In the past decade many population studies have been performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to understand the bulk properties of protoplanetary disks around young... Show moreIn the past decade many population studies have been performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to understand the bulk properties of protoplanetary disks around young stars. These population studies mostly consisted of low mass stars, with relatively few of the disks around the more massive Herbig stars, even though the latter are important formation sites of giant exoplanets. This thesis presents the first systematic view of Herbig disks at millimeter wavelengths, and compares these results to the population of disks around lower mass stars. This thesis obtains the dust and gas mass of Herbig disks and their precursors, compares these values to the accretion rates of their stars, computes their vertical structure, and obtains the first complete millimeter study of Herbig disks in a single star-forming region. The results of this thesis have laid an important foundation for future research, especially for obtaining a full sample of the Herbig disk population and for detailed chemical and high-resolution follow-up studies. Show less
This thesis focusses on the temperature structure in protoplanetary disks. The relation between structures seen in the dust and gas-phase molecules is investigated. This is crucial to understand... Show moreThis thesis focusses on the temperature structure in protoplanetary disks. The relation between structures seen in the dust and gas-phase molecules is investigated. This is crucial to understand the chemical composition of the planet forming material as well as to quantify the amount of gas present in the disk, a crucial parameter to determine if planets are likely present in the disk. One of the important regions in the disk is the water snowline, the midplane location where water freezes-out onto the dust grains. In this thesis, chemical modelling is used to infer the snowline location in a hot disk. This result is confirmed in the next chapter by 2D imaging of the water snow surface, the 2D equivalent of the water snowline. Additionally, the temperature structure across transition disk cavities is investigated to determine the mass of the planets that may be carving that cavity. Finally, the relation between the structures traced in the gas by different molecules and the dust is investigated to show that the chemical composition at the location of the dust rings and dust trap in the HD 100546 and OPH-IRS 48 disks, respectively, are different than in the other disk regions. Show less
High-angular-resolution observations of the circumstellar material have uncovered numerous and very diverse substructures in protoplanetary discs, raising the question of whether they are caused by... Show moreHigh-angular-resolution observations of the circumstellar material have uncovered numerous and very diverse substructures in protoplanetary discs, raising the question of whether they are caused by forming planets or other mechanisms. This dissertation focuses on interpreting gas substructures in discs in the context of disc winds and planet-disc interactions. A special focus is put on transition discs, which show dust (and gas) depleted inner regions and represent ideal laboratories to observe planet formation in action and test disc evolution models. Radiative transfer and hydrodynamical models are combined to investigate photoevaporative winds acting in discs in which volatile carbon is reduced. Compared to solar metallicity discs, photoevaporative winds are stronger in such carbon-depleted discs, resulting in higher mass-loss rates and profiles that extend to larger radii. This may explain more of the observed transition disc population. Furthermore, a large number of transition discs are analysed through CO ALMA observations in terms of substructures in the kinematics and brightness temperatures. In particular, two sources, CQ Tau and HD 100546, are studied in detail and the analysis reveals prominent spiral features in both discs. Together with other substructures, these point towards ongoing planet formation. Show less
This thesis takes steps toward understanding the interaction between gas-phase and solid-state molecules in star- and planet-forming regions. Chapter 1 and 2 provide the reader with an introduction... Show moreThis thesis takes steps toward understanding the interaction between gas-phase and solid-state molecules in star- and planet-forming regions. Chapter 1 and 2 provide the reader with an introduction and in-depth description of methods used in subsequent chapters. Chapter 3 and 4 present the spectroscopic infrared characterization of acetaldehyde, dimethyl ether, ethanol, and methyl formate in the solid state, both pure and mixed in astronomically relevant matrices. This characterization will allow for probing of the solid-state organic inventory of star- and planet-forming regions with the upcoming James Webb Space Telescope. Interferometric observations of the protoplanetary disk around TW Hya with the Atacama Large Millimeter/submillimeter Array are presented in Chapter 5. These results hint that the observed gas-phase formaldehyde is formed in the gas phase, contrary to the generally accepted solid-state formation. Chapter 6 provides an insight to the interaction between gas-phase carbon monoxide and solid-state hydroxyl radicals on the surface of vacuum-UV irradiated water ice. Even tough residence times of carbon monoxide are short, they are sufficient to allow reactions with hydroxyl radicals and produce carbon dioxide. This process could explain the lack of gas-phase carbon monoxide in protoplanetary disks and the presence of carbon dioxide mixed in solid-state water. Show less
