  High-resolution mid-infrared observations carried out by the Spitzer Space
Telescope allowed one to resolve the fine structure of many astrospheres. In
particular, they showed that the astrosphere around the B0.7 Ia star kappa Cas
(HD 2905) has a clear-cut arc structure with numerous cirrus-like filaments
beyond it. Previously, we suggested a physical mechanism for the formation of
such filamentary structures. Namely, we showed theoretically that they might
represent the non-monotonic spatial distribution of the interstellar dust in
astrospheres (viewed as filaments) caused by interaction of the dust grains
with the interstellar magnetic field disturbed in the astrosphere due to
colliding of the stellar and interstellar winds. In this paper, we invoke this
mechanism to explain the structure of the astrosphere around kappa Cas. We
performed 3D magnetohydrodynamic modelling of the astrosphere for realistic
parameters of the stellar wind and space velocity. The dust dynamics and the
density distribution in the astrosphere were calculated in the framework of a
kinetic model. It is found that the model results with the classical MRN size
distribution of dust in the interstellar medium do not match the observations,
and that the observed filamentary structure of the astrosphere can be
reproduced only if the dust is composed mainly of big (micron-sized) grains.
Comparison of the model results with observations allowed us to estimate
parameters (number density and magnetic field strength) of the surrounding
interstellar medium.
