Charting Circumstellar Chemistry of Carbon-rich AGB Stars

Licentiate thesis, 2023

Stars of low to intermediate initial masses (0.8 - 8 M⊙) enter the asymptotic giant branch (AGB) phase during their late evolution. This phase is characterised by intense mass loss from the stellar surface into the interstellar medium (ISM), eventually leading to the formation of an extended circumstellar envelope (CSE) composed of the ejected gas and dust around the star. These stars are also the birthplace of many heavy elements. AGB stars thus contribute heavily to the chemical replenishment of the ISM in galaxies, by enriching it with the nuclear-processed material dredged up from the stellar interiors. This makes AGB CSEs very interesting cosmic chemical laboratories, sites of diverse, and often complex chemistry.
 
Based on the relative abundances of carbon and oxygen in their atmospheres, AGB stars are categorised into C-rich and O-rich, the carbon stars being more chemically complex than their oxygen-rich counterparts. The study of molecular line emission from AGB CSEs is of particular interest, as it can help constrain both the physical and chemical characteristics of the envelopes. However, much of our current knowledge of AGB circumstellar chemistry, particularly of the C-rich type, is based on observations and models of a single object, IRC +10 216, which is often regarded as an archetypical carbon star. Advances in instrumentation, including the development of high angular resolution interferometers, have opened up possibilities for observational studies of additional sources in unprecedented detail.
 
This thesis summarises our current understanding of the chemistry in C-rich CSEs, and presents the first spatially-resolved, unbiased spectral surveys of the circumstellar molecular emission from multiple carbon stars other than IRC +10216. ALMA band 3 spectral surveys of three C-rich AGB stars reveal the morphological and chemical complexity of their CSEs. We compare the results obtained from these surveys with those of IRC +10216, and discuss their implications for the generalised understanding of the chemistry in carbon star CSEs. By obtaining well-constrained estimates of the emission region sizes and circumstellar abundances of a variety of molecular species, this work aims to provide updates to existing chemical models, and put to test the archetype status attributed to IRC +10216 in the literature.