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

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 κ 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 κ 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 (µm-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.

Simulating observable structures due to a perturbed interstellar medium in front of astrospheric bow shocks in 3D MHD

“…Because the dust emission depends on the distance from the star in addition to the number density, the region of the outer astrosheath close to the AP near the inflow axis is brighter in dust emission than the regions farther from the star. This is in agreement with the results of Katushkina et al (2018), where dust was kinetically modelled. The ring of high flux densities in both radiation types with a radius of 0.2°a round the model centre and the jags perpendicular to the radial direction at the BS are artefacts introduced by the interpolation (cf.…”

“…Three-dimensional (3D) magnetohydrodynamics (MHD) have recently come into focus as a viable tool for further examining and understanding the structures of astrospheres (e.g. Katushkina et al 2018;Gvaramadze et al 2018;Scherer et al 2020). By changing these models' orientations with respect to an observer's line of sight (LOS), the generation of synthetic observations produces images similar to genuine data of actual bow shocks (BSs; e.g.…”

MHD-shock structures of astrospheres: λ Cephei -like astrospheres

“…A detailed analysis of the KH in the case of the heliosphere is given by Ruderman & Brevdo (2006), where the authors show that the magnetic field stabilizes the astropause. The feature of a smooth astropause is mainly seen in heliospheric simulations (for example Pogorelov et al 2017a) and in the 3D MHD astrosphere simulations by Katushkina et al (2018) and Gvaramadze et al (2018). Most of the instabilities are discussed in detail for the heliosphere, a special astrosphere.…”

3D MHD astrospheres: applications to IRC-10414 and Betelgeuse

“…Interestingly, the perpendicular orientation of the ISM magnetic field can result in series of multiple arcs normal to the direction of motion of the star (Fig. 11f) as modelled in the context of hot OB stars in Katushkina et al (2017Katushkina et al ( , 2018, see also Baalmann et al (2021). Furthermore, the opening of the bow shock can significantly differ from a projection model to another, with rounder termination shocks (Fig.…”

“…Until recently, most simulations, except very few, were two-dimensional hydrodynamical models. Over the past few years, several threedimensional hydrodynamical (Blondin & Koerwer 1998;Mohamed et al 2012) and three-dimensional (magneto-)hydrodynamical (3D MHD) models have been performed (Gvaramadze et al 2018;Katushkina et al 2018;Scherer et al 2020;Baalmann et al 2021). However, no one so far has investigated the appearance of bow shocks from moving red supergiant stars using 3D MHD simulations.…”