Presentasjon av masteroppgave: Marte Cecilie Wegger

Probing the impact of UV radiation on the isotopic chemistry in the outflow of AGB stars


Low- and intermediate-mass stars (0.8-8 M⊙) lose mass in a late stage of their evolution, called the asymptotic giant branch (AGB) phase. The mass and dust ejected into space enriches the interstellar medium and creates a circumstellar envelope (CSE) around the AGB star. We want to study CSEs in order to gain a better understanding of their chemical composition, but they are challenging to model precisely. The 12CO/13CO isotopologue ratio in CSEs has been used as a tracer for the photospheric 12C/13C ratio in CSEs, which is a good tracer of the stellar nucleosynthesis, but it might not be a reliable tracer for the 12C/13C ratio in UV irradiated regions as it is affected by UV radiation. We want to test if the H12CN/H13CN abundance ratio, a ratio which is not affected by UV radiation, is a better tracer for the 12C/13C ratio than the 12CO/13CO ratio. In this thesis, we model CSEs for two UV active AGB stars, W Hydrae and R Leporis. We perform radiative transfer analysis of observed rotational transitions for H12CN and H13CN lines by use of accelerated lambda iterations, fit the observed data to the calculated models, and measure the H12CN/H13CN isotopologue ratios using initial abundance values provided from a good fit between model and data. For carbonrich R Leporis, we find that the H12CN/H13CN abundance ratio is consistent with the previously reported 12C/13C ratio and show a small discrepancy with the previously reported 12CO/13CO ratio. This result suggests that the H12CN/H13CN ratio can be a more reliable tracer for the C ratio in UV dominated CSEs for carbon-rich AGB stars than the previous commonly used CO ratio. For oxygen-rich W Hydrae, our analysis predicts an H12CN/H13CN ratio consistent with the previously reported 12C/13C and 12CO/13CO ratios. In future work, we suggest observing several oxygen-rich UV active AGB stars and creating a larger sample of HCN ratios. By looking at the overall trend in the comparison between HCN ratios and CO and C ratios, we hope to increase our understanding of the impact of UV radiation on the isotopic chemistry in CSEs for oxygen-rich AGB stars.

This is a hybrid examination. Participants can join in person (Peisestua 304) or by zoom link. 



Associate Professor Sven Wedemeyer, Institute of Theoretical Astrophysics, UiO (main-supervisor)

Postdoctoral Fellow Maryam Saberi, Institute of Theoretical Astrophysics, UiO (co-supervisor)


Intern. assessor: Professor Boris V. Gudiksen, Institute of Theoretical Astrophysics, UiO

Extern. assessor: Postdoctoral researcher Holly Andrews, Department of Space, Earth and Environment, CTH

Publisert 18. mai 2022 13:33 - Sist endret 14. juni 2022 09:30