Stars like the sun are born in large molecular clouds existing from gas and dust. During the formation process, the chemical composition of the material can be altered drastically by the changing... Show moreStars like the sun are born in large molecular clouds existing from gas and dust. During the formation process, the chemical composition of the material can be altered drastically by the changing physical conditions. This thesis focuses on how molecules in young protostellar systems are inherited from molecular clouds. The emphasis lies on so-called complex organic molecules and accretion shocks.Based on observations of complex organic molecules, it can be suggested that the molecular composition of a protostellar disk is (partially) inherited from the molecular cloud. The abundance ratio between various molecules is remarkably constant in various protostellar systems, implying that they form under similar conditions in molecular clouds. Furthermore, absence of complex molecules in observations does not directly mean that they are absent in the protostellar system but rather that they are hidden from us.This thesis also focuses on accretion shocks at the boundary between infalling cloud and protostellar disk. Based on a comparison between detailed numerical simulations and observations it can be suggested that strong accretions are not always present in protostellar systems. In turn, this suggests that the chemical composition in protostellar disks can be directly inherited from the molecular cloud. 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
James, T.A.; Viti, S.; Yusef-Zadeh, F.; Royster, M.; Wardle, M. 2021
To address the fundamental questions of how life on Earth emerged and how common life may be in the Universe, it is crucial to know the chemical composition of the planet-forming material. Planets... Show moreTo address the fundamental questions of how life on Earth emerged and how common life may be in the Universe, it is crucial to know the chemical composition of the planet-forming material. Planets were originally thought to form in protoplanetary disks, but studies of both disks and our Solar System show that planet formation already starts much earlier, in disks that are still embedded in cloud material. These young disks, however, are largely uncharacterised. This thesis presents a number of case studies on the physical and chemical structure of young disks, including the first temperature measurements showing that young disks are too warm for CO ice, unlike protoplanetary disks. In addition, it is shown that young disks around outbursting stars are the ideal sources to probe the the chemical complexity in planet-forming material. Show less
The primary focus of this thesis is the formation of low-mass protostars, specifically the earliest deeply embedded phase, when material from the collapsing envelope is still accreted onto the... Show moreThe primary focus of this thesis is the formation of low-mass protostars, specifically the earliest deeply embedded phase, when material from the collapsing envelope is still accreted onto the growing young star. Rotational transitions of CO and O2 data are obtained by the Herschel Space Observatory key projects, WISH and HOP, together with ground-based observations from APEX and the JCMT. We have found that CO and its isotopologs have different line profiles tracing different materials in the protostellar regions. Our new high-J rotational transitons of CO is key to characterize the warmer parts of the protostellar envelope and quantify feedback of the protostars on their surroundings in terms of shocks, ultraviolet (UV) heating, photodissociation, and outflow dispersal. Radiative transfer modeling was performed to determine the CO abundance structure in the envelope, showing evidence for significant freeze-out in the coldest regions in the parts of the envelope where the temperature exceeds 25 K. A tentative detection of O2 is reported toward the source position of a protostar, which originates from the surrounding cloud. These kind of detailed studies of the physical and chemical structure of low-mass protostars are important for a complete understanding of the evolution of young stellar objects (YSOs). 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
Stars form as a result of gravitational collapse of an interstellar molecular cloud. In the process, a circumstellar disk, often referred to as a protoplanetary disk, is formed as well as a result... Show moreStars form as a result of gravitational collapse of an interstellar molecular cloud. In the process, a circumstellar disk, often referred to as a protoplanetary disk, is formed as well as a result of the net angular momentum of the parental cloud. This thesis addresses several questions about the formation of this disk and, in particular, the evolution of the velocity field surrounding the young star. The composition of the velocity field (i.e., the ratio of infall to rotation) can be use to trace the evolution of young stellar objects. This method of characterizing the evolutionary stage of protostars is applied to two objects, NGC1333-IRAS2A and L1489 IRS, the first of which we find to be a very young object while the latter is significantly more evolved. In addition, one chapter of this thesis presents a new radiation transfer code which has been developed by the author. Show less
