FROM SUPERSONIC WINDS TO ACCRETION - COMMENTS ON THE STABILITY OF STELLAR WINDS AND RELATED FLOWS

For nearly vanishing values of the interstellar pressure a supersonic flow connecting via a shock to the interstellar medium is the only sta tionary state describing the extension of a hot corona into space. We show here that in terms of the relative pressure jump between the coro nal base and distant medium the stationary flow solutions follow an hy steresis-type cycle with two catastrophy points: as the pressure of th e interstellar medium increases, the termination shock moves closer to ward the stellar surface, but when the shock position reaches the soni c point the wind collapses into supersonic accretion with a shock belo w the critical point. If the pressure of the interstellar medium decre ases again, or the pressure at the coronal base increases, the flow ca n evolve continuously into breeze (everywhere subsonic) accretion, but the flow evolves back into a state characterized by a supersonic shoc ked wind, once the pressure difference corresponding to a static strat ification is exceeded.