Trapped Protostellar Winds and Their Breakout

Observations show that high-velocity jets stem from deeply embedded youngstars, which may still be experiencing infall from their parent cloud cores.Yet theory predicts that, early in this buildup, any outgoing wind is trappedby incoming material of low angular momentum. As collapse continues and bringsin more rapidly rotating gas, the wind can eventually break out. Here we modelthis transition by following the motion of the shocked shell created by impactof the wind and a rotating, collapsing envelope. We first demonstrate, bothanalytically and numerically, that our previous, quasi-static solutions aredynamically unstable. Our present, fully time-dependent calculations includecases both where the wind is driven back by infall to the stellar surface, andwhere it erupts as a true outflow. For the latter, we find that the time ofbreakout is sim 50,000 yr for wind speeds of 200 km/s. The reason for the delayis that the shocked material, including the swept-up infall, must be able toclimb out of the star's gravitational potential well. We explore the critical wind speed necessary for breakout as a function ofthe mass transport rates in the wind and infall, as well as the cloud rotationrate Omega0 and time since the start of infall. Breakout does occur forrealistic parameter choices. The actual breakout times would change if werelaxed the assumption of perfect mixing between the wind and infall material.Our expanding shells do not exhibit the collimation of observed jets, butcontinue to expand laterally. To halt this expansion, the density in theenvelope must fall off less steeply than in our model.Comment: 44 pages, 10 figures, accepted to Ap