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High-resolution mid-infrared observations carried out by the Spitzer SpaceTelescope allowed one to resolve the fine structure of many astrospheres. Inparticular, 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 filamentsbeyond it. Previously, we suggested a physical mechanism for the formation ofsuch filamentary structures. Namely, we showed theoretically that they mightrepresent the non-monotonic spatial distribution of the interstellar dust inastrospheres (viewed as filaments) caused by interaction of the dust grainswith the interstellar magnetic field disturbed in the astrosphere due tocolliding of the stellar and interstellar winds. In this paper, we invoke thismechanism to explain the structure of the astrosphere around kappa Cas. Weperformed 3D magnetohydrodynamic modelling of the astrosphere for realisticparameters of the stellar wind and space velocity. The dust dynamics and thedensity distribution in the astrosphere were calculated in the framework of akinetic model. It is found that the model results with the classical MRN sizedistribution of dust in the interstellar medium do not match the observations,and that the observed filamentary structure of the astrosphere can bereproduced only if the dust is composed mainly of big (micron-sized) grains.Comparison of the model results with observations allowed us to estimateparameters (number density and magnetic field strength) of the surroundinginterstellar medium.

An astrosphere around the blue supergiant kappa Cas: possible explanation of its filamentary structure