Discovery and Morphology of Complex Molecules toward Interstellar Molecular Clouds

Up to now, more than 250 individual molecular species have been detected toward both interstellar and circumstellar mediums with astronomical facilities. This dissertation presented two cases of investigations focusing on the chemical composition as well as the molecular spatial distributions of complex astronomical molecules in interstellar clouds, the chemistry in which sets the initial conditions of the chemical processes in the late stages of star formation.

We leveraged the GOTHAM high-sensitivity and high-spectral-resolution survey as well as cutting edge spectral line stacking and match filtering techniques to conduct molecular census towards the Taurus molecular cloud-1 (TMC-1). TMC-1 is a prototypical object of study in the quest for an understanding of dark cloud chemistry. The large observational program GOTHAM (Green Bank Telescope Observations of TMC-1: Hunting for Aromatic Molecules) is undertaking a radio survey with a combined observation time of ~850 hours directed toward TMC-1. The GOTHAM project has provided extremely sensitive data at a sensitivity of 2 mK and a spectral resolution of 0.05 km/s, which is essential for searching complex interstellar molecules. Our work has led to the first astronomical detections of numerous new complex molecules and greatly expanded the molecular inventory in the interstellar medium. Of the newly detected species, this dissertation specifically presents the discoveries of two unsaturated carbon-chain molecules, isocyanodiacetylene (HC4NC) and thiopropadienone (H2C3S) in space. These studies provided new mechanistic insights into CN-vs-NC chemistry as well as the sulfur evolution under dark-cloud conditions.

An innovation of this dissertation is the development of a quantitative data-filtering method for molecular line identification and multiple-transition imaging to represent the intrinsic spatial distribution of a molecule. Guided by these methods, the morphological investigations of terrestrial-type molecules were composed of three projects directed toward two massive molecular clouds, namely Sagittarius (Sgr) B2 and NGC 6334I. Sgr B2(N) is the richest known source for molecular detections and has contributed to 69 new interstellar molecules, (27%) of the interstellar molecular census. The first project revealed the spatial differentiation of the C2H4O2 isomers towards Sgr B2(N), which demonstrated the distinct morphology can help to constrain the formation mechanisms and the chemical relations of the isomeric molecules. The second project, focusing on precisely locating the emission region of acetic acid with a wide range of excitation energy toward NGC 6334I, uncovered the dependence of the molecular emission peak positions on the excitation levels and brought to light the potential unresolved sub-structures of the source. In synergies with VLA observations, the last project performed a multi-wavelength investigation for the origin, distribution, relationships of molecular masers toward Sgr B2(N).