Numerical hydrodynamic simulations of jet-driven bipolar outflows

The jet model for protostellar outflows is confronted with the constraints imposed by CO spectroscopic observations. From three dimensional simulations of a dense molecular medium being penetrated by a denser molecular jet, we simulate line profiles and construct position-velocity diagrams for the (low-J) CO transitions. We find (1) the profiles imply power law variation of integrated brightness with velocity over a wide range of velocities, (2) the velocity field resembles a 'Hubble Law' and (3) a hollow-shell structure at low velocities becomes an elongated lobe at high velocities. Moreover, the leading bow shock produces strong forward motion of the cool gas rather than the expected lateral expansion. We are thus able to satisfy the Lada and Fich (1996) criteria, employing NGC 2264G as an example. Deviations from the simple power 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 NGC 2264G and Cepheus E data confirm these predictions. We identify these two features with a jet-ambient shear layer and the jet itself. A deeper analysis reveals that the power-law index is an indicator of the evolutionary stage: a profile steepens with time. Also, the CO excitation temperature changes along the bow walls and thus a CO line intensity does not directly yield the mass distribution, as often assumed. Instead, the CO emission is enhanced near the excitation peaks.