In recent years it has become clear that the space in between the stars, contains a remarkable amount of highly diverse molecules, ranging from simple diatomics to large complex species.... Show moreIn recent years it has become clear that the space in between the stars, contains a remarkable amount of highly diverse molecules, ranging from simple diatomics to large complex species. Astronomical observations and dedicated laboratory experiments show that icy dust grains play a prominent role in the chemical enrichment of matter in space. Exotic solid state reactions taking place for temperatures as low as -260 degree merge small abundant species to larger and larger compounds until species are formed that are considered to be of relevance for life; sugars, fats and precursors of amino acids. Ultimately this material is embedded in matter from which stars and planets form. So, could it be possible, that the ingredients for life form everywhere, in space, following very similar chemical pathways? In this thesis a dedicated laboratory study is described that studies exactly these processes. With a new experimental setup the physical and chemical processes are characterized that are needed to interpret and guide astronomical observations and that yield parameters needed as input in astrochemical models. It is concluded that the chemical complexity in the solid state reaches out much further than assumed so far. Show less
In this thesis we used numerical simulations to explore the role that chemistry plays in galaxy formation. Simulations of galaxies often assume chemical equilibrium, where the chemical reactions... Show moreIn this thesis we used numerical simulations to explore the role that chemistry plays in galaxy formation. Simulations of galaxies often assume chemical equilibrium, where the chemical reactions between ions and molecules have reached a steady state. However, this assumption may not be valid if the physical conditions of the gas are evolving rapidly. Therefore, we developed a chemical model to follow the non-equilibrium evolution of ions and molecules. We then incorporated this model into hydrodynamic simulations of galaxies. We ran simulations with different metallicities (i.e. different proportions of heavy elements) and UV radiation fields, first using our full non-equilibrium chemical model and then assuming chemical equilibrium. We found that the total star formation rate is higher at higher metallicity and for weaker radiation fields. In contrast, non-equilibrium chemistry does not strongly influence the total star formation rate or outflow properties of the galaxy. However, it does affect the abundances of individual chemical species, for example in molecular outflows. Finally, we explored the properties of molecular clouds in our simulations. At low metallicity, the molecular fraction of young clouds tends to be below equilibrium, as the molecules are still forming. This also affects the observable CO emission from young clouds. 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
This thesis presents the results from the analysis and characterisation of the water and mid-J (J<11) 12CO, 13CO and C18O observations for a large sample of low-, intermediate-, and high-mass... Show moreThis thesis presents the results from the analysis and characterisation of the water and mid-J (J<11) 12CO, 13CO and C18O observations for a large sample of low-, intermediate-, and high-mass young stellar objects (YSOs). The studied molecular transitions have been observed with the HIFI instrument on board of Herschel Space Observatory and within the context of the Herschel key programme __WISH__. These species and transitions constitute unambiguous tracers of specific physical conditions within the inner and warmer regions of the YSO environment. The sample of sources, composed by more than 120 YSOs, covers a large range of bolometric luminosities, several evolutionary stages within the embedded phase, and different physical scales. The aim of this work is to explore the differences and similarities between low- and high-mass star-forming regions. In particular, this study focuses on investigating the physical and dynamical structure of dense warm gas within protostellar environments by characterising the velocity-resolved H2O and CO spectra in terms of line profile and line luminosity. The ultimate goal is to contribute to the understanding of the star formation process without imposing luminosity boundaries, and to put in context these processes on Galactic and extragalactic scales. Show less
This thesis is devoted to the study of regular and deuterated water in ices and on surfaces against an interstellar background. A large network for the formation of regular water has been studied... Show moreThis thesis is devoted to the study of regular and deuterated water in ices and on surfaces against an interstellar background. A large network for the formation of regular water has been studied with the use of a Kinetic Monte Carlo model. A specific reaction has been investigated as well: H2 + O -> OH + H. Furthermore, in the light of deuterium fractionation, a thermal study on deuteron scrambling in the ice has been performed: H2O + D2O -> 2 HDO. Finally, two low-temperature routes relevant to HDO formation have been investigated: H2O + OD -> OH + HDO and D2O + OH -> OD + HDO. Show less
This thesis explores the chemistry of interstellar and circumstellar molecules during star formation and death. From the perspective of chemical physics, the most important outcome of this thesis... Show moreThis thesis explores the chemistry of interstellar and circumstellar molecules during star formation and death. From the perspective of chemical physics, the most important outcome of this thesis lies in that the rates for two important reactions are determined accurately for the first time: N2 photodissociation and reaction rates (both state-to-state and thermal) of OH with H. In particular, accurate N2 photodissociation rate and shielding functions were calculated. The results are very useful in many astrophysical fields. In addition, a new method for accurately calculating molecular shielding functions in full 3D radiation field was proposed, and was employed to give new predictions for molecule distributions in a C-rich AGB star, IRC +10216, and an O-rich AGB star, IK Tau, based on the latest progress from both observations and simulations. By far, these results are the most accurate ones, and can be directly compared to the future observations. Show less
