Ramlal Unnikrishnan Nair,

Stars of low to intermediate initial masses (~0.8 - 8 M_⊙) enter the asymptotic giant branch (AGB) phase in their late evolution. The intense mass loss during the AGB forms an extended circumstellar envelope (CSE) of chemically rich gas and dust around the star. These stars also form heavy elements via the s-process, and contribute substantially to the chemical enrichment of the interstellar medium (ISM). Carbon-rich AGB stars present more chemically complex CSEs than their oxygen-rich counterparts. The chemical characterisation of carbon-star CSEs is crucial for unravelling the astrochemical networks that form complex molecules and dust. However, most of our current knowledge of the circumstellar chemistry in carbon-stars is based on observations and models of a single object, IRC+10 216, often regarded as the archetype carbon-star. To address this lack of sufficient sampling evident in the literature, this thesis presents the first spatially resolved, unbiased spectral survey of circumstellar molecular line emission from carbon-stars other than IRC +10 216, revealing their complex morphologies and chemical contents. We derive the emitting region sizes and LTE fractional abundances for a large number of molecular species. We further employ non-LTE radiative transfer models, constrained using a large number of interferometric and single-dish observations, to determine both the physical and chemical characteristics of these envelopes. We derive the circumstellar abundance distributions of various molecular species (CS, SiO, SiS), and compare these to IRC +10 216, informing state-of-the-art chemical models as well. The large amount of observational constraints available also helps illuminate the limitations inherent to such modelling. Overall, this work leverages the high sensitivity and angular resolution of the ALMA interferometer, combined with detailed radiative transfer and chemical models, to provide a broader, more generalised understanding of the chemistry in carbon-star CSEs, extending beyond IRC +10 216.