Herschel-HIFI spectra of H2O towards low-mass protostars show a distinct velocity component not seen in observations from the ground of CO or other species. The aim is to characterise this component in terms of excitation conditions and physical orig
in. A velocity component with an offset of ~10 km/s detected in spectra of the H2O 110-101 557 GHz transition towards six low-mass protostars in the Water in star-forming regions with Herschel (WISH) programme is also seen in higher-excited H2O lines. The emission from this component is quantified and excitation conditions are inferred using 1D slab models. Data are compared to observations of hydrides (high-J CO, OH+, CH+, C+, OH) where the same component is uniquely detected. The velocity component is detected in all 6 targeted H2O transitions (Eup~50-250K), and in CO 16-15 towards one source, Ser SMM1. Inferred excitation conditions imply that the emission arises in dense (n~5x10^6-10^8 cm^-3) and hot (T~750K) gas. The H2O and CO column densities are ~10^16 and 10^18 cm^-2, respectively, implying a low H2O abundance of 10^-2 with respect to CO. The high column densities of ions such as OH+ and CH+ (both ~10^13 cm^-2) indicate an origin close to the protostar where the UV field is strong enough that these species are abundant. The estimated radius of the emitting region is 100AU. This component likely arises in dissociative shocks close to the protostar, an interpretation corroborated by a comparison with models of such shocks. Furthermore, one of the sources, IRAS4A, shows temporal variability in the offset component over a period of two years which is expected from shocks in dense media. High-J CO gas detected with Herschel-PACS with Trot~700K is identified as arising in the same component and traces the part of the shock where H2 reforms. Thus, H2O reveals new dynamical components, even on small spatial scales in low-mass protostars.
`Water In Star-forming regions with Herschel (WISH) is a key program on the Herschel Space Observatory designed to probe the physical and chemical structure of young stellar objects using water and related molecules and to follow the water abundance
from collapsing clouds to planet-forming disks. About 80 sources are targeted covering a wide range of luminosities and evolutionary stages, from cold pre-stellar cores to warm protostellar envelopes and outflows to disks around young stars. Both the HIFI and PACS instruments are used to observe a variety of lines of H2O, H218O and chemically related species. An overview of the scientific motivation and observational strategy of the program is given together with the modeling approach and analysis tools that have been developed. Initial science results are presented. These include a lack of water in cold gas at abundances that are lower than most predictions, strong water emission from shocks in protostellar environments, the importance of UV radiation in heating the gas along outflow walls across the full range of luminosities, and surprisingly widespread detection of the chemically related hydrides OH+ and H2O+ in outflows and foreground gas. Quantitative estimates of the energy budget indicate that H2O is generally not the dominant coolant in the warm dense gas associated with protostars. Very deep limits on the cold gaseous water reservoir in the outer regions of protoplanetary disks are obtained which have profound implications for our understanding of grain growth and mixing in disks.
