  The properties and the evolution of asymptotic giant branch (AGB) stars are
strongly influenced by their mass loss through a stellar wind. This is believed
to be caused by radiation pressure due to the absorption and scattering of the
stellar radiation by the dust grains formed in the atmosphere. The optical
properties of dust are often estimated using the small particle limit (SPL)
approximation, and it has been used frequently in modelling AGB stellar winds
when performing radiation-hydrodynamics (RHD) simulations. We aim to
investigate the effects of replacing the SPL approximation by detailed Mie
calculations of the size-dependent opacities for grains of amorphous carbon
forming in C-rich AGB star atmospheres. We performed RHD simulations for a
large grid of carbon star atmosphere+wind models with different effective
temperatures, luminosities, stellar masses, carbon excesses, and pulsation
properties. Also, a posteriori radiative transfer calculations for many radial
structures (snapshots) of these models were done, resulting in spectra and
filter magnitudes. We find, when giving up the SPL approximation, the wind
models become more strongly variable and more dominated by gusts, although the
average mass-loss rates and outflow speeds do not change significantly; the
increased radiative pressure on the dust throughout its formation zone does,
however, result in smaller grains and lower condensation fractions (and thus
higher gas-to-dust ratios). The photometric K magnitudes are generally
brighter, but at V the effects of using size-dependent dust opacities are more
complex: brighter for low mass-loss rates and dimmer for massive stellar winds.
Given the large effects on spectra and photometric properties, it is necessary
to use the detailed dust optical data instead of the simple SPL approximation
in stellar atmosphere+wind modelling where dust is formed.
