In their twilight years, solar-like stars in the asymptotic giant branch (AGB) phase enrich the interstellar medium (ISM), the space between the star,s with fresh material (gas and dust) for new stars and planets. AGB stars lose their outer layers to the ISM through a stellar outflow or wind, forming an extended circumstellar envelope (CSE). The wind is thought to be dust-driven, with dust grains forming close to the star. State-of-the-art observations have revealed the composition of the inner CSE, allowing the first identification of gas-phase seeds for dust grains, and the presence of disks around AGB stars. Despite major knowledge gains over the past three decades, it is still not fully understood how dust forms, grows, and drives the stellar wind, limiting our understanding of both stellar evolution and the chemical enrichment of the ISM. Moreover, the structure and chemistry of AGB disks is unknown; if similar to protoplanetary disks, second generation planet formation around old stars may be possible therein.
Solving these puzzles requires new and sophisticated models that connect dust formation with chemistry and couple gas and dust chemistry throughout the wind and in the disk. With this project, we have quantified the organic refractory coating of interstellar dust grains delivered by AGB outflows to the ISM. This was done by merging previous model developments into the most comprehensive chemical model of an AGB outflow so far: the model can account for the effects of a clumpy outflow and the presence of a companion star and includes an extensive dust-gas chemical network. We find that the organic refractory coating of dust is significantly affected by these factors, potentially leading to a surface coverage of several monolayers. This will affect the sticking/coagulation properties of this dust, as well as its efficiency in absorbing and reflecting stellar light. These theoretical results guide future observations to test their predictions.
We also developed the first chemical model of an AGB disk. To do so, we retrieved the density and temperature profile of the disk from archival observations. This new physical model was necessary as the results of a model available in the literature could not be reproduced. It also allowed for the fellow to be trained in working with interferometric observations and retrieval using three-dimensional radiative transfer models, expanding her skillset. In an upcoming publication, the unique chemistry of the AGB disk will be discussed and compared to that of PPDs. Both the physical and chemical model will be made publicly available.
At the University of Leeds, the fellow was able to continue her collaboration with theoretical and experimental chemists. This led to a currently ongoing project where dust clustering reactions are included in the chemical network, the initial step in developing the first chemical model that is applicable to the entire AGB outflow, from stellar surface to where the outflow merges with the ISM.
