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D. Gobrecht - 15 ottobre 2014, h.10.30 Stampa E-mail
martedì 30 settembre 2014
 

"Dust Formation in AGB stars"

 

Dr. David Gobrecht

Universita' di Basilea

 

 Abstract:

The purpose of the study is to understand the formation processes underpinning the formation of dust in oxygen-rich AGB stars, reproduce molecular abundances in the dust formation zone, and derive dust mass yields for specific condensates. We model the inner winds O-rich AGB star at different evolutionary stages  by considering layers of gas above the stellar photosphere that are periodically crossed by pulsation-induced shocks. These gas layers experience a wide range of temperatures and number densities and are still gravitationally bound to the star. The formation of molecules and dust clusters follows a chemical kinetic approach, and is described by a chemical network, which includes the formation pathways to small clusters of silicates (e.g., enstatite, MgSiO3, and forsterite Mg2SiO4), clusters of alumina (Al2O3), and metal oxides. The dust nucleation phase is then coupled to the condensation phase, described by a Brownian formalism, to derive grain size distributions and mass yields. Our results for molecules (CO, H2O, SiO, SO,  SiS, PN) agree well with the most recent observations with Herschel, and confirm the crucial role of shocks in the making of carbon-rich molecules, such as HCN, CO2 and CS, in O-rich AGB inner winds. The nucleation of small silicate grains with forsterite stoichiometry proceeds through a new chemical route involving the dimerization of HSiO. These clusters grow and condense around 4R*. They keep growing over several pulsation periods and shock passages as the gas layers move to larger radii. At 5R*, 65% of silicate clusters condense into grains of average radius 40 Angstroems, while at 8R*, 95% of all silicate clusters have condensed to form small grains with average radius  100 Angstroems. Our final dust-to-gas mass ratio for silicates is 2 x 10^(-3), and in good agreement with values characteristic of O-rich AGB stars (10^(-3)). In a clumpy wind test model with a density increased by a factor of 10 we derive a silicate dust-to-gas mass ratio of  6 x 10^(-3) with average radius of 0.1 microns. Our results indicate that the wind acceleration due to silicate grains will take place at radii larger than 5R*, in good agreement with recent observations of CO and HCN emission lines.

 

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