Studies of the Envelopes of Evolved Stars using Millimetre and Submillimetre Spectroscopy

Abstract

Asymptotic giant branch (AGB) stars are low- to intermediate-mass stars at the end of their life. At this stage of stellar evolution, these objects shed large amounts of matter in the form of gas and dust, forming a circumstellar envelope around them and enriching the interstellar medium. This thesis presents the work where we probe these envelopes using carbon-bearing species, the C atom, and various molecules, with observations from different single-dish telescopes. We use the rotational lines of the HCN molecule, arising from its ground and vibrationally excited states, to study the inner and hotter parts of the envelope, whereas fine-structure lines of atomic and ionised carbon (C⁰ and C⁺) are used to constrain their distribution in the envelope of the archetypal AGB star, IRC +10216. Furthermore, we focus on AGB stars as a population and analyse 445 stars as part of the Nearby Evolved Stars Survey (NESS) collaboration. We observed transitions of the CO molecule from multiple rotational states to derive their physical parameters along with mass-loss rates. We explore the statistical distributions of the inferred stellar parameters and demonstrate how the statistical analysis of a stellar population confirms and extends our understanding of its contribution to the cosmic cycle of matter.