Water in low-mass star-forming regions with Herschel: HIFI spectroscopy of NGC1333

TL;DR

This study uses Herschel's HIFI instrument to analyze water emission in low-mass star-forming regions, revealing complex line profiles and shock-related origins, and constraining water's abundance and physical conditions.

Contribution

It provides detailed spectroscopic analysis of water in NGC1333, showing the shock origin of water emission and quantifying water abundance in protostellar environments.

Findings

Water emission profiles are complex and decomposed into broad, medium, and narrow components.

H2-18O detected only in broad lines, indicating similar origin as broad H2-16O.

Water abundance relative to H2 is estimated to be 10^-5 to 10^-4.

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 NGC1333 in several H2-16O, H2-18O, and CO transitions. Line profiles are resolved for five H16O transitions in each source, revealing them to be surprisingly complex. The line profiles are decomposed into broad (>20 km/s), medium-broad (~5-10 km/s), and narrow (<5 km/s) components. The H2-18O emission is only detected in broad 1_10-1_01 lines (>20 km/s), indicating that its physical origin is the same as for the broad H2-16O 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 H2O abundance as a function of velocity within these shocked regions. The H2O/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 H2. 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.