Water in low-mass star-forming regions with Herschel: HIFI spectroscopy of NGC 1333

Abstract

"Water In Star-forming regions with Herschel" (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 Herschel Space Observatory observed three deeply embedded protostars in the low-mass star-forming region NGC 1333 in several H_(2)^(16)O, H_(2)^(18)O, and CO transitions. Line profiles are resolved for five H_(2)^(16)O transitions in each source, revealing them to be surprisingly complex. The line profiles are decomposed into broad (>20 km s^(-1)), medium-broad (~5-10 km s^(-1)), and narrow (<5 km s^(-1)) components. The H_(2)^(18)O emission is only detected in broad 1_(10)–1_(01) lines (>20 km s^(-1)), indicating that its physical origin is the same as for the broad H_(2)^(16)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 H2O line profiles are compared to CO line profiles to constrain the H_(2)O abundance as a function of velocity within these shocked regions. The H_(2)O/CO abundance ratios are measured to be in the range of ~0.1-1, corresponding to H2O abundances of ~10^(-5)–10^(-4) with respect to H_2. 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.

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