It is now generally agreed that low velocity molecular outflows represent ambient gas that is somehow accelerated by highly collimated and partly ionized jets, at least in the case of low and intermediate mass YSOs. Most of this acceleration is thought to be achieved at the head of the jet through the leading bow shock (the so-called prompt entrainment mechanism). In this paper we have examined through many axially symmetric simulations the efficiency of the prompt entrainment mechanism as a means of transferring momentum to ambient molecular gas without causing dissociation. It is found, as one would expect, that the fraction of jet momentum transferred to the ambient environment depends on the jet/ambient density ratio. More importantly, we see that cooling, which is particularly important at higher densities, decreases the fractional jet momentum that goes into ambient molecules . It would seem on the basis of the simulations presented here that both heavy and equal density (with respect to the environment) jets with radiative cooling have very low efficiencies at accelerating ambient molecules without causing dissociation. In part this is because cooled jets have more aerodynamic bow shocks than the corresponding adiabatic ones (i.e. they present a smaller cross sectional area to the ambient medium). The actual shape, however, of the bow shock also seems to to be important as the decrease in momentum transferred to the ambient medium seems to affect the acceleration of molecules more so than atoms/ions.
We have also tested whether pulsed jets are more efficient at transferring momentum to the ambient medium than the corresponding steady jet with the same average velocity. Somewhat surprisingly we found that even relatively large velocity variations do not give rise to significant changes in the amount of momentum being deposited in ambient gas. Fundamentally this is because in high Mach number jets, even with cooling, there is very little coupling between the jet's cocoon and the "sheath" (i.e. the post-shock ambient gas). This lack of coupling is also the reason why turbulent entrainment is not significant in YSO jets.
Our simulations were also used to model the expected variation of mass with velocity in molecular flows. We found relatively large values, i.e. mass should decline steeply with velocity, in line with observations. Interestingly we found that a wide shear layer and an increasing molecular component in the jet reduced . If, as one might expect, such conditions are common among outflows from low luminosity YSOs, this could explain their observed lower values for . Finally we have shown that the so-called Hubble Law for molecular outflows is almost certainly a local effect in the vicinity of a bow shock.
© European Southern Observatory (ESO) 1999
Online publication: April 28, 1999