This thesis treats the chemical behaviour of carbonaceous molecules in water-dominated interstellar ices. VUV photons are considered as the chemical trigger to induce solid state chemistry as it is... Show moreThis thesis treats the chemical behaviour of carbonaceous molecules in water-dominated interstellar ices. VUV photons are considered as the chemical trigger to induce solid state chemistry as it is omnipresent. Lyman- radiation occurs even in dense molecular clouds as a result of cosmic ray excitation of H2 and subsequently emitting its excess energy at 122 nm (Mathis et al. 1983). It comprises the addition of new tools to laboratory astrochemistry, expanding knowledge on the behaviour of PAHs in interstellar ices and research into the role of C 2H2 and polyynes in interstellar ice VUV photochemistry. It provides with spectroscopic tools for observers to enable the identification of the hydrocarbons encountered in interstellar ices, and it provides modellers with reaction dynamic information that can be used as an input for their models. Show less
Protostars interact violently with their natal cocoons within dense molecular clouds. Characterizing this feedback is key to understanding the efficiency of the star formation process and the... Show moreProtostars interact violently with their natal cocoons within dense molecular clouds. Characterizing this feedback is key to understanding the efficiency of the star formation process and the chemical processing of material that will be available for planet formation. In this thesis, the imprints of physical processes on molecular gas are analyzed using state-of-the-art far-infrared spectroscopy from Herschel / PACS. Interpretation of the origin of far-infrared line emission allows us to quantify the physical conditions and the role of shocks and ultraviolet radiation during the 'kindergarten years' of low- and high-mass protostars. Show less
Star and planet formation is intimately linked through the protostellar disk. Understanding the formation and evolution of this disk is crucial to determine the physical and chemical processes that... Show moreStar and planet formation is intimately linked through the protostellar disk. Understanding the formation and evolution of this disk is crucial to determine the physical and chemical processes that occur from the formation of dense molecular clouds to the emergence of life. Yet, the formation and early evolution of the protostellar disk are still not well explored. This thesis presents both observational and theoretical aspects of the early stages of disk formation and evolution. Hydrodynamical simulations of disk formation are coupled with multi-frequency continuum radiative transfer to determine the dust temperature. The detailed dust temperature structure is crucial for the construction of chemical structure. Observational predictions are simulated through molecular line radiative transfer methods to be compared with spectrally and spatially resolved data. By comparing these predictions with observational data, it is possible to link the disk formation process with planet formation 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
The importance of ice in the interstellar medium is indisputable. Gas phase reactions relying on three-body collisions are exceedingly rare in the sparse medium between the stars. On solid surfaces... Show moreThe importance of ice in the interstellar medium is indisputable. Gas phase reactions relying on three-body collisions are exceedingly rare in the sparse medium between the stars. On solid surfaces, atoms and molecules can reside and rove the surface until a reaction takes place. Upon reaction, the released energy is dissipated into the grain, allowing the new species to form. Solid surfaces thus act as sites for chemical processes, that would otherwise be very slow, or not take place at all. This thesis is dedicated to the study of the composition and physical characteristics of interstellar ices using a variety of experimental observational techniques. The overall goal is to shed light on the processes that chemically enrich planet-forming regions. The specific objectives are to characterize morphological changes and molecular composition in interstellar ices, to explore new experimental techniques to study solid state reactions, and to use complex molecules to probe large scale astronomical phenomena. 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
It has been a long standing problem in astrochemistry to explain how molecules can form in a highly dilute environment as the interstellar medium. In recent years it has become clear that solid... Show moreIt has been a long standing problem in astrochemistry to explain how molecules can form in a highly dilute environment as the interstellar medium. In recent years it has become clear that solid state reactions on icy grains play an important role in the formation of both simple and rather complex molecules. Laboratory based experiments that simulate the UV processing or the impact of H-atoms on interstellar ice analogues are needed in order to investigate the underlying processes. This is the topic of this PhD thesis that mainly summarizes research on SURFRESIDE, one of the ultra-high vacuum setup in the Sackler Laboratory for Astrophysics. It is shown how under fully controlled conditions molecules form when CO and O2 containing ices are bombarded by hydrogen atoms. Surface reaction schemes for methanol, water, carbon dioxide and formic acid formation at low temperatures in space are presented, and it is discussed how species may be chemically linked in space. 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
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
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
