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Molecule and dust synthesis in the inner winds of oxygen-rich AGB stars

Gobrecht, David Leon. Molecule and dust synthesis in the inner winds of oxygen-rich AGB stars. 2015, PhD Thesis, University of Basel, Faculty of Science.

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Official URL: http://edoc.unibas.ch/diss/DissB_11980

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Abstract

This thesis aims to explain the masses and compositions of prevalent molecules, dust clusters, and dust grains in the inner winds of oxygen-rich AGB stars. In this context,
models have been developed, which account for various stellar conditions, reflecting all the evolutionary stages of AGB stars, as well as different metallicities. Moreover, we aim to gain insight on the nature of dust grains, synthesised by inorganic and metallic clusters
with associated structures, energetics, reaction mechanisms, and finally possible formation
routes. We model the circumstellar envelopes of AGB stars, covering several C/O ratios below unity and pulsation periods of 100 - 500 days, by employing a chemical-kinetic
approach. Periodic shocks, induced by pulsation, with speeds of 10 - 32 km/s enable
a non-equilibrium chemistry to take place between 1 and 10 R* above the photosphere.
The various models include the well-studied, galactic Mira variables like IK Tau and TX
Cam, galactic S-stars, semi-regular variables of type SRa and SRb, as well as Mira stars
of lower metallicity in the Magellanic clouds. In addition, we perform quantum-chemical
calculations on the Density Functional Theory (DFT) level for several alumina and silicate
clusters, in order to obtain structures, electronic properties, and infrared (IR) spectra of the
potential dust components.
The results for the gas phase agree well with the most recent observational data for IK Tau
and VY CMa. Major parent molecules form in the shocked gas under non-equilibrium
conditions and include CO, H2O, SiO, SiS, SO and SO2, as well as the unexpected carbon-bearing
species HCN, CS and CO2, and the recently detected phosphorous species PO and
PN. In the galactic models, small alumina clusters form and condense efficiently close to
the star. In the case of galactic Miras, silicate clusters with forsterite mineralogy form and
coalesce around 4 R*. In the lower metallicity and semi-regular models, the dust formation
is hampered by the unavailability of the critical elements (Si and Al), low densities, and
high temperatures. The dust/gas mass ratio ranges from 10^(-9) to 10^(-5) for alumina, and from
10^(-6) to 10^(-3) for forsterite, and agrees with the dust-to-gas mass ratio derived for oxygen-rich
AGB stars.
For the first time, a complete non-equilibrium model - including gas phase chemistry, cluster
growth and dust formation - is built up self-consistently, and explaining successfully the
most recent observations.
Advisors:Thielemann, F.-K. and Cherchneff, Isabelle and Talbi, Dahbia
Faculties and Departments:05 Faculty of Science > Departement Physik > Physik > Theoretische Physik Astrophysik (Thielemann)
Item Type:Thesis
Thesis no:11980
Bibsysno:Link to catalogue
Number of Pages:1 Online-Ressource (117 Seiten)
Language:English
Identification Number:
Last Modified:28 Dec 2016 11:33
Deposited On:28 Dec 2016 11:33

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