This work is part of an ongoing effort aiming at identifying the actual
wind-drivers among the dust species observed in circumstellar envelopes. In
particular, we focus on the interplay between a strong stellar radiation field
and the dust formation process. To identify critical properties of potential
wind-driving dust species we use detailed radiation-hydrodynamical models which
include a parameterized dust description, complemented by simple analytical
estimates to help with the physical interpretation of the numerical results.
The adopted dust description is constructed to mimic different chemical and
optical dust properties in order to systematically study the effects of a
realistic radiation field on the second stage of the mass loss mechanism. We
see distinct trends in which combinations of optical and chemical dust
properties are needed to trigger an outflow. Dust species with a low
condensation temperature and a NIR absorption coefficient that decreases
strongly with wavelength will not condense close enough to the stellar surface
to be considered as potential wind-drivers. Our models confirm that metallic
iron and Fe-bearing silicates are not viable as wind-drivers due to their
near-infrared optical properties and resulting large condensation distances.
TiO2 is also excluded as a wind-driver due to the low abundance of Ti. Other
species, such a SiO2 and Al2O3, are less clear-cut cases due to uncertainties
in the optical and chemical data and further work is needed. A strong candidate
is Mg2SiO4 with grain sizes of 0.1-1 micron, where scattering contributes
significantly to the radiative acceleration, as suggested by earlier
theoretical work and supported by recent observations.Comment: 15 pages, 12 figure
