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F.P. Wilkin (Caltech/IPAC), S.W. Stahler (Astronomy Dept., UC Berkeley)
Jets from embedded young stars may be collimated by the anisotropic infall of their cloud envelopes. To model this effect, we have followed numerically the motion of the shocked shell created by the impact of a spherical wind and a rotating, collapsing cloud. We first demonstrate, both analytically and numerically, that our previous, quasi-static solutions are dynamically unstable. Our present, fully time-dependent calculations include cases both where the wind is driven back by infall to the stellar surface, and where it erupts as a true outflow. For the latter, we find that the time of breakout can be surprisingly late, of order 105\,{\rm yr}, at least for wind speeds of 200\,{\rm km}\,{\rm s}-1 or less. The reason is that the shocked material must be able to climb out of the star's gravitational potential well. We explore the critical wind speed necessary for breakout as a function of the mass transport rates in the wind and infall, as well as the cloud rotation rate \Omega\circ and time since the start of infall. For realistic parameter choices, we do find that breakout can occur. Our actual breakout times would change if we relaxed the assumption of perfect mixing between the wind and infall material. In any case, our erupting shells do not exhibit the collimation of observed jets, but continue to expand laterally. To halt this expansion, the pressure in the envelope must fall off less steeply than in our model.