This thesis focuses on protoplanetary disks: flattened structures of gas and dust around young stars in which planets are expected to form and grow. Physical-chemical models that compute the... Show moreThis thesis focuses on protoplanetary disks: flattened structures of gas and dust around young stars in which planets are expected to form and grow. Physical-chemical models that compute the thermal structure and chemical composition of protoplanetary disks are compared to observations to increase our understanding of the processes that shape these disks.Chapters two and three investigate the sizes of protoplanetary disks in the context of evolution of the dust. A gas disk that is observed to be four times more extended than the dust disk is found to be a clear indication that the dust has drifted inward. Detailed modeling reveals that five out of a sample of 10 disks in the Lupus star-forming region show evidence for dust evolution. Chapter four shows that observed gas outer radii are consistent with disks evolving viscously, assuming disks start out small and evolve slowly. Chapter five reveals that the chemical conversion of CO into more complex species cannot by itself explain the low observed CO isotopolog line fluxes. Finally, Chapter six uses non-detections of the HD emission line to put an upper limit on the total mass of disks and rules out that they are currently gravitationally unstable. Show less
My work focuses on a class of astronomical objects called protoplanetary disks. These flattened structures rotating around young stars are made of gas and dust and are the places where planets,... Show moreMy work focuses on a class of astronomical objects called protoplanetary disks. These flattened structures rotating around young stars are made of gas and dust and are the places where planets, like our own Earth, are formed. One of the main properties needed to explain the process of planet formation is the mass of protoplanetary disks. There is however not yet a consensus on how such masses can be reliably measured from disk observations. In this thesis, I investigate weather less abundant isotopologues of carbon monoxide (CO) are good candidates for tracing disk masses. Initially I tackle the problem from a theoretical point of view by running a grid of physical-chemical disk models. Subsequently I compare my model predictions with recently acquired observations of protoplanetary disks in the Lupus star-forming region. The conclusion of my work is that CO isotopologues are good disk mass tracers, but they need to be calibrated. Observations of other molecules like hydrogen deuteride (HD), atomic carbon and hydrocarbons can serve this cause. Show less
Star formation occurs when a dense cloud of interstellar gas and dust gravitationally collapses. Rotation during this collapse leads naturally to the formation of a flattened circumstellar disk... Show moreStar formation occurs when a dense cloud of interstellar gas and dust gravitationally collapses. Rotation during this collapse leads naturally to the formation of a flattened circumstellar disk around the forming star. These disks are additionally known as protoplanetary disks because the orbiting circumstellar dust and cold gas represent the building blocks for planets. How long this material survives and how it evolves in this time will determine the propensity for (and the diversity of) planetary systems. This thesis is split into three parts that analyze different aspects of disk evolution and the circumstellar environment. In part one, we use observations at millimeter wavelengths to probe (and then model and compare) the dust and gas properties around low-mass Sun-like stars. We conclude that high-resolution spatial and spectral imaging of optically thinner molecular lines will provide the most robust description of the disk structure and evolution using future instrumentation. During these routine observations, we report recurring millimeter flares resulting in part two. We attribute this phenomenon to synchrotron emission from relativistic electrons trapped in the (colliding) magnetospheres of a young binary system. Finally, we present a microgravity experiment to probe the collisional growth mechanism for the first steps of planet formation. Show less
The planets, comets, asteroids... all objects in the Solar system have become from a single disc of matter at the time Sun was a young star. In the recent years it has become possible to take... Show moreThe planets, comets, asteroids... all objects in the Solar system have become from a single disc of matter at the time Sun was a young star. In the recent years it has become possible to take images of such discs elsewhere in our Galaxy, but it soon became clear that each different 'camera' we use tells a different story about the disc. In this thesis, the state-of-the-art telescopes around the globe are used to image the distribution of matter in such discs around young distant stars. We combine different pieces of information together, in a single model, and test this model against images that probe the basic disc properties: its size, mass and shape. These hi-tech snapshots of the childhood of a planetary system show that a disc may extend much further and not be as smooth as previously thought. This thesis concludes that the structure of the disc as a whole cannot be constrained without the interferometric images, and demonstrates that it is essential to combine them with the existing techniques and theory. Show less
