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

Abstract

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 H 2 O line emission from low-mass protostars and their associated outflows performed with HIFI onboard the Herschel Space Observatory have highlighted the complexity of H 2 O line profiles, in which different kinematic components can be distinguished. Aims. The goal is to study the spatial distribution of H 2 O, in particular of the different kinematic components detected in H 2 O emission, at two bright shocked regions along IRAS 4A, one of the strongest H 2 O 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 H 2 O 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 H 2 O 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 H 2 O and CO studies, the LVG analysis of the H 2 O 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 H 2 ∼ (1−4) × 10 5 cm −3 , whereas the extended (10 −17 , corresponding to 2400−4000 AU) one traces a warm (T ∼ 300−500 K) and dense gas (n H 2 ∼ (3−5) × 10 7 cm −3 ). Finally, using the CO (16−15) emission observed at R2 and assuming a typical CO/H 2 abundance of 10 −4 , we estimate the H 2 O/H 2 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.

Water in star-forming regions withHerschel(WISH)

“…Figure 11 shows, in solid lines, a comparison of the 1σ χ 2 contours for fits between grids of radex models over N H 2 O and n H 2 at a range of temperatures from 100−1500 K to the average line ratios for cavity and spot shocks given in Table 5 Aside from the 100 K model, there is a slight trend towards higher n H 2 and lower N H 2 O with increasing temperature but the contours and emitting region sizes derived from the different models are effectively the same within the uncertainties. This insensitivity of low-J water emission lines to temperature within the post-shock density and column density regime present towards our sources has also been seen in other studies (see also Kristensen et al 2013;Santangelo et al 2014). The temperature and density cannot both be the same as traced by low-J CO, which traces temperatures of ∼100 K but is insensitive to density (Yıldiz et al 2013), because otherwise water would trace similar gas (and thus have similar line profiles), which is not the case (e.g.…”

“…While Gaussian decomposition similar to that used here has not been presented for those observations, some additional slightly offset features can be seen in some line profiles which are reminiscent of the onsource spot shock component. At some locations, particularly away from the brightest parts of the outflow, the line shape becomes more like the classical triangular shape of some CO outflows (Santangelo et al 2014). In addition, there can be significant differences in line shape between H 2 O transitions, resulting in line ratios (and thus excitation conditions) which vary with velocity.…”

“…Nisini et al 2010;Santangelo et al 2012) with the dominant extended component having similar velocity distributions (e.g. Santangelo et al 2014) as this component. As cavity shocks also dominate the on-source line profiles, we assume that it is elongated along the outflow direction but does not fill the beam parallel to the outflow axis, as in the spectrally unresolved PACS H 2 O observations.…”

“…In addition, there can be significant differences in line shape between H 2 O transitions, resulting in line ratios (and thus excitation conditions) which vary with velocity. Table 7 presents a summary of recent determinations of the excitation conditions towards such regions for some of the sources present in our study (Santangelo et al , 2014Busquet et al 2014). Aside from the differences in isolating which emission to integrate over, these studies used similar Large Velocity Gradient models in their analysis, sometimes simultaneously fitting emission from water and other species also expected to originate in the outflow.…”

Jet multiplicity in the proto-binary system NGC 1333-IRAS4A

“…5) A1 lobes seen in our SiO map. Since there is no H 2 or single-dish CO outflow signature further along the A1 jet axis, as can be seen on larger scales from the IRAC and APEX maps in Santangelo et al (2014), it appears that here we are tracing the terminal shocks, where the A1 outflow impacts the ambient cloud. Remarkably, our observations detect a sharp bend to the south-west of the blue-shifted A2 jet at a distance of about 4 from the driving source.…”