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
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
Context. The physical and chemical conditions in Class 0/I protostars are fundamental in unlocking the protostellar accretion process and its impact on planet formation.Aims: The aim is to... Show moreContext. The physical and chemical conditions in Class 0/I protostars are fundamental in unlocking the protostellar accretion process and its impact on planet formation.Aims: The aim is to determine which physical components are traced by different molecules at subarcsecond scales (<100-400 au).Methods: We used a suite of Atacama Large Millimeter/submillimeter Array (ALMA) datasets in band 6 (1 mm), band 5 (1.8 mm), and band 3 (3 mm) at spatial resolutions 0.″5-3″ for 16 protostellar sources. For a subset of sources, Atacama Compact Array (ACA) data at band 6 with a spatial resolution of 6″ were added. The availability of low- and high-excitation lines and data on small and larger scales, is important to understand the full picture.Results: The protostellar envelope is well traced by C18O, DCO+, and N2D+, which stems from the freeze-out of CO governing the chemistry at envelope scales. Molecular outflows are seen in classical shock tracers such as SiO and SO, but ice-mantle products such as CH3OH and HNCO that are released with the shock are also observed. The molecular jet is a key component of the system. It is only present at the very early stages, and it is prominent not only in SiO and SO, but occasionally also in H2CO. The cavity walls show tracers of UV-irradiation such as C2H, c-C3H2 and CN. In addition to showing emission from complex organic molecules (COMs), the hot inner envelope also presents compact emission from small molecules such as H2S, SO, OCS, and H13CN, which most likely are related to ice sublimation and high-temperature chemistry.Conclusions: Subarcsecond millimeter-wave observations allow us to identify these (simple) molecules that best trace each of the physical components of a protostellar system. COMs are found both in the hot inner envelope (high-excitation lines) and in the outflows (lower-excitation lines) with comparable abundances. COMs can coexist with hydrocarbons in the same protostellar sources, but they trace different components. In the near future, mid-infrared observations with JWST-MIRI will provide complementary information about the hottest gas and the ice-mantle content, at unprecedented sensitivity and at resolutions comparable to ALMA for the same sources. Show less
