On the motion of wind-driving stars relative to the ambient interstellar medium

Abstract

In a series of foregoing papers we have argued that each wind-driving star in relative motion with respect to the ambient interstellar medium which has formed an adapted astropause is subject to the action of thrust forces exerted on its wind-generating central body. The instantaneous magnitude of the resulting net force depends on the actual geometry of the counterflow configuration of stellar and interstellar winds in the particular kinematic situation, especially depending on the instantaneous velocity relative to the ambient medium. We investigate how sensitive this configuration is to whether the interstellar flow is sub- or supersonic. It will be demonstrated here that the resulting net force varies in a complicated, non-monotonic manner with the actual velocity, however, for subsonic motions it is generally of an accelerating nature, thus operating like a rocket thrust motor, whereas for supersonic motions at supercritical Mach numbersm _s≥μ _s,cit is of a decelerating nature. For an adequate description of a time-dependent circumstellar flow configuration, we shall use an analytic, hydrodynamic modelling of the counterflow configuration representing the case of a stellar wind system in an adiabatically adapted subsonic or supersonic motion with respect to the local interstellar medium. Hereby we assume irrotational and incompressible flows, after passage through the respective shocks, and can give quantitative numbers for the accelerating forces acting on the motion of such a star. We describe the long-period evolution of star motions and can give typical acceleration time periods for different stars.