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The outflow force of molecular bipolar outflows is a key parameter in theories of young stellar feedback on their surroundings. The focus of many outflow studies is the correlation between the outflow force, bolometric luminosity and envelope mass. However, it is difficult to combine the results of different studies in large evolutionary plots over many orders of magnitude due to the range of data quality, analysis methods and corrections for observational effects such as opacity and inclination. We aim to determine the outflow force for a sample of low luminosity embedded sources. We will quantify the influence of the analysis method and the assumptions entering the calculation of the outflow force. We use the James Clerk Maxwell Telescope to map 12CO J=3-2 over 2'x2' regions around 16 Class I sources of a well-defined sample in Ophiuchus at 15" resolution. The outflow force is then calculated using seven different methods differing e.g. in the use of intensity-weighted emission and correction factors for inclination. The results from the analysis methods differ from each other by up to a factor of 6, whereas observational properties and choices in the analysis procedure affect the outflow force by up to a factor of 4. For the sample of Class I objects, bipolar outflows are detected around 13 sources including 5 new detections, where the three non-detections are confused by nearby outflows from other sources. When combining outflow forces from different studies, a scatter by up to a factor of 5 can be expected. Although the true outflow force remains unknown, the separation method (separate calculation of dynamical time and momentum) is least affected by the uncertain observational parameters. The correlations between outflow force, bolometric luminosity and envelope mass are further confirmed down to low luminosity sources., 24 pages, 13 figures, Accepted by A&A

High resolution solar spectra have permitted the measurement with great accuracy of equivalent widths of vibration-rotation lines of OH in the X2Pi state near 3-micron wavelength. Using recent theoretical results for the transition probabilities, a solar oxygen abundance of (8.93 + or - 0.02) is derived which is in perfect agreement with the abundance deduced from the OH pure rotation lines. The solar abundance of oxygen is therefore A(O) = 8.92 + or - 0.035, as inferred from the analysis of 43 vibration-rotation lines and 81 pure rotation lines of the OH molecule. It is confirmed that the dipole moment function of Werner, Rosmus and Reinsch (1983) together with the Holweger-Mueller (1974) solar atmosphere model are to be preferred in the analysis of the data.

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 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.

Thanks to the advent of the Atacama Large Millimeter/submillimeter Array (ALMA), large surveys of protoplanetary disks in different star forming regions have been carried out to study the gas and dust components simultaneously. Carbon monoxide (CO) and its less abundant isotopologues have been observed to trace the bulk of the gas, while the dust was traced by the (sub-)mm continuum. A result that is common to these surveys is that CO emission from disks is fainter than expected. As a consequence, the overall CO-based gas-masses are very low, often smaller than one Jupiter mass and global gas/dust mass ratios are much lower than the expected interstellar-medium value of 100. This may be interpreted as lack of gas due to fast disk dispersal, or as lack of volatile carbon that leads to faint CO lines. After summarizing the results from different ALMA disk surveys and their implications, I will present alternative observational strategies which may help us to disentangle between the gas dispersal scenario and the chemical evolution hypothesis. More specifically I will show ALMA C2H observations that allow us to constrain the C/O ratio and to confirm that chemical evolution is at play in disks with faint CO lines.

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) 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.

This thesis presents observations of exoplanets orbiting very close to their parent star, with a particular focus on a novel technique for characterizing their atmospheres. This is based on the use of high-resolution spectroscopy from the ground. The first detection of the atmosphere of a non-transiting planet is presented, together with the determination of its mass and orbital inclination. Moreover, it is shown that high-dispersion spectroscopy is very effective in recognizing molecular species, measuring their relative abundances, and determining whether temperature increases or decreases with altitude in the observed planetary atmospheres. The method also led to the measurement of the rotational period of a transiting exoplanet, which was found to be tidally locked, in line with theoretical predictions. Finally, the evidence for the disintegration of a small, rocky planet candidate in the Kepler database is presented. This result was obtained by fitting the light curve of the object with a model of a trailing tail of dust.

Unlike the formation of Solar-type stars, the formation of massive stars (M>8 Msun) is not yet well understood. For Solar-type protostars, the presence of circumstellar or protoplanetary disks which provide a path for mass accretion onto protostars is well established. However, to date only few cases of young massive stars show the evidence of circumstellar disks which support the idea that a scaled-up version of low-mass star formation could be applied to young massive stars. To what extend this hypothesis can be applied is still unclear and more observational evidences are required to characterize the physical properties of the disk-like structures around massive stars in order to understand how exactly massive stars gain their masses in the equatorial regions. In this thesis, we present high-resolution (sub)millimeter interferometric and single-dish observations of massive star-forming cores. We suggest that massive stars may form via disk accretion in some relatively isolated star-forming cores, while massive stars may form through competitive accretion in the central part of massive star-forming clusters. Our studies serve as the basis for the coming Atacama Large Millimeter/submillimeter Array (ALMA) studies at very high angular resolution.