The formation of complex organic molecules that consist of more than four atoms in space is one of the main questions in the field of astrochemistry and star formation. Although the exact formation... Show moreThe formation of complex organic molecules that consist of more than four atoms in space is one of the main questions in the field of astrochemistry and star formation. Although the exact formation mechanisms are not yet known, they are expected to form in thin ice layers on the surfaces of small interstellar dust grains through successive addition of H, C, N or O atoms to CO (carbon monoxide). In this thesis the formation of these molecules is studied in two different ways: simulation of interstellar ices analogues in the laboratory and observations of the same molecules after evaporation toward star forming regions. The laboratory experiments are high and ultra high vacuum setups in which ices of e.g. CO, CO2, HCOOH and CH3CHO are frozen out on an inert surface. The spectroscopy and the thermal behavior of pure and layered ices have been studied. Furthermore, the ices have been bombarded with H-atoms to test reactions schemes relevant for astronomical environments. In the second part of this thesis the same molecules have been observed with the single dish submillimeter telescopes the __James Clerk Maxwell Telescope__ at Hawaii and the Institut de Radioastronomie Millim_trique in Spain toward a sample star forming regions as well as with interferometer the SubMillimeter Array at Hawaii toward two sources. The relative abundances of molecules in different star forming regions measured with the single dish telescopes as well as the spatial extent of the emission detected with the interferometer has been used to determine the chemical relations between complex organics that have also been studied in the laboratory. Show less
In this thesis we study the dust around solar-type young stars. In particular, we focus on one specific species of dust, namely the Polycyclic Aromatic Hydrocarbons (PAHs), a family of large... Show moreIn this thesis we study the dust around solar-type young stars. In particular, we focus on one specific species of dust, namely the Polycyclic Aromatic Hydrocarbons (PAHs), a family of large molecules, or small grains, that are widely observed in nearby star-forming regions. We address the following questions. What happens to PAHs in the embedded phase of a forming star? Are PAHs present in low-mass young star systems? Does the PAH emission originate from the envelope or from the disk? What do they tell us about disk structure and evolution and grain growth? What can we say about the evolution of PAHs during star formation and their typical size? We present mid-infrared spectroscopy and imaging surveys combined with 3D radiative transfer models to constrain the presence and location of PAH emission toward embedded young stellar objects and circumstellar disks around young solar-type stars. PAHs are detected toward a small fraction (11-14%) of young solar-type stars with disks and toward a minority of embedded objects (<3%), with derived abundances of 10-100 times lower than standard interstellar values. A new class of disks with weak mid-IR continuum emission and very strong PAH features is found. Show less
Planets form in disks of gas and dust around young stars. Since the gas makes up 99 % of the disk mass, it is critical for our understanding of planet formation to gain direct information from the... Show morePlanets form in disks of gas and dust around young stars. Since the gas makes up 99 % of the disk mass, it is critical for our understanding of planet formation to gain direct information from the gas, independently of what can be learned from dust emission. In this thesis, calculations are presented of the chemistry and gas temperature in disks, and the resulting atomic and molecular emission lines are investigated. The main focus of the thesis is on the effects of dust settling on gas-phase emission lines of disks around T-Tauri and Herbig Ae stars. It is found that dust settling has little effect on the overall chemistry and molecular lines; the main effect is a decrease in the gas temperature, which is reflected in atomic fine-structure lines and especially in the [O I] lines. The chemistry, and especially the CO abundance and HCN/CN ratio, is affected more by the total gas mass than by the dust gas ratio in a disk. The models were also applied to the disk around HD 141569A, which is in a transitional stage between a gas-rich Herbig Ae disk and a debris disk. Using chemical models to fit the observed CO rotational lines it is concluded that gas and small dust particles have an approximately interstellar mass ratio, and that gas is still present in the inner hole in the dust distribution Show less
The formation of snow and ice has always intrigued humans and challenged them to study these phenomena. Every snowflake has its own unique history of formation, but no two are alike. Like snow... Show moreThe formation of snow and ice has always intrigued humans and challenged them to study these phenomena. Every snowflake has its own unique history of formation, but no two are alike. Like snow-crystals, interstellar ices consist predominantly of water (H2O), but also contain significant fractions of other molecules such as carbon monoxide (CO), carbon dioxide (CO2), and methanol (CH3OH), and traces of dinitrogen (N2) and ammonia (NH3). The presence, or absence, of a molecule in the ice strongly depends on the environmental conditions. Vice versa, these molecules have an influence on their environment as well. Hence, the chemical composition and the structure of interstellar ices are thought to contain valuable information about the past and the future of interstellar regions, and it is for this reason that interstellar ices are simulated and studied under laboratory conditions. The present thesis contains a study of laboratory analogs of interstellar ices and presents a newly developed apparatus that provides a novel laboratory route to investigate the properties of these ices in more detail than has previously been possible. Show less
