Water distribution in shocked regions of the NGC 1333-IRAS 4A protostellar outflow.

Context. Water is a key molecule in protostellar environments because its line emission is very sensitive to both the chemistry and the physical conditions of the gas. Observations of H₂O line emission from low-mass protostars and their associated outflows performed with HIFI onboard the Herschel Space Observatory have highlighted the complexity of H₂O line profiles, in which different kinematic components can be distinguished. Aims. The goal is to study the spatial distribution of H₂O, in particular of the different kinematic components detected in H₂O emission, at two bright shocked regions along IRAS 4A, one of the strongest H₂O emitters among the Class 0 outflows. Methods. We obtained Herschel-PACS maps of the IRAS 4A outflow and HIFI observations of two shocked positions. The largest HIFI beam of 38" at 557 GHz was mapped in several key water lines with different upper energy levels, to reveal possible spatial variations of the line profiles. A large velocity gradient (LVG) analysis was performed to determine the excitation conditions of the gas. Results. We detect four H₂O lines and CO (16–15) at the two selected shocked positions. In addition, transitions from related outflow and envelope tracers are detected. Different gas components associated with the shock are identified in the H₂O emission. In particular, at the head of the red lobe of the outflow, two distinct gas components with different excitation conditions are distinguished in the HIFI emission maps: a compact component, detected in the ground-state water lines, and a more extended one. Assuming that these two components correspond to two different temperature components observed in previous H₂O and CO studies, the LVG analysis of the H₂O emission suggests that the compact (about 3" , corresponding to about 700 AU) component is associated with a hot (T ~ 1000 K) gas with densities n_H2 ~ (1–4) × 10⁵ cm^-3, whereas the extended (10"–17" , corresponding to 2400–4000 AU) one traces a warm (T ~ 300–500 K) and dense gas (n_H2 ~ (3–5) × 10⁷ cm^-3). Finally, using the CO (16–15) emission observed at R2 and assuming a typical CO/H₂ abundance of 10^-4, we estimate the H₂O/H₂ abundance of the warm and hot components to be (7–10) × 10^-7 and (3–7) × 10^-5. Conclusions. Our data allowed us, for the first time, to resolve spatially the two temperature components previously observed with HIFI and PACS. We propose that the compact hot component may be associated with the jet that impacts the surrounding material, whereas the warm, dense, and extended component originates from the compression of the ambient gas by the propagating flow. [ABSTRACT FROM AUTHOR]