Interaction of the Supersonic Stellar Wind with Free Stream of the Interstellar Medium: the Effect of the Azimuthal Magnetic Field of the Star

Abstract

The problem of the interaction of a hypersonic stellar wind with the surrounding interstellar medium is considered. The media are assumed to be fully ionized and are accounted for within the framework of ideal magnetohydrodynamics. The scientific novelty of the study consists in taking into account the star’s magnetic field. The magnetic field modifies qualitatively the shape of the astropause under certain flow parameters. The astropause is a tangential discontinuity that separates the stellar wind from the interstellar medium. Instead of the classical paraboloidal shape, the astropause acquires a tube (or cylindrical) shape. It is shown that the tube shape takes place for slowly moving stars or, in the star’s coordinate system, for free streams with the Mach number M_∞ less than a threshold one. The flow regime bifurcates and the astropause changes the shape from the tube to the classical one when a threshold flow Mach number \({\text{M}}_{\infty }^{*}\) is reached. For stars with the strong magnetic field, the bifurcation takes place at the higher Mach numbers as compared with stars with the weak magnetic field. It is also shown that one more qualitative flow restructuring occurs at M_∞ = 1. In this case, the astropause shape does not change, but a bow shock and a Mach disk are formed.