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

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.