We have investigated with the Long Wavelength Spectrometer (LWS) of the Infrared Space Observatory (ISO) the far-infrared spectra (43-197 μm) of a sample of 17 class 0 sources and their associated outflows. In addition to [O I] 63 μm, the pure rotational lines of abundant molecules such as CO, H2O, and OH are frequently observed in these sources, at variance with more evolved young stellar objects. We found, in agreement with previous studies conducted on individual sources, that the molecular line excitation arises from small regions, with typical sizes of 10-9 sr, of warm (200 T 2000 K) and dense gas (104 n 107 cm-3), compressed after the passage of shocks. In particular, we found slow, nondissociative shocks as the main mechanism at the origin of the molecular gas heating, while the bulk of the [O I] 63 μm line emission is due to the dissociative J-shock component arising from the Mach disk at the head of the protostellar jet, as testified by the fact that this line emission happens to be a good tracer of the source mass-loss rate. Large abundances of gas-phase H2O are commonly estimated, with values that appear to be correlated with the gas temperature. The total far-infrared (FIR) line cooling LFIR = L(O ) + L(CO) + L(H2O) + L(OH), which amounts to ~10-2 to 10-1 L☉, is roughly equal to the outflow kinetic luminosity as estimated by means of millimeter molecular mapping. This circumstance demonstrates that the FIR line cooling can be a valid direct measure of the power deposited in the outflow, not affected by geometrical or opacity problems like the determination of Lkin or by extinction problems like the near-infrared shocked H2 emission. We finally remark that the strong molecular emission observed, and in particular H2O emission, is a peculiarity of the environments of class 0 sources. The present analysis shows that the ratio between FIR molecular line luminosity and bolometric luminosity (Lmol/Lbol) is always larger than ~10-3 in class 0 objects. We suggest that this parameter could be used as a further criterion for identifying future class 0 candidates.
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