Supervisor: Josef Hron (Department of Astrophysics) in collaboration with Franz Kerschbaum (Department of Astrophysics), Claudia Paladini (ESO, Chile), and Susanne Höfner (Uppsala University)

Funding Situation: potentially via a VISESS PhD fellowship

Project outline: Low-mass and intermediate-mass stars on the asymptotic giant branch (AGB) are important contributors to the enrichment of the ISM with heavy elements and dust. In particular, Carbon is dredged-up from deeper zones with Helium-burning which may lead to an overabundance of C with respect to Oxygen. Therefore, the cool atmospheres of these so-called Carbon stars are dominated by C-rich molecules like C2, CN, C2H2 or HCN. C2H2 is of special relevance as it is considered to be the main building block of (amorphous) carbonaceous stardust. Understanding the formation of this type of dust, its effect on the structure of the atmosphere and its importance for ISM enrichment requires studies of the abundance and spatial distribution of C2H2. This molecule has its strongest signatures in the thermal IR (beyond 2mm), a wavelength region which is now well accessible via ground-based spectroscopy and interferometry thanks to instruments like CRIRES+ and MATISSE at the ESO VLT(I). The pronounced stellar pulsations of AGB stars and the complex opacities due to molecules and dust pose a major challenge for modelling the atmospheres but this challenge is currently tackled within the ERC-grant EXWINGS (PI Susanne Höfner). The aim of the PhD project is to acquire and model spectra and interferometric data for representative Carbon stars in order to deduce the spatial stratification of C2H2 , and thus get a solid picture of the processes acting in the atmospheres of Carbon stars and their importance for stellar and galactic chemical evolution. We are partners in MATISSE as well as in EXWINGS and thus have access to all the necessary data and expertise for this project. Furthermore. the collaboration with Claudia Paladini offers the possibility to extend/complement the project with an ESO-studentship to Chile.