Astron. Astrophys. 348, 584-593 (1999)

1. Introduction

Outflows are an essential part of star formation and cloud evolution. They remove excess angular momentum from the protostellar system, and can also contribute to turbulent support for the cloud. However, there is little consensus on the dynamics of the jets and outflows. Observations suggest that the outflows consist of jet-accelerated molecular gas, thought to be powered by the accretion process (Königl 1995). The outflows generally have two components, a highly collimated high velocity jet, and a more poorly collimated low velocity outflow, both of which are usually bipolar.

As the material in the jet impacts with the surrounding ambient medium, shocks form, which allow the kinetic energy of high velocity material to be dissipated and radiated away. Ambient material is swept up around the jet, in an entrainment process, forming the molecular outflow. Episodic outflow activity, or jet instabilities, can lead to a series of knots in the jet.

Jets have been observed in vibrational molecular hydrogen lines, which are thought to be emitted in cooling regions behind shocks. The excitation pattern in molecular hydrogen is indicative of the type of excitation mechanism. Emission in vibrational transitions of molecular hydrogen is only significant for temperatures 1000 K, and since the cooling timescales are short compared to the dynamical timescales of outflows, molecular hydrogen emission traces non dissociative shocks, or regions heated by some other energetic means such as UV excitation (Fernandes & Brand 1995; Hora & Latter 1994).

The Lynds 483 dark cloud contains IRAS 18148-0440, a deeply embedded, very young source, with a luminosity of (Fuller et al. 1995). It is assumed to be at a distance of 200 pc, and has an age, estimated from the outflow velocity, and including a correction for inclination angle, of 4 yrs. The bolometric temperature (Myers & Ladd 1993), calculated from the spectral energy distribution, is 57 K, which reinforces the young age estimate.

Dense gas surrounds the source, which has a low velocity bipolar molecular outflow, and a collimated jet which displays a knotty structure. The outflow is inclined close to the plane of the sky, and has been observed in CO J=21 (Hatchell et al. 1999; Parker et al. 1991), 32 (Fuller et al. 1995) and 43 (Hatchell et al. 1999) transitions. There are two spatially separated lobes, extending east and west of the source, with an aspect ratio of  5:1. The lobes extend over  AU, with a maximum velocity of  km s^-1, and a temperature of at least 20 K to 60 K (Hatchell et al. 1999), consistent with the predictions for a jet-driven outflow model (Masson & Chernin 1993). The jet is visible in the blue-shifted lobe, and extends over 12 AU. It has been mapped in , which shows a knot of emission connected by a weaker, jet-like structure to the embedded source (Fuller et al. 1995).

In this paper, we present the results of longslit spectroscopy of the jet/outflow system in L483. Although spectroscopy has been carried out on a number of young sources (e.g. Fernandes & Brand 1995; Wright et al. 1996), our study is important, as it is the first to target a Class 0 source. The dynamics of these less evolved jets may be simpler to understand than their more complex evolved counterparts. In Sect. 2, we describe the observations and data reduction. In Sect. 3 we present the results, and in Sect. 4 we analyse the resulting temperature, density and spatial structure. In Sect. 5 we look at the emission from the peaks in the light of recent shock models, and in Sect. 6 we discuss the processes underlying the continuous emission. Finally, in Sect. 7 we summarise our findings.

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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