SiS line emission as a probe of chemistry and grain formation in circumstellar envelopes of AGB stars
by
Jeanette Bast(SU Fysikum)
→
Europe/Stockholm
FA31
FA31
Description
The stellar interiors of low mass stars are considered to be one of the most important factories in galaxies that are responsible for the formation of more heavy atoms like, e.g., carbon.
During the AGB phase these nucleosynthesised species get transported to the surface of the star where they will be expelled into the interstellar medium through extensive mass loss from the star.
In addition, the CSEs around AGB stars can themself have a rich and complex chemistry.
The various chemical processes occuring in AGB stars are therefore very important to understand as they relate to the chemical evolution of galaxies.
17 AGB stars have been studied in this diploma thesis
project with the help of multi-transition SiS line observations in the radio regime.
An extensive and detailed radiative transfer modelling
has been performed in order to estimate reliable abundances and look for possible trends with stellar parameters such as photospheric C/O ratio and mass loss rates.
The results clearly show that the SiS abundance is sensitive to the C/O-ratio in the photosphere and that
the production of SiS is favoured in the carbon stars relative to the M-type AGB stars.
Also a correlation between the mass loss rate and
the SiS abundance have been found for the carbon stars, corroborating earlier research on the related molecule SiO.
This indicates that SiS molecules are effectively accreted onto dust grains at higher mass loss rates.
The results open for the possibility that CSEs include processes that can protect newly formed
molecules from eventual interstellar photodissociation.
This also means that some of the molecules possibly can be redistributed back into the interstellar medium where they will be able to contribute to the enrichment and evolution of more complex molecules in star and planet forming regions.
