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Astron. Astrophys. 344, 687-695 (1999)

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1. Introduction

All low mass stars are thought to drive massive molecular outflows for some period of their early evolution. Yet the mechanism by which these outflows are generated and driven remains uncertain (eg. review by Bachiller 1996). The process by which the molecular gas is accelerated to the observed outflow velocities is likely also to heat the molecular gas. The observational signature of this heating provides a potential probe of the acceleration mechanism. Using molecular line excitation studies to determine the gas temperature in protostellar outflows can provide a test, in addition to the outflow dynamics, of outflow acceleration models.

Several models for outflows have been put forward. These generally fall into one of two categories: wide-angle winds; and jet-driven outflows. Non-isotropic radial winds may explain both jet and molecular outflow features as different components of the same stellar wind (Shu et al. 1991; Mellema & Frank 1997). Models of jet-driven outflows, on the other hand, assume that the stellar wind originates from close to the star as a highly collimated jet which then entrains molecular material forming the molecular outflow. Momentum can be transferred from the jet to the molecular gas either through shocks or through turbulent mixing along the length of the jet (De Young 1986; Cantó & Raga 1991; Taylor & Raga 1995). Bow shocks can be either at the head of the jet (Masson & Chernin 1993, Chernin & Masson 1995) or at working surfaces within the jet (Raga & Cabrit 1993). Hydrodynamic and magnetohydrodynamic numerical models which contain one or more of these features have been developed (eg. Smith et al. 1997).

Of these, one of the most promising models is the jet-driven bow shock model, which succeeds in explaining both the clumps of shocked H2 commonly seen at the end of outflows, and the shell structures observed in young sources such as L1157 (Gueth et al. 1996), L1448 (Bachiller et al. 1995) and HH111 (Nagar et al. 1997). Masson & Chernin (1993; hereafter MC93) calculated the dynamics of such a bow-shock driven shell using a simple analytical model. The MC93 model outflows produces features similar to observed CO lobes: sharp velocity peaks behind the bow shock; velocity increasing with distance from the star; shell structures; and high collimation. Although the model predicts a narrow range of velocities at each position, once turbulence and beam dilution are taken into account the observed mass distributions can also be reproduced (MC93).

In this paper we show how predictions for the temperature of the outflow can be made from MC93's shell model, and compare with an observational study of the young outflow in the Lynds dark cloud, L483. Sect. 2 describes the model in detail and displays the predicted temperature distributions. Sect. 3 contains the observational results and the comparison with the model's predictions. We summarize the conclusions in Sect. 4.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999
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