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
During solar-type star formation, the chemistry evolves towards the formation of complex organic molecules, eventually setting the stage for the origin of life. This astrochemical evolution depends... Show moreDuring solar-type star formation, the chemistry evolves towards the formation of complex organic molecules, eventually setting the stage for the origin of life. This astrochemical evolution depends on the interaction between gas and microscopic interstellar grains, producing icy grain mantles. This thesis combines ice and gas-phase observations with astrophysically relevant laboratory simulations to constrain some of the key gas-grain processes. From Spitzer observations, the first simple ices, e.g. water and methane, form sequentially through condensation followed by an active surface chemistry, with more source-to-source variation the later in the sequence an ice forms. Close to the protostar the ices are heated. Experiments and their modeling have provided a generalized, quantitative understanding of the induced ice mixture evaporation and segregation, based on relative diffusion barriers alone. When no heat is available UV-induced evaporation still connects the ice and gas; UV photodesorption is found experimentally to be efficient with a similar yield for most ices. UV irradiation also converts simple ices into more complex species and this formation process has been quantified in situ for the first time. Based on these experiments, observations of complex organic molecules around protostars and in comets are readily explained by ice photochemistry. Show less