The Spectral Energy Distributions (SEDs) of star-forming regions and starburst galaxies are unique tracers of the star formation processes in these environments, since they contain information on the escaping and processed photons emitted by newly formed massive stars. Understanding these internal processes is crucial in our physical interpretation of observations of unresolved star formation in the Universe. In the __rst part of this thesis, we study the physical conditions in resolved starburst regions using Bayesian __tting of their spatially integrated infrared SEDs, including both the thermal continuum and the atomic emission lines. We then apply the method to unresolved starburst to learn about their star formation physics. Our approach leads to robust constraints on physical parameters such as age, compactness, and amount of currently ongoing star formation in starburst, which are otherwise biased by model degeneracies, and allows us to link the resolved properties of giant H II regions to the star formation process at larger scales. In the second part of this thesis, we discuss the wavelength calibration of the next instrument to study the midinfrared spectral properties of starbursts, with improved resolution and sensitivity: the mid-infrared instrument (MIRI), which will __y onboard the James Webb Space Telescope in 2018.

In this thesis we study the earliest stages of high-mass star formation. Class II methanol masers are only associated with massive star formation and are a unique probe of these environments. Through observations we have studied where and when the methanol maser emission occur in relation to the protostar. We have found that for a fair fraction of the sources the methanol masers appear on size scales of ca. 1000 AU, in the equatorial region of the massive protostar. It appears that infall, rather than rotation, is the dominant motion. We propose that the maser emission occur close to or in a shock interface, possibly related to the accretion flow of the more extended gas in the protostellar envelope onto an accretion disk. The morphology and kinematics of the thermal methanol gas support the hypothesis that the maser region is also the region where the methanol molecules are released from the icy mantles of the du st grains. We have also estimated the temperature and column density of the methanol gas in the outflows and find evidence for radiative excitation of the methanol gas at the location of the maser emission.

My PhD research focuses on the maser emission which occurs in different regions of the circumstellar envelopes (CSEs) of evolved stars and can be studied at high angular resolution using radio interferometers. These masers are useful probes of the dynamics and kinematics of the outflow from AGB stars. Moreover, the masers can be important tracers of the magnetic field strength and morphology at various distances from the central stars. It is expected that the magnetic field plays an important role in transforming spherically symmetric AGB stars into a-spherical PNe. Theoretical modeling indicated that magnetically collimated jets may be responsible for the formation of the a-spherical PNe. We performed polarimetric observations of the masers (OH, H2O and SiO) in different classes of evolved stars using several radio interferometers as well as single dish radio and sub-mm telescopes, to study the role of the magnetic field throughout the AGB evolution.

This thesis focuses on the interplay of the young star and its protoplanetary disk, on the evolution of the dust particles that make up the protoplanetary disk surrounding the young star, and thus on the very first stage of the formation of planets like those that compose our own Solar system.

Star formation occurs when a dense cloud of interstellar gas and dust gravitationally collapses. Rotation during this collapse leads naturally to the formation of a flattened circumstellar disk around the forming star. These disks are additionally known as protoplanetary disks because the orbiting circumstellar dust and cold gas represent the building blocks for planets. How long this material survives and how it evolves in this time will determine the propensity for (and the diversity of) planetary systems. This thesis is split into three parts that analyze different aspects of disk evolution and the circumstellar environment. In part one, we use observations at millimeter wavelengths to probe (and then model and compare) the dust and gas properties around low-mass Sun-like stars. We conclude that high-resolution spatial and spectral imaging of optically thinner molecular lines will provide the most robust description of the disk structure and evolution using future instrumentation. During these routine observations, we report recurring millimeter flares resulting in part two. We attribute this phenomenon to synchrotron emission from relativistic electrons trapped in the (colliding) magnetospheres of a young binary system. Finally, we present a microgravity experiment to probe the collisional growth mechanism for the first steps of planet formation.

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 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).

Starburst galaxies undergo a phase of vigorous star formation. In these galaxies, gas is turned into stars at such a high rate, that the reservoir of available gas is quickly emptied. Numerous massive, bright star clusters are created from the giant molecular clouds. They form an ideal laboratory for the study of rapid star formation. In this thesis, a detailed study of active star-forming regions in the starburst galaxies NGC 4038/4039 and M83 is presented. Sincethe stars are hidden from view by large quantities of dust, we observe them at infrared wavelengths. We address the properties of the newly formed star clusters and the surrounding interstellar matter.

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 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 (

Chemistry plays an important role in the study of the physics and evolution of the warm dense Interstellar Medium (ISM). This is found in very different environment, e.g. in the disks around low-mass young stellar objects (YSOs), in the envelopes of massive-YSOs, and in the nuclei of Ultra Luminous Infrared Galaxies (ULIRGS). The excitation temperatures and abundances of the observed molecules provide the observer with a physicochemical snapshot of the regions. The abundances of the molecules depend strongly on temperature, density, radiation field (UV, Infrared, and X-ray) and initial composition of the gas. Infrared spectroscopy is instrumental in the study of dense ISM as many species have transitions in the infrared. Space based instruments, unhindered by the earth atmosphere, allow us to study these. The absorption and emission features are often weak. Therefore a major challenge in using these instruments is to understand the instrumental characteristics and translating that knowledge into usable reduction algorithms. Most of the data used in this thesis are obtained with space based spectrographs, ISO-SWS and Spitzer-IRS, pushing them to their limits.

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 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