A 3-D radiative transfer analysis of AGB winds

Abstract

The chemical enrichment of the interstellar medium is driven by stars of low to intermediate masses shedding their external layers through powerful stellar winds. Eventually, after many years in this phase, the core of the star ionises these expelled layers and becomes a planetary nebula, where the shape and morpology of the surrounding gas was imprinted during the previous mass loss phase of its life. In this project, two aims are established. The first, to test two 3-D radiative transfer codes, MOLLIE and LIME, which were written for cold molecular clouds in their ability to model the conditions present in stellar winds of these asymptotic giant branch stars. The second is then using a code to model observational data of such a star. Two models were constructed for this work in accordance with the two aims established. The first model is a representation of the conditions present within the circumstellar envelope of an asymptotic giant branch star while the second is a model of the star IK Tauri, using literature and the code itself to constrain the parameters for accurately modelling the out flow. Both models use carbon monoxide as the rotational molecule due to its proliferation throughout stars and that it closely follows hydrogen lines while also being much visible than hydrogen itself. From the analysis of the test model, we determined MOLLIE unfit for the purpose of this work, so further modelling was carried out by LIME alone. With LIME we obtained accurate fits to the IK Tauri data for each rotational line, isotopolgue, and resolution studied. This ver ifies that the relatively simple model could produce line profiles supported by observational data while being less complex in comparison to 1-D radiative transfer models for similar results.