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Astron. Astrophys. 345, 977-985 (1999)

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On the transfer of momentum from stellar jets to molecular outflows

T.P. Downes 1,2 and T.P. Ray 3

1 Sterrenkundig Instituut, Postbus 80000, 3508 TA Utrecht, The Netherlands
2 School of Mathematics, Trinity College, Dublin 2, Ireland
3 Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland

Received 22 October 1998 / Accepted 16 February 1999


While it is generally thought that molecular outflows from young stellar objects (YSOs) are accelerated by underlying stellar winds or highly collimated jets, the actual mechanism of acceleration remains uncertain. The most favoured model, at least for low and intermediate mass stars, is that the molecules are accelerated at jet-driven bow shocks. Here we investigate, through high resolution numerical simulations, the efficiency of this mechanism in accelerating ambient molecular gas without causing dissociation . The efficiency of the mechanism is found to be surprisingly low suggesting that more momentum may be present in the underlying jet than previously thought. We also compare the momentum transferring efficiencies of pulsed versus steady jets. We find that pulsed jets, and the corresponding steady jet with the same average velocity, transfer virtually the same momentum to the ambient gas. The additional momentum ejected sideways from the jet beam in the case of the pulsed jet only serves to accelerate post-shock jet gas which forms a, largely atomic, sheath around the jet beam.

For both the steady and pulsing jets, we find a power law relationship between mass and velocity ([FORMULA]) which is similar to what is observed. We also find that increasing the molecular fraction in the jet decreases [FORMULA] as one might expect. We reproduce the so-called Hubble law for molecular outflows and show that it is almost certainly a local effect in the presence of a bow shock.

Finally, we present a simple way of overcoming the numerical problem of negative pressures while still maintaining overall conservation of energy.

Key words: hydrodynamics – shock waves – ISM: jets and outflows – ISM: molecules

Send offprint requests to: T.P. Downes

Correspondence to: School of Mathematics, Trinity College, Dublin 2, Ireland

© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999