High velocity bipolar gas flows are observed very frequently towards regions of star formation, and it has been conjectured that almost every star of mass larger than 1 M⊙ could undergo a phase of mass ejection during its formation (see e.g. Lada 1985). To explain the collimation of gas flows at interstellar scale, models have been proposed based on a stellar wind or violent isotropic ejection of matter by the protostar, that is pressure collimated by the molecular cloud in which it is embedded (Canto 1980, Königl 1982). In some cases, a molecular disk is observed perpendicular to the direction of the flow, but it is not quite sufficient to collimate it, see for example the high resolution CS observations of Takano et al. (1984) and Kawabe et al. (1984). Also, collimation occurs at a distance smaller than 3x1013m, according to the optical emission line observations of Mundt and Fried (1983).