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

TL;DR

This study models the filamentary structure of the astrosphere around kappa Cas using 3D magnetohydrodynamics and dust dynamics, revealing that large dust grains are necessary to reproduce observed features and estimating interstellar medium parameters.

Contribution

It introduces a detailed 3D MHD model incorporating dust dynamics to explain filamentary structures around kappa Cas, emphasizing the role of large dust grains.

Findings

Filamentary structures are best reproduced with micron-sized dust grains.

The model estimates interstellar medium density and magnetic field strength.

Classical dust size distribution does not match observed structures.

Abstract

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.