Where the stellar wind impinges the interstellar medium

Massive stars have strong radiation-driven winds, losing part of their mass. Where these winds encounter the interstellar medium, a separatrix surface can form, called astropause. The astropause separates the stellar wind region, the astrosphere, from the outer streaming interstellar medium. At the apex of the astropause the stagnation point is located, where the streaming velocity of both flows, namely the flow of the interstellar medium and the stellar wind flow, vanishes. The velocity fields in the astropause regions can be described by potential theory, while the m-CAK theory is used to describe the radiation-driven winds, in the proximity of the star. In this work, we aim to connect these two theories to describe the interaction between the stellar wind and their surroundings. We propose a criterion for delimiting validity regions for both theories, and we found a relation between the stellar wind parameters (mass-loss rate and terminal velocity), the interstellar medium conditions, and the distance between the
star and the stagnation point, called the stagnation distance.