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
Stars like our Sun are formed in large, tenuous clouds of gas and dust. As the star is formed at the centre, the remaining material collapses into a thick disk around it. The chemical composition... Show moreStars like our Sun are formed in large, tenuous clouds of gas and dust. As the star is formed at the centre, the remaining material collapses into a thick disk around it. The chemical composition of such a cloud changes dramatically during this process. Spherical models have always been used to model this chemical evolution, but they cannot properly describe the disk. This thesis presents the first model that follows the entire chemical evolution from a pre-stellar core to a circumstellar disk in two spatial dimensions. It follows material as it falls in from the cloud to the star and disk. The density, temperature and UV flux along these trajectories serve as input for a gas-phase chemical network -- including freeze-out onto and evaporation from cold dust grains. The model offers new insights into the chemical history of disks, in particular of the region where planets and comets are formed. Applications of the model include the gas/ice ratios of carbon monoxide and water (Chapter 2), the abundances of key gas-phase molecules (Chapter 3), the crystallinity of the dust (Chapter 4), the isotope-specific photodissociation of carbon monoxide (Chapter 5) and the charge balance of polycyclic aromatic hydrocarbons (PAHs; Chapter 6). Show less
We present Submillimeter-Array (SMA) observations of two embedded young stellar objects, Elias 29 and IRS 63. The masses of the central stars, the discs, and the envelopes are determined for such... Show moreWe present Submillimeter-Array (SMA) observations of two embedded young stellar objects, Elias 29 and IRS 63. The masses of the central stars, the discs, and the envelopes are determined for such young objects for the first time. A survey of bright T-Tauri stars in the southern constellations Lupus and Chamaeleon was carried out with the Australia Telescope Compact Array (ATCA) at 3 mm. The Lupus sources were subsequently observed with the SMA at 1 mm. Comparison with the 10-micron silicate features of seven of the sources suggested a correlation between the infrared and radio data. A large survey of T-Tauri stars was carried out with the SMA, the ATCA, the Combined Array for Research in Millimeter-wave Astronomy (CARMA), and the Very Large Array (VLA). The correlation between the infrared and radio data was confirmed, suggesting that planet formation in discs is more uniform then heretofore thought. Three bright T-Tauri stars were followed up with the ATCA at 7 mm, 1.6, 3.5, and 6.3 cm. Dust emission up to 7 mm was detected around CS Cha and RU Lup and even up to 1.6 cm for WW Cha, indicating that grains have grown up to sizes of at least several centimetres. Show less
