Context. As material from an infalling protostellar envelope hits the forming disk, an accretion shock may develop which could (partially) alter the envelope material entering the disk.... Show moreContext. As material from an infalling protostellar envelope hits the forming disk, an accretion shock may develop which could (partially) alter the envelope material entering the disk. Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) indicate that emission originating from warm SO and SO2 might be good tracers of such accretion shocks.Aims. The goal of this work is to test under what shock conditions the abundances of gas-phase SO and SO2 increase in an accretion shock at the disk-envelope interface.Methods. Detailed shock models including gas dynamics were computed using the Paris-Durham shock code for nonmagnetized J-type accretion shocks in typical inner envelope conditions. The effect of the preshock density, shock velocity, and strength of the ultraviolet (UV) radiation field on the abundance of warm SO and SO2 is explored. Compared with outflows, these shocks involve higher densities (similar to 10(7) cm(-3)), lower shock velocities (similar to few km s(-1)), and large dust grains (similar to 0.2 mu m) and thus probe a different parameter space.Results. Warm gas-phase chemistry is efficient in forming SO under most J-type shock conditions considered. In lower-velocity (similar to 3 km s(-1)) shocks, the abundance of SO is increased through subsequent reactions starting from thermally desorbed CH4 toward H2CO and finally SO. In higher velocity (greater than or similar to 4 km s(-1)) shocks, both SO and SO2 are formed through reactions of OH and atomic S. The strength of the UV radiation field is crucial for SO and in particular SO2 formation through the photodissociation of H2O. Thermal desorption of SO and SO2 ice is only relevant in high-velocity (greater than or similar to 5 km s(-1)) shocks at high densities (greater than or similar to 10(7) cm(-3)). Both the composition in the gas phase, in particular the abundances of atomic S and O, and in ices such as H2S, CH4, SO, and SO2 play a key role in the abundances of SO and SO2 that are reached in the shock.Conclusions. Warm emission from SO and SO2 is a possible tracer of accretion shocks at the disk-envelope interface as long as a local UV field is present. Observations with ALMA at high-angular resolution could provide further constraints given that other key species for the gas-phase formation of SO and SO2, such as H2S and H2CO, are also covered. Moreover, the James Webb Space Telescope will give access to other possible slow, dense shock tracers such as H-2, H2O, and [SI} 25 mu m. Show less
Multiple stars, that is two or more stars composing a gravitationally bound system, are common in the universe.They are the cause of many interesting phenomena, from supernovae and planetary... Show moreMultiple stars, that is two or more stars composing a gravitationally bound system, are common in the universe.They are the cause of many interesting phenomena, from supernovae and planetary nebulae, to binary black hole mergers. Observations of main sequence stars, young stars and forming protostars show that multiplicity is common, and that multiple stars are born. This thesis focuses on several of the open questions on the formation and evolution of multiple stars, namely when do rotationally supported disks form, the factors leading to fragmentation of the cloud core and the physico-chemical structure of multiple protostars. For this purpose, radio interferometric observations of dust continuum and molecular line emission, coupled with chemical and physical models are used to study several young, deeply embedded prototstars. The results of this thesis contribute useful pieces to the puzzle of multiple star formation, demonstrating that rotationally supported disks can form early in the star formation process, while temperature and the presence of disks can alter the physico-chemical protostellar structure. Furthermore, the results of this thesis indicate that mass, rather than temperature, could be an important factor in fragmentation of cloud cores, and the formation of multiple stars. 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 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
