self-gravity, which mode first becomes gravitationally unstable and thus determines the onset criterion and nature of the gravitational instability in stratified discs. We also contrast the more exact instability criterion obtained with our 3D model with that of density waves in 2D discs. Based on these findings, we discuss the origin of 3D behaviour of perturbations involving noticeable disc surface distortions, as seen in some numerical simulations of self-gravitating discs.

Previous studies have shown that thermal feedback from protostars plays a key role in the process of low-mass star formation. In this thesis, we explore the effects of protostellar heating on the formation of stellar clusters. We describe new methods for modelling protostellar accretion luminosities and protostellar evolution in calculations of star formation. We then present results of a series of numerical simulations of stellar cluster formation which include these effects, and examine their impact. We begin by investigating the dependence of stellar properties on the initial density of molecular clouds. We find that the dependence of the median stellar mass on the initial density of the cloud is weaker than the dependence of the thermal Jeans mass when radiative effects are included. We suggest that including protostellar accretion luminosities and protostellar evolution may weaken this dependence further, and may account for the observed invariance of the median stellar mass in Galactic star-forming regions. Next, we investigate the effects of including accretion feedback from sink particles on the formation of small stellar groups. We find that including accretion feedback in calculations suppresses fragmentation even further than calculations that only include radiative transfer within the gas. Including feedback also produces a higher median stellar mass, which is insensitive to the sink particle accretion radius used. Finally, we compare calculations of small stellar clusters which model the evolution of protostars using a live stellar model with those which use a fixed stellar structure. We find that the dynamics of the clusters are primarily determined by the accretion luminosities of protostars, but that the relative effects of protostellar evolution depend on the accretion rate and advection of energy into the protostar. We also demonstrate how such calculations may be used to study the properties of young stellar populations.

Radio-loud active galactic nuclei (AGN) are one class of objects associated with accretion activity onto supermassive black holes in centers of massive galaxies. They are believed to be in a radiatively-inefficient accretion mode with low accretion rate. To understand this accretion mode, it is important to measure its radiative output at high energies (> 13.6eV), which can be traced through optical emission lines. However, little is known about their true radiative output. This is because no correlation between optical emission-line and radio luminosity has been found for the majority of low-luminosity radio AGN, which are often classified as low-excitation radio galaxies, or Fanaroff-Riley Class I (FR-I) radio galaxies. We demonstrate that most of the line emission found in these galaxies is not powered by the central AGN, but likely powered by some old stellar population. Only when this component is subtracted or otherwise taken into account can we estimate the true line emission associated with the AGN. These emissions may show interesting correlations with the radio luminosities in some cases.