UV Chemistry in the Circumstellar Envelopes of Evolved Stars
Doctoral thesis, 2019

Low- and intermediate-mass stars eject a substantial amount of their material into space during a late phase of stellar evolution, the asymptotic giant branch (AGB) phase. Therefore, they impact the chemistry of the interstellar medium. Due to the intense mass loss, a circumstellar envelope (CSE), rich in gas and dust, forms around the AGB star. Observations of molecular species and dust content in CSEs help us to broaden our knowledge on late phases of stellar evolution, mass-loss processes, the CSE chemistry, and the stellar properties.

For instance, observations of carbon monoxide (CO) have been extensively used to determine the mass-loss rate and the overall CSE properties. Ultraviolet (UV) photodissociation of CO from the interstellar radiation field (ISRF) is the dominant process that determines the CO distribution and extent in CSEs. Therefore, a precise calculation of the CO photodissociation rate is crucial to determine the mass-loss rates. Subsequently, the value adopted for the mass-loss rate in further modelling of the CSEs will affect the abundances derived for all other molecules. Thus, an estimation of the CO photodissociation rate affects the estimates of the amount of all the recycled material. In this thesis, we present the most updated calculations of the depth dependency of the CO photodissociation rate in CSEs using the latest laboratory measurements.

Generally, it is well known that UV radiation impacts the CSE chemistry and the influence of UV radiation from the ISRF has been considered in the models of CSEs. However, there has been little discussion on the impact of internal sources of UV radiation. Recent Galaxy Evolution Explorer observations reveal the presence of strong internal UV radiation for a large sample of AGB stars. The internal UV radiation can originate from stellar chromospheric activity, a hot binary companion, and/or accretion of matter between two stars in a binary system. This thesis seeks to address the impact of both the internal and external sources of UV radiation on the CSE chemistry.

To trace the impact of UV radiation, we present two approaches. First, observations of the main UV photodissociation and photoionization products, such as CI and CII. We present, for the first time, detections of CI around a UV-bright oxygen-rich AGB star, omi Ceti. In the second approach, we investigate the isotopologue ratio of molecules with different photodissociation mechanisms. We expect variations in the isotopologue ratio of molecules that dissociate through lines. However, there should not be any variation by UV radiation in the isotopologue ratio of molecules with continuum dissociation.

Astrochemistry – molecular processes – stars: abundances – AGB – binaries – circumstellar matter – chromospheres – ultraviolet

Room EA, 4th floor, Hörsalsvägen 11, Chalmers University of Technology
Opponent: Prof. Albert Zijlstra, Jodrell Bank Centre for Astrophysics, The University of Manchester, UK


Maryam Saberi

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics

(HCN)-C-12 and (HCN)-C-13 excitation analysis in the circumstellar outflow of R Sculptoris

Astronomy and Astrophysics,; Vol. 599(2017)

Journal article

Detection of CI line emission towards the oxygen-rich AGB star omi Ceti

Astronomy and Astrophysics,; Vol. 612(2018)

Journal article

Photodissociation of CO in the outflow of evolved stars

Astronomy and Astrophysics,; Vol. 625(2019)

Journal article

M. Saberi, W. H. T. Vlemmings, E. De Beck, and H. Olofsson - CO and HCN isotopologue ratios in the outflows of evolved stars

Sun-like stars evolve towards the Asymptotic Giant Branch (AGB) phase, where they eject a substantial amount of heavy chemical elements to the interstellar medium (ISM), created through stellar nucleosynthesis in the stellar interior. Therefore, AGB stars are among the main contributors to the enrichment of the ISM. A large fraction of essential elements necessary for the formation of rocky planets, life in the universe, and the human body are created in the deep layers of AGB stars.

A circumstellar envelope (CSE) rich in molecular gas and dust will be formed as a consequence of an intense mass loss during the AGB phase. Observations of molecular species and dust particles in the CSEs are crucial to broaden our understanding of the late phases of the stellar evolution, the physical processes of the mass-loss, the chemistry active in the CSEs, and the amount of matter returned to the ISM.

In this thesis, we have used state-of-the art observational facilities to observe several chemical species in the CSEs around AGB stars at submillimetre wavelengths. We performed astrochemical and radiative transfer analysis to model the observational data and improve the current models of the CSEs chemistry.

Subject Categories

Astronomy, Astrophysics and Cosmology


Onsala Space Observatory



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4614



Room EA, 4th floor, Hörsalsvägen 11, Chalmers University of Technology

Opponent: Prof. Albert Zijlstra, Jodrell Bank Centre for Astrophysics, The University of Manchester, UK

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8/5/2019 1