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

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1. Introduction

It has been proposed that molecular outflows, at least from low and intermediate mass young stars, may be driven by highly collimated jets (see, for example, Padman, Bence & Richer 1997). Although the likely mechanism by which such jets transfer their momentum to the ambient medium remains unknown, a number of ideas have been put forward (for a review of models the reader is referred to Cabrit, Raga & Gueth 1997). Of these the most promising seems to be the so-called "prompt entrainment" mechanism. According to this model the bulk of the molecular outflow is accelerated ambient gas near the head of the jet or more precisely along the wings of its associated bow shock. Observational support for prompt entrainment comes from the spatial coincidence of shocked molecular hydrogen bows with peaks in the CO outflow emission (e.g. Davis & Eislöffel 1995).

Smith, Suttner & Yorke (1997) and Suttner et al. (1997) have carried out a number of 3-D simulations of dense molecular jets propagating into a dense medium in order to test the prompt entrainment hypothesis. These authors found that their simulations reproduced many of the observational characteristics of molecular flows including the so-called `Hubble law' (see, e.g. Padman et al. 1997) and strong forward, as opposed to sideways, motion (Lada & Fich 1996). While such results are encouraging for jet-driven models, it is still fair to say that no individual model has yet been able to plausibly account for all the observations (Lada & Fich 1996). Moreover, alternatives to the jet model may be better at explaining the observational characteristics of some molecular flows (e.g. Padman et al. 1997and Cabrit et al. 1997).

The limited resolution of the 3-D jet simulations of Smith et al. (1997), and Suttner et al. (1997), along with the high densities used by these authors, meant that they could not resolve the post-shock cooling regions in the flow. In addition it was not possible to explore parameter space as only a few such simulations could be performed. Here we take a somewhat different approach by assuming low density atomic/molecular jet mixtures, a low density ambient medium and cylindrical symmetry. Although such an approach obviously has its limitations, it does allow us to explore parameter space more fully and to resolve post-shock cooling regions (this might be important, for example, if one is to gauge the importance of certain instabilities). The primary goal of this work is to investigate the efficiency of YSO jets in accelerating ambient molecular gas without causing dissociation of its molecules.

An additional question we address in this paper is whether velocity variations (pulsing) of the jet might enhance transfer of momentum from the jet to its surroundings and thus help to accelerate ambient gas. Pulsing induces internal shocks which can squeeze jet gas sideways (Raga et al. 1993). This gas does not interact with the ambient medium directly, but is instead squirted into the cocoon of processed (post-shock) jet gas, which separates the jet from the "shroud" of post-shock ambient gas. Chernin & Masson (1995) however argue that, through the cocoon, momentum from the jet may be continuously coupled to the ambient flow.

The properties of the simulated systems in which we are interested are as follows:

  • How much momentum is transferred to the ambient molecules?

  • Is there a power-law relationship predicted between mass in the molecular flow and velocity?

  • What are the proper motions of the molecular `knots', and how does their emission behave with time?

  • Is the so-called `Hubble law' of molecular outflows reproduced under reasonable conditions?

  • Is there extra entrainment of ambient gas along the jet due to velocity variations?

We will discuss each of these points in turn when presenting our results.

Our numerical model is presented in Sect. 2 and our results in Sect. 3. Conclusions from this work are presented in Sect. 4 and a simple way of overcoming the numerical problem of negative pressures while still maintaining overall energy conservation is given in the Appendix.

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999