Understanding the formation and evolution of planetary systems is one of the most fundamental challenges in astronomy. To directly image and study young exoplanets and the circumstellar disks they... Show moreUnderstanding the formation and evolution of planetary systems is one of the most fundamental challenges in astronomy. To directly image and study young exoplanets and the circumstellar disks they form from, dedicated high-contrast imaging instruments are built. Several of these instruments have polarimetric modes that are particularly powerful to reach the large contrasts required to directly image these objects as well as to characterize them. This thesis aims to improve the polarimetric sensitivity, accuracy, and capabilities of high-contrast imaging polarimeters for the detection and characterization of exoplanets and circumstellar disks. In addition, this thesis presents the first direct detections of linear polarization from self-luminous planetary mass companions. The focus of this thesis is mostly on ground-based high-contrast imaging, in particular with the instrument SPHERE-IRDIS at the Very Large Telescope. This thesis covers many aspects of high-contrast imaging polarimetry, ranging from theoretical work, calibrations, and the development of new observing techniques to actual scientific polarimetric measurements and astrophysical interpretation. Show less
Circumstellar discs are the reservoirs of gas and dust that surround young stars and have the potential to become planetary systems. Their evolution will determine the time and material available... Show moreCircumstellar discs are the reservoirs of gas and dust that surround young stars and have the potential to become planetary systems. Their evolution will determine the time and material available to form planets. Studying the evolution of circumstellar discs can then help us understand planet formation and the diversity of observed planetary systems. These discs develop almost immediately after star formation, as a direct consequence of the collapse of a molecular cloud and angular momentum conservation. Their surroundings are rich in gas and neighbouring stars, which can be hostile to the discs and affect their evolution in different ways: dynamical encounters with nearby stars can truncate the discs; stellar winds and supernovae explosions can truncate, tilt, or completely destroy the discs; and the presence of bright, massive stars in the vicinity of circumstellar discs can heat their surface enough to evaporate mass from them. This process, known as external photoevaporation, is arguably one of the most important environmental mechanisms in depleting mass from young circumstellar discs. The work performed for this thesis consisted of simulating the early evolution of circumstellar discs in star clusters and the effects of the environment, in particular, truncations due to close encounters and photoevaporation. The results show that photoevaporation is extremely efficient in removing mass from the discs, greatly limiting the amount of material and time available to form planets. Show less
The focus of this thesis is how stars like our Sun and planets like Jupiter, Saturn, and Earth are formed. With arrays of radio telescopes, I observed the environments where the first stages of... Show moreThe focus of this thesis is how stars like our Sun and planets like Jupiter, Saturn, and Earth are formed. With arrays of radio telescopes, I observed the environments where the first stages of star and planet formation occur. This thesis focuses on characterizing different components of young protostellar systems, most notably their jets and disks. Using interferometric radio observations with ALMA array, I provided information on key chemical tracers of different components of the protostellar systems. By characterizing the radio signal from young stars with ALMA and VLA interferometers, I was able to disentangle an emission from the jet and the disk. This led to an unexpected development: I was able to compare dust masses of young disks with those of older disks for the first time. By comparing this information with masses of the extrasolar planets detected so far I showed that the solid cores of gas giants must form in the first 0.1 Myr of stellar life. That is an important time constrain, that pushes the onset of planet formation earlier and highlights the importance of characterization of the youngest protostars in understanding the origin of Solar System and Earth. Show less
Mulders, G.D.; Drążkowska, J.; Marel, N. van der; Ciesla, F.J.; Pascucci, I. 2021
The work presented in this thesis is based on ALMA surveys of protoplanetary disks in three star-forming regions: Lupus, OMC-2, and NGC 2024. The motivation for this thesis is to study the... Show moreThe work presented in this thesis is based on ALMA surveys of protoplanetary disks in three star-forming regions: Lupus, OMC-2, and NGC 2024. The motivation for this thesis is to study the evolution of protoplanetary disks from the population level.The first two chapters focus on the Lupus clouds, a low-mass star-forming region. It has been the subject of a large survey with ALMA, targeting bright gas lines and the emission from millimeter-sized grains. This allows us to answer important questions on disk evolution: how common are >200 AU-sized disks with continuum substructure, and how are these substructures formed? Do compact disks observed in the continuum also correspond to compact gas disks?The chapters focusing on Orion deal with the impact of massive stars on disks. OMC-2 provides a view of a population of disks that are formed in a massive cloud, but isolated from the radiation of massive stars. They link disks that do form near these massive stars to those in low-mass YSOs. NGC 2024 also hosts massive stars, and is the youngest region surveyed; the presence of multiple populations of young stars has been suggested. ALMA allows us to independently test the complexity of this environment. Show less
This thesis examines the link between simple molecules and the underlying structure and chemistry within protoplanetary disks - the birthplaces of planets. The chapters describe the analysis and... Show moreThis thesis examines the link between simple molecules and the underlying structure and chemistry within protoplanetary disks - the birthplaces of planets. The chapters describe the analysis and interpretation of data obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) interferometer, primarily in two disks around the young stars HD 163296 and HD 169142. Observations of dust and molecular gas probe the relationship between the dust structure, the gas distribution, and the chemical processes that give rise to the gaseous species. In the disk around HD 169142, substructure in the millimeter dust and carbon monoxide gas strongly suggests the presence of giant planets sweeping up disk material. Meanwhile, molecular ions reveal previously hidden structure in the gas deep within the disk beyond the millimeter dust edge. In the disk around HD 163296, carbon monoxide and the simple organic molecule formaldehyde show radial variation connected to the millimeter dust edge. The organic molecule methanol is not detected in the disk, suggesting differences in the production of formaldehyde and methanol. This thesis concludes that the distribution of simple molecules is connected to the dust size distribution in disks, while more complex molecules remain elusive but can still provide constraints on disk chemistry. Show less
