Water along the outflow from a young nearby protostar: a Herschel view

Jets and outflows represent direct evidence of the earliest phases of star formation, being closely associated with disk accretion. Strong collimated winds and/or stellar jets cause ejection of material that then interacts with the dense parent cloud generating strong interstellar shocks. These processes strongly modify the chemical composition of the surrounding gas, enhancing the abundance of several species (such as SiO and H₂O).

Among the different tracers of shocks, water (H₂O) is a key molecule and a unique diagnostic tool of local conditions and energetic processes, since its line emission is very sensitive to both the chemistry and the physical conditions of the gas. H₂O abundance varies by many orders of magnitude during the shock lifetime. In particular, it is expected to increase from <10^-7 in cold regions to about 10^-4 in warm gas, due to the combined effects of dust grain destruction and high-temperature chemical reactions. However, the observations of most protostellar outflow regions show H₂O abundances of at least an order of magnitude lower than expected. This is an important and highly debated topic since H₂O content can give constraints on the physics of shocks and thus, indirectly, on the star formation process.

A study conducted by a team of astronomers led by Gina Santangelo (postdoc at the Arcetri Observatory) with Claudio Codella (Observatory of Arcetri), Brunella Nisini (Rome Observatory), and other collaborators from Italy, Sweden, The Netherlands, Spain, and USA, present observations of several transitions of H₂O performed with the high-resolution spectrometer HIFI and the imaging spectrometer PACS on board the Herschel Space Observatory to characterize the water kinematics and spatial distribution at selected shocked positions along the outflow powered by the young protostar IRAS4A. This young object is a binary system resolved into two components with a separation of only 2 arcseconds (<500 AU) with a well-collimated outflow, extending over arcminute scales (Fig. 1).

Figure 1 shows a detailed view of the IRAS4A outflow through the comparison between the H₂O emission and the emission from several other molecular tracers of gas with different excitation conditions: H₂, CO(3-2), CO(14-13), and CO(6-5). The figure nicely reveals where shocks deposit energy into the molecular cloud, lighting up the water emission along the outflow. Strong H₂O emission peaks are indeed found at the position of the IRAS4A source and at the location of active shocked regions. The H₂O emission is spatially correlated with that of H₂ and of high-excitation CO, whereas the low-excitation CO traces a more extended emission.

Fig. 1. Herschel-PACS map of the H2O emission at 1670 GHz (image on the right and black contours on the left) of the IRAS4 region compared with the Spitzer-IRAC emission at 4.5 ÞÌm and the CO(14-13) emission in the left-panel and with the JCMT CO(3-2) and APEX CO(6-5) emission in the right panel. The positions of the IRAS4A and IRAS4B binary sources are marked with yellow symbols. The regions mapped in the water lines with Herschel-HIFI around the R1 and R2 shock positions are indicated (dashed rectangles). Strong H2O emission peaks are found at the position of the IRAS4A source and at the location of active shocked regions (e.g. R1 and R2). The H2O emission is spatially correlated with that of H2 and of high-excitation CO.

The R2 shock position, far from the driving source of the outflow, is a very interesting laboratory to study the H₂O distribution around shocks. Figure 2 shows the spectra of the H₂O transition at 1113 GHz observed with HIFI. The figure highlights the complex water line profiles and their significant variation around the shocked region. In particular,a triangularly shaped outflow wing up to about 60 km/s is uniformly present across the region; in addition, a secondary emission peak at high velocity (+35 km/s) is observed only at the bright shock peak. These line profiles are interpreted with the presence of two different gas components in the water emission: a compact gas component, located at the shock peak and associated with the high-velocity peak, and a more diffuse emission component.

Fig. 2. Herschel-HIFI spectra around R2 of the H2O line at 1113 GHz. Spectra are overlaid on the Herschel-PACS H2O map at 1670 GHz (grey scale and white contours). Two different gas components are identified in the H2Oline profiles: a compact one at the shock position, associated with the high-velocity peak, and a more diffuse one (triangularly shaped wing).

The analysis of the H₂O line ratios shows that the extended component (2400-4000 AU) is associated with a warm (300-500 K) and dense gas with H₂O abundance <10^-6 and the compact component (700 AU) corresponds to a hot (~1000 K) and more tenuous gas with higher H₂O abundance of the order of a few 10^-5. For the first time, these two temperature components, already identified in other protostellar outflows observed with Herschel, have been here spatially resolved. It is proposed 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. To confirm this scenario high-angular resolution observations are crucial to probe the structure of the investigated shock region in depth.

The results of this study are published in ò??Water distribution in shocked regions of the NGC1333-IRAS4A protostellar outflowò??, Santangelo et al., A&A 568, A125.

Edited by Gina Santangelo and Anna Gallazzi, 3/10/2014