W50 - a stellar-wind bubble in a three-phase interstellar medium

The interaction between the relativistic jets of SS433 and the radio nebula W50 is analysed on the basis of recent observations. It is found that SS433 may be surrounded by a cloudy interstellar medium, of the type described by the three-phase model of McKee and Ostriker (1977. Astrophys. J. 218,148). In addition, it is concluded that W50 is more likely to represent an expanding interstellar bubble, driven by a strong stellar wind from the binary companion of SS433, than a supernova remnant. It is argued that the dynamics of the expansion are probably controlled by the inertia of evaporated interstellar clouds within the bubble. (author)