This thesis addresses the chemical processes that determine the compositions of giant planet atmospheres. Connecting the observed composition of exoplanets to their formation sites often involves... Show moreThis thesis addresses the chemical processes that determine the compositions of giant planet atmospheres. Connecting the observed composition of exoplanets to their formation sites often involves comparing the observed planetary atmospheric carbon-to-oxygen (C/O) ratio to a disk midplane model with a fixed chemical composition. In this scenario chemistry during the planet formation era is not considered, and the C/O ratios of gas and ice in disk midplane are simply defined by volatile icelines in a midplane of fixed chemical composition. However, kinetic chemical evolution during the lifetime of the gaseous disk can change the relative abundances of volatile species, thus altering the C/O ratios of planetary building blocks. In my chemical evolution models I utilize a large network of gas-phase, grain-surface and gas-grain interaction reactions, thus providing a comprehensive treatment of chemistry. In my talk I will show how chemical evolution can modify disk miplane chemistry and how this affects the C/O ratio of giant planet-forming material. I will argue that midplane chemical evolution needs to be addressed when predicting the chemical makeup of planets and their atmospheres. And as an extra, I will propose that chemical evolution can help constrain the formation histories of comets. Show less
To study how planetary systems come into existence we study much younger systems still in formation. Gas and dust rich disks surrounding young stars are thought to be the precursors of... Show moreTo study how planetary systems come into existence we study much younger systems still in formation. Gas and dust rich disks surrounding young stars are thought to be the precursors of planetary systems and therefore known as protoplanetary disks. In this thesis, I study large-scale structures in protoplanetary disks through high-contrast imaging of the scattering surfaces of these disks; and I calibrated two high-contrast imagers. To observe these disks at optical wavelengths, we need to take into account that the central star is much brighter than the (star)light reflected by the disk surface: i.e., high contrast between star and disk. Additionally, light coming from the star & disk is disturbed by the Earth’s atmosphere. Therefore, specialized high-contrast imaging instruments are required to correct for atmospheric disturbance of the stellar light in order to allow the highest possible spatial resolution and contrast between the star and its nearby surroundings. Improving our understanding of these high-contrast imagers will allow for a better interpretation of the data recorded with these instruments, while the interpretation of disk structures detected at high spatial resolution forms a crucial step in our understanding of the general principles that govern disk evolution and planet formation. Show less
This thesis discusses the structure of gas and dust in protoplanetary disks around young stars, in which the planets are formed, using ALMA (Atacama Large Millimeter/submillimeter Array)... Show moreThis thesis discusses the structure of gas and dust in protoplanetary disks around young stars, in which the planets are formed, using ALMA (Atacama Large Millimeter/submillimeter Array) observations. Primary targets of this study are the so-called 'transition disks', with a central cavity in the dust disk. A possible explanation for the presence of this cavity is the recent formation of a young planet which has cleared its own orbit. ALMA can for the first time zoom in onto the structure of both gas and dust and answer this question. The thesis presents the first ALMA observations of cold molecular gas and dust in transition disks. These data show that millimeter-dust grains are concentrated in a 'dust trap', allowing the dust particles to grow to larger sizes, an important step in the planet formation process. Also, it turns out that gas is still present in the dust cavity of the disks in this study, its structure points indeed towards the planet clearing mechanism. These discoveries form a giant leap in our understanding of planet formation. In the coming years, ALMA will be completed and allow us to see even smaller details in these disks, possibly even the planets itself. Show less
With over 1800 exoplanets detected, we now believe the majority of stars is orbited by one or more planets. If we want to understand the observed abundance and diversity of planetary systems, we... Show moreWith over 1800 exoplanets detected, we now believe the majority of stars is orbited by one or more planets. If we want to understand the observed abundance and diversity of planetary systems, we have to understand their formation history. The first step in the planet-formation process is the collisional coagulation of microscopic dust grains into kilometer-size planetesimals (the building blocks of planetary systems). The goal of this thesis has been to achieve a better understanding of the microphysics that govern this early growth. Show less
The different chapters cover studies in which the physical structures of the gas such as temperature, densities and movements of the gas are estimated. In addition chemical characteristics of the... Show moreThe different chapters cover studies in which the physical structures of the gas such as temperature, densities and movements of the gas are estimated. In addition chemical characteristics of the gas such as different molecular abundances and their spatial distribution are defined. This information is discussed in the context of how the chemical evolution of the gas in the planet-forming region progress and how this affects which type of planets that can form there. The results are mainly based on infrared observations and radiative transfer disk models. Show less
Planets form in disks that are commonly found around young stars. The intimate relationship that exists between planet and disk can account for a lot of the exotic extrasolar planetary systems... Show morePlanets form in disks that are commonly found around young stars. The intimate relationship that exists between planet and disk can account for a lot of the exotic extrasolar planetary systems known today. In this thesis we explore disk-planet interaction using numerical hydrodynamical simulations. We study the growth and migration of embedded planets, as well as the condition for gap formation in the disk. These planetary gaps provide an important link to future observations of circumstellar disks. Show less
