Dust production in Thermally Pulsing Asymptotic Giant Branch Stars

In this thesis I discuss the dust chemistry and growth in the circumstellar envelopes (CSEs) of Thermally Pulsing Asymptotic Giant Branch (TP-AGB) stars computed with the COLIBRI code at different initial metallicities (Z=0.001, 0.008, 0.02, 0.04, 0.06) and stellar masses. I adopt a formalism of dust growth coupled with a stationary wind for both M and C-stars CSEs. In the original version of this formalism, the most efficient destruction process of silicate dust in M-giants is chemisputtering by H2 molecules. For these stars I find that dust grains can only form at relatively large radial distances (r ~ 5 Rstar), where they cannot be efficiently accelerated, in agreement with other investigations. In the light of recent laboratory results, I also consider the alternative case that the condensation temperature of silicates is determined only by the competition between growth and free evaporation processes (i.e. no chemisputtering). With this latter approach I obtain dust condensation temperatures that are significantly higher (up to Tcond~1400 K) than those found when chemisputtering is included (Tcond~900 K), and in better agreement with condensation experiments. As a consequence, silicate grains remain stable closer to the stellar photosphere (r~2 Rstar), where they rapidly grow and are efficiently accelerated. With this modification, the models nicely reproduce the observed trend between terminal velocities and mass-loss rates of Galactic M-giants. For C-stars the formalism is based on the homogeneous growth scheme where the key role is played by the carbon over oxygen excess. The models reproduce fairly well the terminal velocities of Galactic stars and there is no need to invoke changes in the standard assumptions. At decreasing metallicity the carbon excess becomes more pronounced and the efficiency of dust formation increases. This trend could be in tension with recent observational evidence in favour of a decreasing efficiency, at decreasing metallicity. If confirmed by more observational data, it would indicate that either the amount of the carbon excess, determined by the complex interplay between mass loss, third dredge-up and hot bottom burning, or the dust growth scheme should be revised. This comparison also shows that the properties of TP-AGB CSEs are important diagnostic tools that may be profitably added to the traditional calibrators for setting further constraints on this complex phase of stellar evolution. I first compute the dust ejecta integrated along the all TP-AGB phase at solar and sub-solar metallicities (Z=0.001, 0.008, 0.02) comparing the results obtained with the two formalisms (with and without chemisputtering) and with other results in the literature. I find that, in spite of the differences in the expected dust stratification, for a given set of TP-AGB models, the ejecta are only weakly sensitive to the specific assumption. On the other hand, the results highly depend on the adopted TP-AGB models. I thus extend this formalism to the case of super-solar metallicity stars considering the preferred scheme for dust growth: for M-stars, I neglect chemisputtering by H2 molecules and, for C-stars, I assume the homogeneous growth scheme. At super-solar metallicity, dust forms more efficiently than in the solar and sub-solar cases and silicates tend to form at significantly inner radii, and thus at higher temperatures and densities, than at solar and sub-solar metallicity values. In such conditions, the hypothesis of thermal decoupling between gas and dust becomes questionable and dust heating due to collisions become important. This heating mechanism delays dust condensation to slightly outer regions in the circumstellar envelope. By calculating the dust ejecta at super-solar metallicities I find that the main dust products metallicities are silicates. I finally present the total dust-to-gas ejecta for different values of the stellar initial masses and for all the metallicity values considered: Z=0.001, 0.008, 0.02, 0.04, 0.06, finding that the total dust-to-gas ejecta of intermediate-mass stars are much less dependent on the metallicity than what is usually assumed.