Molecular diversity in early stage high-mass protostars

Abstract

During star formation, the molecular gas undergoes significant chemical evolution with the emergence of dense cores harboring hot cores and hot corinos, rich in complex organic molecules (COMs). The formation pathways, even for many simpler molecules, are, however, debated. Based on the 870µm ATLASGAL survey of the inner Galaxy, the SPARKS project discovered 6 high-mass protostellar envelopes that are isolated down to 400au scales. Since these targets have been selected due to their low bolometric luminosity, the discovery of these early stage high-mass protostars offers new, previously unrecognized laboratories to study the early warm-up phase chemistry leading to the emergence of hot cores. In this thesis, we performed a complete unbiased spectral survey covering the frequency range between 159GHz and 374GHz with the APEX telescope towards these sources with the aim to perform an unprecedentedly detailed study of their molecular composition constraining chemical formation pathways. <br /> From this sample we first investigated the hot core precursor, G328.25, in particular detail because its inner envelope structure is relatively well constrained on <1000au scales based on high angular-resolution observations with ALMA. Our results using the unbiased spectral survey uncover a remarkable molecular richness consisting of more than 40 species (plus 26 isotopologues). We could pin down their location within the envelope combining a detailed analysis of the line profiles and local thermodynamic equilibrium (LTE) modeling. Our results show that the G328.25 source is particularly rich in sulfur bearing molecules in terms of number of species identified, but also considering their high molecular abundances pointing to a low sulfur depletion. We could also confirm and characterize the accretion shocks revealed by ALMA observations and extend the number of COMs originating from these shocks. <br /> Considering all 6 sources of the sample, we identify on average about 40 species (60 including isotopologues). These infrared-quiet massive sources thus exhibit a great chemical richness despite their early evolutionary stage. However, while some objects exhibit a clear structure with a well-defined warm gas phase, we find that about half of the sample remains predominantly cold with warm gas traced only by two species, CH₃OH and CH₃CN. The molecular composition of the sample is remarkably similar: all sources have a common molecular content composed of the simple molecules as well as COMs. The largest differences in the molecular emission are found in the deuterated molecules, S-bearing molecules and the COMs. Towards the warm component, the comparison of the relative molecular abundances shows an emerging component with an average temperature of 100 K. <br /> Finally, based on the unbiased spectral line surveys we elaborate qualitative and quantitative criteria to compare the molecular composition of our objects to that of a sample of hot corinos and radiatively heated hot cores from the literature. The quantitative comparison reveals that our objects and hot corinos have similar molecular abundances in the cold gas phase for O, N- bearing molecules, deuterated molecules and COMs. In the warm gas, hot corinos and our objects have similar relative abundances for O-bearing molecules relative to dimethyl ether while the complex cyanides show remarkably higher abundances by a factor of 10 relative to methyl cyanide compared to hot corinos. A comparison of the sizes and temperatures of the warm gas reveals that our objects have a more compact warm gas phase compared to hot cores with a temperature similar to hot corinos. <br /> Altogether this work explores the molecular reservoir of both the cold and warm gas and the physical properties of the heated gas component from which hot cores emerge. It uncovers an extremely rich chemical reservoir of the star forming gas, where the molecular composition and molecular abundances can be well used to characterize the deeply embedded protostellar object.