1999A&A...342L..21C

Using the Long Wavelength Specrometer aboard ISO, we have detected far infrared rotational H₂O emission lines in five low-mass young stellar objects in a survey of seven such sources. The total H₂O fluxes are well correlated with the 1.3mm continuum fluxes, but - surprisingly - not with the SiO millimeter emission originating in the outflows, suggesting that the water emission arises in the circumstellar envelopes rather than in the outflows. In two of the sources, NGC 1333-IRAS 4 and IRAS 16293-2422, we measured about ten H₂O lines, and used their fluxes to put stringent constraints on the physical conditions (temperature, density and column density) of the emitting gas. Simple LVG modelling implies that the emission originates in a very small (∼200AU), dense (≥10⁷cm^–3) and warm (∼100K) region, with a column density larger than about 10¹⁶cm^–2. The detected H₂O emission may be well accounted for by thermal emission from a collapsing envelope, and we derive constraints on the acccretion rate and central mass of NGC 1333-IRAS 4. We also discuss an alternative scenario in which the H₂O emission arises in an extremely dense shock very close to the central object, perhaps caused by the interaction of the outflow with the innner regions of the circumstellar envelope.