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Context: Outflows are an important part of the star formation process as boththe result of ongoing active accretion and one of the main sources ofmechanical feedback on small scales. Water is the ideal tracer of these effectsbecause it is present in high abundance in various parts of the protostar.Method: We present \textit{Herschel} HIFI spectra of multiple water-transitionstowards 29 nearby Class 0/I protostars as part of the WISH Survey. These aredecomposed into different Gaussian components, with each related to one ofthree parts of the protostellar system; quiescent envelope, cavity shock andspot shocks in the jet and at the base of the outflow. We then constrain theexcitation conditions present in the two outflow-related components. Results:Water emission is optically thick but effectively thin, with line ratios thatdo not vary with velocity, in contrast to CO. The physical conditions of thecavity and spot shocks are similar, with post-shock H$_{2}$ densities of order10$^{5}-$10$^{8}$\,cm$^{-3}$ and H$_{2}$O column densities of order10$^{16}-$10$^{18}$\,cm$^{-2}$. H$_{2}$O emission originates in compactemitting regions: for the spot shocks these correspond to point sources withradii of order 10-200\,AU, while for the cavity shocks these come from a thinlayer along the outflow cavity wall with thickness of order 1-30\,AU.Conclusions: Water emission at the source position traces two distinctkinematic components in the outflow; J shocks at the base of the outflow or inthe jet, and C shocks in a thin layer in the cavity wall. Class I sources havesimilar excitation conditions to Class 0 sources, but generally smallerline-widths and emitting region sizes. We suggest that it is the velocity ofthe wind driving the outflow, rather than the decrease in envelope density ormass, that is the cause of the decrease in H$_{2}$O intensity between Class 0and I.

astronomy and astrophysics

Water in star-forming regions with Herschel (WISH) V. The physical  conditions in low-mass protostellar outflows revealed by multi-transition  water observations