Probing the effect of UV radiation on the chemistry in circumstellar envelopes around evolved stars

Licentiate thesis, 2016

Low to intermediate mass (0.8 - 8 M⊙) stars lose a substantial amount of their mass in the last phase of stellar evolution, the asymptotic giant branch (AGB) phase. Hence, they are one of the main suppliers of dust and gas in the interstellar medium and important for the enrichment of galaxies. Due to the intense mass loss, a circumstellar envelope (CSE), rich in gas and dust, forms around AGB stars. The exact mechanism(s) responsible for the mass loss is not yet fully established, although a radiatively driven dust wind appears to play a major role. CSEs are excellent laboratories to understand the mass loss process as well as to constrain stellar properties. It is known that ultra-violet (UV) photons play a critical role in the chemical composition and evolution of the CSEs. The UV photons that can impact the chemistry of the CSE originate, in the inner part, from stellar chromospheric activity or hot binary companions. In the outer part they come from the interstellar radiation field (ISRF). However, the ISRF is the only UV source which has been con- sidered to affect the photo-chemistry of the CSEs. It was long expected that the stellar UV emission is weak and that its effects would not be noticeable, especially not using single-dish telescope observations. Now, high-spatial resolution interferometric observa- tions and increasing number of detections of stellar UV sources indicate the importance of considering the internal UV radiation in the CSEs chemistry. In order to study this effect, we have started an analysis of the carbon-type AGB star, R Scl. Previous studies show a variation of the 12CO/13CO ratio in the circum- stellar envelope and a discrepancy between the circumstellar 12CO/13CO ratio and the photospheric 12C/13C ratio, which has been explained as a result of internal UV radi- ation. Our detailed radiative transfer modelling of H12CN and H13CN isotopologues around R Scl confirms this proposed scenario. This result highlights the importance of considering all the potential UV sources on the chemistry of the CSE of R Scl. Applying our suggested method of quantifying the effect of the internal UV radiation field to a large number of AGB stars might help to investigate the binarity rate of AGBs. This will also improve our understanding of the stellar chromospheric activity. These are both open questions in the field of research on AGB stars.