UV Chemistry in the Circumstellar Envelopes of Evolved Stars
Doctoral thesis, 2019
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
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics
(HCN)-C-12 and (HCN)-C-13 excitation analysis in the circumstellar outflow of R Sculptoris
Astronomy and Astrophysics,; Vol. 599(2017)
Detection of CI line emission towards the oxygen-rich AGB star omi Ceti
Astronomy and Astrophysics,; Vol. 612(2018)
Photodissociation of CO in the outflow of evolved stars
Astronomy and Astrophysics,; Vol. 625(2019)
M. Saberi, W. H. T. Vlemmings, E. De Beck, and H. Olofsson - CO and HCN isotopologue ratios in the outflows of evolved 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.
Astronomy, Astrophysics and Cosmology
Onsala Space Observatory
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4614
Chalmers University of Technology
Room EA, 4th floor, Hörsalsvägen 11, Chalmers University of Technology
Opponent: Prof. Albert Zijlstra, Jodrell Bank Centre for Astrophysics, The University of Manchester, UK