5 Conclusions

We presented a spectral survey of the protobinary system IRAS 4 in the NGC 1333 cloud, using ISO-LWS in grating mode. We targeted the source as well as two adjacent positions, NE-red and SW-blue, that encompass the red and blue lobes of the outflow emanating from IRAS 4, respectively. The three spectra are dominated by the [OI] 63$\mu$m and CII [157] $\mu$m lines, that likely originate in the PDR associated with the parental cloud. On the contrary, water emission is only detected towards the on-source position, whereas no significant water emission is detected towards the NE-red and SW-blue positions. This suggests that the bulk of the water emission is due to the thermal emission of the protostellar envelopes around the two protostars. Using an accurate model of the chemistry, thermal balance and radiative transfer in protostellar envelopes (CHT96), we modeled the water line emission due to two identical envelopes surrounding IRAS 4A and B respectively. We found that the observations are consistent with the CHT96 model, which implicitly assumes the "inside-out'' theory (Shu 1977). The best fit of the model allows us to estimate the four main model parameters: the accretion rate, $5 \times 10^{-5}$  ${M}_{\odot }$ $~{\rm yr}^{-1}$, the central mass, 0.5  ${M}_{\odot }$, the water abundance in the outer envelope, $5 \times 10^{-7}$, and in the inner envelope, $5 \times 10^{-6}$ (this last parameter is the least constrained with about a factor 3 of uncertainty). This gives an age of 10 000 years, assuming that the accretion rate remains constant during the collapse. Based on this model, we derived the density and temperature profiles of the gas in the envelopes. We also reviewed the suggestion by Blake et al. (1995) that CO is depleted by about a factor ten in the envelope of IRAS 4. We could not confirm or rule out this hypothesis but caution that the transitions used by this study, C¹⁸O 3-2 and 2-1, can hardly probe the inner regions, where the CO abundance may be "canonical''.

A comparison with several previous studies of the same source (Blake et al. 1995; Neufeld et al. 2000; DiFrancesco et al. 2001, JSD02) shows that the derived parameters are reasonable and consistent with the available literature, hence re-enforcing the thesis that the observed water emission is indeed due to the thermal emission from the envelopes. A by-product of the present study is the prediction of the existence of a hot core like region in the inner parts of the envelope, where grain mantles evaporate, releasing large amounts of water (about a factor ten) in the gas phase. Such a hot core has already been proposed to exist around IRAS 16293-2422, where a similar study as been carried out (Ceccarelli et al. 2000a; Ceccarelli et al. 2000b). Comparison between the two protostars, show that IRAS 4 is younger and surrounded by a more massive envelope. This explains the larger continuum emission and the larger depletion factors observed in IRAS 4. Finally, this study emphasis the necessity of ground based observations, where higher spatial and spectral resolutions are achievable. H₂CO and CH₃OH are of particular interest as they are among the most abundant components of grain mantles, and are therefore expected to evaporate in the inner parts of the envelope. Appropriate transitions can hence be used to constrain the physical and chemical conditions in the innermost part of protostellar envelopes (see Ceccarelli et al. 2000b).

Acknowledgements

We wish to thank Edwin A. Bergin for frank and constructive discussions on the SWAS data. We thank Edwin A. Bergin and Jes K. Jø rgensen for providing us with their papers prior to publication. The referee Neal Evans is thanked for his useful comments. Most of the computations presented in this paper were performed at the Service Commun de Calcul Intensif de l'Observatoire de Grenoble (SCCI).