Formation and desorption of organic molecules: Resolving their emission in a prominent protostellar hot core and its outflow

Abstract

Star-forming regions are known to be sites of a rich chemistry involving a variety of molecules. Interstellar complex organic molecules (COMs) are thereby of particular interest. They might act as precursors to larger and potentially prebiotic species that eventually get incorporated into planets or smaller bodies (such as comets) and could participate in the emergence of life. <br /> Many COMs have first been detected and, since then, extensively studied in protostellar environments such as Sgr B2 (N)orth, which is one of the most prominent high-mass star-forming regions in the Milky Way located in the Galactic centre region. It contains several hot (molecular) cores. A hot core represents the heated region of gas and dust that surrounds a high-mass protostar. COMs are highly abundant in the gas of hot cores and their emission is thus bright. The occurrence of the high gas-phase abundances of COMs in these sources results from their formation at high temperature in the gas phase itself or from their sublimation from icy dust-grain surfaces into the gas phase. Many molecules are in fact believed to form in the ice mantles of dust grains, which can either occur during the cold phase prior to the birth of a stellar embryo or during the so-called warm-up phase, when the young protostar gradually heats the surrounding material. Subsequently, the sublimation of COMs can either proceed thermally (i.e. be induced by a gradual increase in dust grain temperature) or non-thermally (e.g. through shocks or interaction with cosmic rays). In hot cores the bulk of the COMs that are formed in the solid phase desorbs thermally. There are essentially two ways in which COMs could desorb thermally: alongside water, which is the most abundant species of the grains’ ice mantles, or at their individual desorption temperature, which depends on how strongly they are bound to the grain surface. Which process occurs is yet to be confirmed by observations, which was one of the goals of this thesis. <br /> To tackle this question, we used data taken with the Atacama Large Millimetre/submillimetre array (ALMA) towards Sgr B2 (N). The high angular resolution provided by ALMA allowed us to spatially resolve the COM emission in our target source, Sgr B2 (N1), which is the most massive hot core in the region. This enabled us to study variations in molecular composition by deriving abundances of a dozen of COMs at various positions within the source. Besides abundances, we derived gas temperatures and could thus investigate the relation between the two quantities. We found a steep increase of the COM abundances around 100K, leading us to conclude that COMs co-desorb with water. Furthermore, we observed non-zero abundances of COMs below 100 K, which suggests a different desorption process at low temperature. With these two results, it is the first time that COM desorption is spatially resolved in the interstellar medium. Comparing our observational results to predictions of astrochemical models, we could infer the formation of certain COMs on dust-grain surfaces. <br /> The second goal of this thesis was to investigate the impact of shocks, provoked by the outflow of Sgr B2 (N1), on the molecular composition. Comparing the observed abundances to the chemical composition at positions that are not exposed to the outflow revealed segregation between O- and N-bearing molecules, with the former being depleted at the outflow positions. Based on the comparison with astrochemical models and with observed chemical compositions of other sources that are exposed to shocks, we proposed a scenario that may be able to explain the chemical segregation, which was also previously observed in various sources. Our scenario presents a new perspective on this process that, however, still needs to be confirmed by chemical models. <br /> Similar studies of other protostellar sources at high angular resolution in the future could probe whether the results obtained for Sgr B2 (N1) (e.g. regarding desorption processes or the impact of the outflow on the chemical composition) are common at this stage of star formation or are specific to this hot core. It would be of particular interest to compare Sgr B2 (N1), which is a high-mass young stellar object located in the Galactic centre region, with low-mass analogues, on the one hand, and with sources located in the Galactic disk, where physical conditions greatly differ, on the other hand.