Water in low-mass star-forming regions with<i>Herschel</i>

Abstract

“Water In Star-forming regions with <i>Herschel<i/>” (WISH) is a key programme dedicated to studying the role of water and related species during the star-formation process and constraining the physical and chemical properties of young stellar objects. The Heterodyne Instrument for the Far-Infrared (HIFI) on the <i>Herschel<i/> Space Observatory observed three deeply embedded protostars in the low-mass star-forming region NGC 1333 in several H<sub>2<sub/><sup>16<sup/>O, H<sub>2<sub/><sup>18<sup/>O, and CO transitions. Line profiles are resolved for five H<sub>2<sub/><sup>16<sup/>O transitions in each source, revealing them to be surprisingly complex. The line profiles are decomposed into broad (<i>><i/>20 km s<sup>-1<sup/>), medium-broad (~5-10 km s<sup>-1<sup/>), and narrow (<i><<i/>5 km s<sup>-1<sup/>) components. The H<sub>2<sub/><sup>18<sup/>O emission is only detected in broad 1<sub>10<sub/>–1<sub>01<sub/> lines (<i>><i/>20 km s<sup>-1<sup/>), indicating that its physical origin is the same as for the broad H<sub>2<sub/><sup>16<sup/>O component. In one of the sources, IRAS4A, an inverse P Cygni profile is observed, a clear sign of infall in the envelope. From the line profiles alone, it is clear that the bulk of emission arises from shocks, both on small (1000 AU) and large scales along the outflow cavity walls (~10 000 AU). The H<sub>2<sub/>O line profiles are compared to CO line profiles to constrain the H<sub>2<sub/>O abundance as a function of velocity within these shocked regions. The H<sub>2<sub/>O/CO abundance ratios are measured to be in the range of ~0.1-1, corresponding to H<sub>2<sub/>O abundances of ~10<sup>-5<sup/>–10<sup>-4<sup/> with respect to H<sub>2<sub/>. Approximately 5–10% of the gas is hot enough for all oxygen to be driven into water in warm post-shock gas, mostly at high velocities.