Physics
Scientific paper
Jun 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt........19c&link_type=abstract
Ph.D. Thesis, University of Amsterdam
Physics
36
Agb Stars, Post-Agb Stars, Molecules, Dust, Infrared, Iso, Mineralogy
Scientific paper
The first part of this thesis analyses the ISO/SWS spectra of a sample of oxygen-rich AGB stars with optically thin dust shells. We present an inventory of the molecular bands in the 2.3 -- 10 micron region (CO, CO2, SiO, OH, water vapour and SO2) and use simple models to characterize these bands. The various molecular bands originate from different layers in the circumstellar environment, making them a very useful tool to probe the temperature and density structure in the innermost circumstellar layers. Water vapour plays an essential role, as the Mira variables in our sample are shown to be surrounded by a very optically thick layer of water vapour. The infrared molecular templates used to model these molecular bands are presented in an appendix.
We also present an inventory of the dust features in the 8-25 micron region of the same sample of O-rich AGB stars. We tentatively identify simple oxides (alumina, MgFeO) as carriers for the 11 and 19.5 micron features respectively. We discuss the apparent sequence in the dust spectra from simple oxides to silicates in the framework of the theoretical dust condensation sequence.
The second part of this thesis deals with the enigmatic post-AGB star HR 4049. In the ISO/SWS spectrum of this star, emission bands of various CO2 isotopomers indicate highly anomalous oxygen isotope ratios in the circumbinary disk surrounding the system, suggesting that the binary nature of this system might play an important role in nucleosynthesis. The Spectral Energy Distribution of HR 4049 shows an infrared excess which can be fitted from the near-IR to the submm by a single-temperature blackbody. We show that such an infrared excess can only be explained if the circumbinary disk is extremely optically thick.
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