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| Paper: |
Dust Formation in the Inner Wind of the Oxygen-rich AGB Star IK Tau |
| Volume: |
497, Why Galaxies Care about AGB Stars III: A Closer Look in Space and Time |
| Page: |
321 |
| Authors: |
Gobrecht, D.; Cherchneff, I.; Sarangi, A. |
| Abstract: |
The purpose of this study is to understand the processes underpinning
the formation of dust in oxygen-rich AGB stars, reproduce molecular
abundances in the dust-formation zone, and derive dust-to-gas mass
ratios for specific condensates. We model the inner wind of the O-rich
Mira IK Tau by considering layers of gas above the stellar photosphere
that are periodically crossed by pulsation-induced shocks. The formation
of molecules and dust clusters follows a chemical kinetic approach,
which includes the formation pathways to small clusters of silicates,
alumina, and metal oxides. In order to derive grain size distributions,
the dust nucleation phase is coupled to the condensation phase, which
is described by a Brownian formalism.
Our results on molecules agree well with the most recent observations
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.
Alumina grains readily form in the shocked photosphere and the gas
layers just above it. The nucleation of small silicate grains with
forsterite stoichiometry proceeds through a new chemical route
involving the dimerisation of HSiO. These clusters grow and condense
around 4 R* and keep growing over several pulsation periods. The
dust-to-gas mass ratio for silicates is ∼ 2 × 10–3,
in good agreement with values characteristic of O-rich AGB stars. |
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