摘要：Context. The formation of dust in winds of cool and highly evolved stars
and the rate of injection of dust into the interstellar medium is not yet completely
understood, despite the importance of the process for the evolution of stars and galaxies.
This holds in particular for oxygen-rich stars, where it is still not known which process
is responsible for the formation of the necessary seed particles of their silicate dust.
Aims. We study whether the condensation of silicate dust in Mira
envelopes could be caused by cluster formation by the abundant SiO molecules.
Methods. We solve the dust nucleation and growth equations in the
co-moving frame of a fixed mass element for a simplified model of the pulsational motions
of matter in the outer layers of a Mira variable, which is guided by a numerical model for
Mira pulsations. It is assumed that seed particles form through the clustering of SiO. The
calculation of the nucleation rate is based on published experimental data. The quantity
of dust formed is calculated via a moment method and the calculation of radiation pressure
on dusty gas is based on a dirty silicate model.
Results. Dust nucleation occurs in the model at the upper culmination of
the trajectory of a gas parcel where it stays for a considerable time at low temperatures.
Subsequent dust growth occurs during the descending part of the motion and continues after
the next shock reversed motion. It is found that sufficient dust forms that radiation
pressure exceeds the gravitational pull of the stars such that the mass element is finally
driven out of the star.
Conclusions. Nucleation of dust particles by clustering of the abundant
SiO molecules could be the mechanism that triggers silicate dust formation in Miras.
关键词：circumstellar matter;stars: mass-loss;stars: winds,
outflows;stars: AGB and post-AGB;solid state: refractory
