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Astron. Astrophys. 350, 529-540 (1999)

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1. Introduction

The Class 0 source L1448-mm (also called L1448C) is a deeply embedded, young low mass protostellar object ([FORMULA] 10 [FORMULA]) located in the core of the L1448 dense cloud (at a distance d = 300 pc, Cernis 1990). It emits strongly at millimeter and centimeter wavelengths, while the IRAS satellite was only able to detect weak far infrared emission (Curiel et al. 1990; Bachiller et al. 1991a; Barsony et al. 1998). H2O maser emission has also been detected from this source, with an intensity which is significantly higher than that usually observed in young sources of similar luminosity (Chernin 1995). This source drives a young and highly collimated bipolar outflow (Bachiller et al. 1990) which has been mapped by means of different molecular tracers. These observations reveal a complex structure where components with different physical properties co-exist and probably are interacting together. CO J=2[FORMULA]1 and 1[FORMULA]0 observations, in particular, have revealed that in addition to the high velocity gas which outline the overall outflow structure, there are several spatially and spectrally separated clumps, aligned with the outflow axis, which have extreme velocities up to 70 km s-1 and sizes of [FORMULA] 0.03 pc (the so-called "molecular bullets", Bachiller et al. 1990). These clumps lie in pairs located symmetrically with respect to the mm source and have similar but opposite velocities. This has lead to the suggestion that these bullets are produced by successive ejections from the central source. In addition, their alignment along the outflow axis al so suggests that they trace the jet which entrains the ambient molecular material in a biconical cavity, which is observed through lower velocity gas (Bachiller et al. 1995). SiO emission has also been observed associated with these clumps, with an abundance enhancement of 104-105 times greater than typical values of quiescent molecular clouds (Bachiller et al. 1991b; Guilloteau et al. 1992; Dutrey et al. 1997). Such high SiO abundance could be generated by shocks with velocities greater than 25 km s-1, that are capable of removing silicon from dust grains (Schilke et al. 1997). High angular resolution images in SiO J=2[FORMULA]1 close to the mm source (Guilloteau et al. 1992) show that this emission is distributed along a jet-like structure, where it is possible to define different sub-clumps of size [FORMULA] 4", whose kinematical timescales are [FORMULA] 100 yr.

The outflow from L1448 has also been mapped extensively in H2 emission (e.g. Bally et al. 1993; Davis et al. 1994; Davis & Smith 1995). The strongest H2 features are seen in the blue lobe of the outflow, with an arc-like structure which surrounds the high velocity molecular jet. H2 emission is also observed at the end of the redshifted lobe, but is not detected from the innermost molecular clumps which lie close to the mm source, presumably due to higher exctinction in this region.

The observational scenario outlined above shows that along the L1448 outflow and in particular in the environment close to the mm source, different excitation regimes are probed by the different molecular lines; in particular, low velocity CO traces relatively low density gas at [FORMULA][FORMULA]10-30 K, the CO and SiO emission in the molecular clumps indicates higher densities (105-106 cm-3) and temperatures of about one hundred K, and finally the H2 emission traces material at temperatures in excess of 2000 K. It is therefore clear that there is an observational gap in our ability to trace gas with excitation temperatures between 100 and 2000 K; this is because the gas cooling in this temperature regime occurs mainly through the emission of atomic and molecular lines like [OI ], H2O, high-J CO, OH and H2 transitions, which all fall in the mid to far infrared spectral range, which is not accessible from the ground. Observations with the Infrared Space Observatory (ISO, Kessler et al. 1996) allow this gap to be filled, giving for the first time the opportunity to spectroscopically investigate the entire spectral range from 3 to 200 µm. Spectra obtained with ISO towards a number of young sources driving molecular outflows have already shown that the circumstellar gas in such sources is cooled mainly by line emission from O0 and CO (Nisini et al. 1999; Nisini et al. 1998; Saraceno et al. 1998). Emission from gas phase H2O has been observed in some sources (e.g. Saraceno et al. 1998), but even if the derived abundance has been found higher than the expected interstellar value in quiescent clouds (van Dishoeck et al. 1993), its contribution to the gas cooling is never found to be as important as expected in the warm gas around protostars, where the water abundance can be significantly enhanced due to either the evaporation of icy grain mantles or to molecular formation in high temperatures ([FORMULA] 300 K) gas-phase chemical reactions. Here we present the first case in which cooling by water dominates over that of any other species which is present in the investigated region.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999