The distribution of water in the high-mass star-forming region NGC 6334 I

Abstract

Aims. We present observations of twelve rotational transitions of H216O, H218O, and H217O toward the massive star-forming region NGC 6334 I, carried out with Herschel/HIFI as part of the guaranteed time key program Chemical HErschel Surveys of Star forming regions (CHESS). We analyze these observations to obtain insights into physical processes in this region.Methods. We identify three main gas components (hot core, cold foreground, and outflow) in NGC 6334 I and derive the physical conditions in these components. Results. The hot core, identified by the emission in highly excited lines, shows a high excitation temperature of ~200 K, whereas water in the foreground component is predominantly in the ortho- and para- ground states. The abundance of water varies between 4 × 10-5 (outflow) and 10-8 (cold foreground gas). This variation is most likely due to the freeze-out of water molecules onto dust grains. The H218O/H217O abundance ratio is 3.2, which is consistent with the 18O/17O ratio determined from CO isotopologues. The ortho/para ratio in water appears to be relatively low (1.6±1) in the cold, quiescent gas, but close to the equilibrium value of three in the warmer outflow material (2.5±0.8).