Astron. Astrophys. 343, 571-584 (1999)

1. Introduction

Jet-driven outflows are now becoming the most popular explanation for the bipolar molecular outflows emanating from young low-mass protostars (e.g. Bachiller 1996). Simplified models have been developed to explain how the propagation of a large bow-shock in a protostellar jet can put into motion ambient molecular gas and therefore produce an outflow (e.g. Raga & Cabrit 1993; see Cabrit et al. 1997 for a review of all the proposed outflow formation mechanisms). The most powerful molecular outflows are associated with the youngest protostars, the so-called Class 0 YSOs (Bontemps et al. 1996), but from an observational point of view the jets themselves have remained rather elusive in these objects (see however Richer et al. 1992). Optical jets have been detected in more evolved (Class I) sources (e.g. Reipurth et al. 1997), but the lack of an appropriate jet tracer for the most embedded objects, as well as the high angular resolution needed for such studies, make it very difficult to observe the primary jet in the youngest sources.

Interferometric observations of the SiO  line emission have provided evidence for a jet in the very young outflow L 1448 (Dutrey et al. 1997) and to a lesser extent in the L 1157 flow (Gueth et al. 1998). However, because the chemistry of SiO is rather peculiar, with SiO abundances enhanced in shocks by several orders of magnitudes as compared to the interstellar medium (e.g. Schilke et al. 1997), the basic parameters of the jets (density, velocity, mass loss rate) are still poorly known. Moreover, L 1448 and L 1157 present striking differences and complex morphologies. Clearly, interferometric observations of a larger number of young outflows is highly desirable. The development of wide-field infrared cameras, equipped with narrow-band filters centered around the 2.12 µm vibrationally excited H₂ transition (which is an excellent tracer of shocked gas) has led to the discovery of several new, possibly younger outflows (e.g. Hodapp & Ladd 1995, Davis et al. 1997, Zinnecker et al. 1998), which are potentially better targets for interferometric studies.

The well collimated Herbig-Haro jet HH 211 (Fig. 1) is one of these recently discovered objects (McCaughrean et al. 1994). It is located in a dense clump of the IC 348 molecular complex in Perseus (see Bachiller et al. 1987), at an estimated distance of 315 pc (Herbig 1998). The HH 211 system has an extremely low kinematical age (lower than 1000 years if one assumes a velocity km  s ^-1) and could thus be one of the youngest IR jets ever discovered. The inclination of the jet axis on the plane of the sky is unknown, but the lack of strong differential extinction in the H₂ brightness distribution suggests it is not too large. McCaughrean et al. (1994) reported the detection of an associated molecular outflow. In this paper, we present an interferometric study of the CO emission in this outflow. The angular resolution is ( AU) for the CO  line, about 15 times better than that of available single-dish observations (McCaughrean et al. 1994).

Fig. 1. The Herbig-Haro jet HH 211, as observed in the H2 v=1-0 S(1) line by McCaughrean et al. (1994) with an angular resolution of . The cross denotes the position of the central exciting source. The circles indicate the half-power primary beams of the 9 fields observed in the CO  line (simultaneous CO  observations were performed, with a two-times larger primary beam). The dotted circles represent the fields observed in the H13CO+  line.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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