At the end of their main sequence lifetimes, stars similar to our sun will ascend the asymptotic giant branch (AGB) of the H-R diagram. The AGB phase is a period of intense mass-loss as the star ejects its outer layers. The ejecta go on to form molecules and condense into dust in a circumstellar envelope. The composition of the dust and the molecular chemistry of the circumstellar envelope depend on the chemical make-up of the star, and most specifically on the carbon to oxygen ratio.
We will be presenting our research on S type AGB stars, which have C/O close to 1. These stars are in a period of transition from oxygen rich to carbon rich and exhibit some characteristics of both types. We have conducted detailed radiative transfer modelling of several S stars based on observations obtained from Herschel/HIFI as part of the HIFISTARS and SUCCESS guaranteed time key projects and will present our results.
By studying the mass-loss properties and circumstellar environments of AGB stars we can estimate the extent to which AGB stars contribute to the chemical enrichment of the ISM.