Title: Molecular diagnostics of the circumstellar envelopes of asymptotic giant branch stars: mass loss and chemistry.
Other Titles: Moleculaire diagnostieken van AGB-sterenveloppes: massaverlies en chemie.
Authors: De Beck, Elvire; S0105134
Issue Date: 8-Nov-2011
Abstract: The asymptotic giant branch (AGB) is part of the late evolution of stars with initial masses of 0.8-9 solar masses. It is characterised by a steady wind carrying mass from the stellar surface into the interstellar medium (ISM), creating an extended circumstellar envelope (CSE) of gas and dust around the star. This matter will ultimately enrich the ISM with new chemical elements produced in the stellar interior. The work presented in this thesis is dedicated to the study of molecular gas in the CSEs of AGB stars, by means of detailed radiative-transfer analysis. This is essential in order to constrain and understand the physical and chemical characteristics of the CSEs. The launch of the Herschel Space Observatory (Herschel) on 14 May 2009 has given astronomers access to a wavelength range which is extremely difficult to observe from ground-based observatories. A substantial part of the work presented in this thesis is based on high-quality data obtained with the HIFI instrument, a high-resolution spectrometer on board Herschel.The study of carbon monoxide (CO) provides strong constraints on basic physical parameters of a star and its CSE. I show the strength in analysing CO data and apply it to a large sample of evolved stars, thereby determining the rates at which they lose mass. The star's mass-loss rate is an essential parameter in any study of late stellar evolution on the AGB. I further perform detailed radiative-transfer modelling of other molecules, such as e.g. SiO, HCN, and CCH, leading to a better understanding of the prevalent physical conditions and chemical reactions. The chemical characteristics of the CSE of an AGB star are largely determined by the composition of the central star. Hence, this work contributes new and unique insights into the structure and evolution of stars on the AGB.
ISBN: 978-90-8649-455-2
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Institute of Astronomy

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