NUMERICAL HYDRODYNAMIC SIMULATIONS OF JET-DRIVEN BIPOLAR OUTFLOWS

The jet model for protostellar outflows is confronted with the constra ints imposed by CO spectroscopic observations. From three dimensional simulations of a dense molecular medium being penetrated by a denser m olecular jet, we simulate line profiles and construct position-velocit y diagrams for the (low-J) CO transitions. We find (1) the profiles im ply power law variation of integrated brightness with velocity over a wide range of velocities, (2) the velocity field resembles a 'Hubble L aw' and (3) a hollow-shell structure at low velocities becomes an elon gated lobe at high velocities. Moreover, the leading bow shock produce s strong forward motion of the cool gas rather than the expected later al expansion. We are thus able to satisfy the Lada and Fich (1996) cri teria, employing NGC 2264G as an example. Deviations from the simple p ower law dependence of integrated brightness versus velocity occur at high velocities in our simulations. The curve first dips to a shallow minimum and then rises rapidly and peaks sharply. Reanalysis of the NG C 2264G and Cepheus E data confirm these predictions. We identify thes e two features with a jet-ambient shear layer and the jet itself. A de eper analysis reveals that the power-law index is an indicator of the evolutionary stage: a profile steepens with time. Also, the CO excitat ion temperature changes along the bow walls and thus a CO line intensi ty does not directly yield the mass distribution, as often assumed. In stead, the CO emission is enhanced near the excitation peaks.