Water line emission in low-mass protostars

Abstract

Using the Long Wavelength Specrometer aboard ISO, we have detected far infrared rotational H_2O emission lines in five low-mass young stellar objects in a survey of seven such sources. The total H_2O fluxes are well correlated with the 1.3 mm 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, NGC1333-IRAS4 and IRAS16293-2422, we measured about ten H_2O 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 ( ~ 200 AU), dense (>= 10(7) cm(-3) ) and warm ( ~ 100 K) region, with a column density larger than about 10(16) cm(-2) . The detected H_2O 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 NGC1333-IRAS4. We also discuss an alternative scenario in which the H_2O 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. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.