Are interstellar toroids the focusing agent of the bipolar molecular outflows?

Abstract

The phenomenon of bipolar mass outflow in regions of star formation is an important phase in the early stages of stellar evolution. In this paper we address observationally the nature of the focusing agent. We searched the (J,K) = (1,1) inversion transition of ammonia in 10 sources of bipolar outflow and detected and mapped the emission in seven of them. Since ammonia is a tracer of high-density gas, n(H/sub 2/)> or approx. =5 x 10/sup 3/ cm/sup -3/, we conclude that bipolar molecular outflows are commonly associated with dense condensations. These condensations have internal velocity dispersions larger than those of dark clouds; however, they are in approximate virial equilibrium. The ammonia data show the correlation between internal velocity dispersion and cloud size and mass previously obtained from carbon monoxide observations. The molecular condensations associated with the bipolar outflows are generally elongated and engulf the compact object suspected to be the source of energy of the system. For nine sources where clear orientations could be assigned to both the major axis of the condensation and the direction of the outflow, we found seven cases where these axes were nearly perpendicular. This suggests that these elongted clouds may be toroid-shaped with dimensionsmore » of tenths of parsecs and may provide the focusing mechanism in bipolar outflows. In this model, the stellar wind of the central object has created a bipolar cavity in an originally pancake-shaped cloud. The wind, which could be isotropic near the star, is stopped in the plane of the dense toroid but escapes along the poles, accelerating the surrounding lower density molecular gas.« less