Water emission in NGC 1333 IRAS 4 - The physical structure of the envelope

We report ISO-LWS far infrared observations of CO, water and oxygen lines towards the protobinary system IRAS 4 in the NGC 1333 cloud. We detected several water, OH, CO rotational lines, and two [OI] and [CII] fine structure lines. Given the relatively poor spectral and spatial resolution of these observations, assessing the origin of the observed emission is not straightforward. In this paper, we focus on the water line emission and explore the hypothesis that it originates in the envelopes that surround the two protostars, IRAS 4 A and B, thanks to an accurate model. The model reproduces quite well the observed water line fluxes, predicting a density profile, mass accretion rate, central mass, and water abundance profile in agreement with previous works. We hence conclude that the emission from the envelopes is a viable explanation for the observed water emission, although we cannot totally rule out the alternative that the observed water emission originates in the outflow. The envelopes are formed by a static envelope where the density follows the r(-2) law, at r greater than or equal to 1500 AU, and a collapsing envelope where the density follows the r(-3/2) law. The density of the envelopes at 1500 AU from the center is similar to4 x 10(6) cm(-3) and the dust temperature is similar to30 K, i.e. about the evaporation temperature of CO-rich ices. This may explain previous observations that claimed a factor of 10 depletion of CO in IRAS 4, as those observations probe the outer similar to30 K region of the envelope. The water is similar to5 x 10(-7) less abundant than H-2 in the outer and cold envelope, whereas its abundance jumps to similar to5 x 10(-6) in the innermost warm region, at r less than or equal to 80 AU where the dust temperature exceeds 100 K, the evaporation temperature of H2O-rich ices. We derive a mass of 0.5 M-circle dot for each protostar, and an accretion rate of similar to5 x 10(-5) M-circle dot yr(-1), implying an age of about 10000 years, if the accretion rate remains constant. We finally discuss the difference between IRAS 4 and IRAS 16293-2422, where a similar analysis has been carried out. We found that IRAS 4 is probably a younger system than IRAS 16293-2422. This fact, coupled with the larger distance of IRAS 4 from the Sun, fully explains the apparent difference in the molecular emission of these two sources, which is much richer in IRAS 16293-2422.