2002ApJ...574..246N

The far-infrared spectra (45-197 µm) of 28 low-luminosity young embedded objects have been studied in order to search for possible evolutive trends in the observed spectral features. The low-resolution spectra from 45 to 197 µm of 17 Class 0 and 11 Class I sources taken with the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory have been used for this analysis. The most prominent features presented by these spectra are the [O I] 63 and 145 µm fine-structure lines and pure rotational lines from the abundant molecules CO, H₂O, and OH. Clear differences are found, however, between the spectra of the two classes of objects. Water lines, which are prominent in the spectra of Class 0 sources, are not observed in Class I objects, with an upper limit ≲10^–5 on the H₂O abundance. Furthermore, the total cooling due to molecular emission in Class 0 sources is on average significantly larger than in Class I sources, while the cooling due to atomic oxygen is fairly constant among the two classes of objects. Finally, the total gas cooling as traced by the far-infrared lines (L_FIR) is correlated with the bolometric luminosity for the Class 0 sample of sources, with an L_FIR/L_bol ratio (∼10^–2) of about an order of magnitude larger than in Class I sources. We suggest that most of the observed emission lines originate from shocks at the base and along the source outflows. In such a case these results can be interpreted in terms of a change in the modality of the interaction between the protostellar jet and the circumstellar environment. During the Class 0 phase the impact of energetic flows with the dense ambient medium gives rise to a strong component of nondissociative C-type shock, while during the Class I phase such impact produces less energetic shocks with an enhanced dissociative J-type component. Finally, the low H₂O abundance found in Class I sources can be explained by the action of the progressively less shielded interstellar UV field.