The Breakout of Protostellar Winds in the Infalling Environment
  The time of protostellar wind breakout may be determined by the evolution of the infalling flow, rather than any sudden change in the central engine. I examine the transition from pure infall to outflow, in the context of the inside-out collapse of a rotating molecular cloud core. I have followed numerically the motion of the shocked shell created by the impact of a stellar wind and infalling gas. These fully time-dependent calculations include cases both where the shell falls back to the stellar surface, and where it breaks out as a true outflow. Assuming a wind launched from the protostellar surface, the breakout time is determined in terms of the parameters describing the wind (Mdot_w, V_w) and collapsing cloud core (a_0, Omega). The trapped wind phase consists of a wind sufficiently strong to push material back from the stellar surface, but too weak to carry the heavy, shocked infall out of the star's gravitational potential. To produce a large-scale outflow, the shocked material must be able to climb out of the star's gravitational potential well, carrying with it the dense, swept-up infall. 
Francis P. Wilkin