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Project

The gas-phase chemistry in the circumstellar environments of AGB stars

The Asymptotic Giant Branch (AGB) phase is one of the final evolutionary phases of low- to intermediate mass stars. During this phase, AGB stars lose their mantle to the interstellar medium (ISM) by means of a slow and dense stellar outflow, creating an extended circumstellar envelope (CSE). The composition of the material that is expelled into the ISM is determined by the nucleosynthesis within the AGB star and the chemistry throughout its CSE. The stellar outflow contributes to the chemical enrichment of the ISM, providing part of the building blocks of a new generation of stars. Thanks to their relatively simple thermodynamic structure, CSEs are ideal astrochemical laboratories. The study of the various chemical processes occurring throughout the CSE contributes to astrochemistry in general, as these results are applicable to other astronomical objects with a more complex structure. Moreover, it also adds to our understanding of the CSE itself, since dynamics and chemistry are closely coupled throughout the outflow.

In this thesis, we aim to contribute to our insight in the different chemical processes throughout the CSE by studying its gas-phase chemistry. This is done by using both an observational and a theoretical approach. Using observations, we retrieve the abundances of HCN and SiO throughout the CSE of the oxygen-rich AGB star R Dor. The inner wind abundance of HCN probes the non-thermodynamic equilibrium (TE) character of the chemistry within this region, whereas SiO can condense onto dust grains, tracing dust-gas interactions. The retrieved HCN abundance confirms the importance of non-TE chemistry in the inner wind. The SiO abundance does decrease in the dust formation region, but the underlying chemical process cannot be unambiguously identified. Moreover, these results are complementary to the abundances retrieved in the CSE of IK Tau. Both R Dor and IK Tau are oxygen-rich, but have respectively a low and high mass-loss rate. By comparing the chemical compositions of their CSEs, the sensitivity of the chemistry to the overall density can be determined.

Non-uniform density structures have been widely observed in CSEs. A clumpy density distribution affects the chemistry in two ways: the interstellar ultraviolet (UV) radiation field can penetrate deeper into the wind through porous channels between the clumps, i.e. porosity, and chemistry occurs faster within a density enhancement, i.e. clumping. We improve upon an existing theoretical chemical kinetics model by taking into account the effects of a clumpy outflow on its chemistry. This is done by implementing a porosity formalism, which is a mathematical framework that takes into account both porosity and clumping. This model is used to study the chemical composition in the inner wind. The non-TE character of the chemistry in this region is proposed to be a result of shocks caused by the pulsating AGB star. However, shock-induced non-TE chemical models are not able to produce all observed species. We find that the clumpy nature of the outflow has a large effect on the inner wind abundances, with our clumpy chemical model producing all observed species. To reproduce their observed abundances, a combination of shock-induced non-TE and clumpy models is likely needed. Our clumpy chemical model can also be used to study the outer region of the CSE. We apply the model to the distribution of cyanopolyynes and hydrocarbon radicals within the inhomogeneous CSE of the carbon-rich AGB star IRC+10216. The cyanopolyynes show a radial distribution, where longer carbon-chains are found further out in the outflow. The hydrocarbon radicals are observed to be cospatial, even though their chemical formation pathways are very similar. Despite being well-studied, these distributions are yet to be explained. Previous models have included either the effect of porosity or the effect of clumping. This is the first time that both are taken into account, revealing their relative importance to the chemical formation and destruction pathways of the cyanopolyynes and hydrocarbon radicals. The clumpy outflow model increases our understanding of the chemistry within the CSE of IRC+10216 and paves the way towards a comprehensive model.

Date:1 Oct 2014 →  23 May 2018
Keywords:Astronomy, Astrophysics, Astrochemistry
Disciplines:Astronomy and space sciences
Project type:PhD project