Context. Water is a potential tracer of outflow activity because it is heavily depleted in cold ambient gas and is copiously produced in shocks. Aims: We present a survey of the water emission in... Show moreContext. Water is a potential tracer of outflow activity because it is heavily depleted in cold ambient gas and is copiously produced in shocks. Aims: We present a survey of the water emission in a sample of more than 20 outflows from low-mass young stellar objects with the goal of characterizing the physical and chemical conditions of the emitting gas. Methods: We used the HIFI and PACS instruments on board the Herschel Space Observatory to observe the two fundamental lines of ortho-water at 557 and 1670 GHz. These observations were part of the ''Water In Star-forming regions with Herschel'' (WISH) key program, and have been complemented with CO and H$_{2}$ data. Results: The emission of water has a different spatial and velocity distribution from that of the J = 1-0 and 2-1 transitions of CO. On the other hand, it has a similar spatial distribution to H$_{2}$, and its intensity follows the H$_{2}$ intensity derived from IRAC images. This suggests that water traces the outflow gas at hundreds of kelvins that is responsible for the H$_{2}$ emission, and not the component at tens of kelvins typical of low-J CO emission. A warm origin of the water emission is confirmed by a remarkable correlation between the intensities of the 557 and 1670 GHz lines, which also indicates that the emitting gas has a narrow range of excitations. A radiative transfer analysis shows that while there is some ambiguity in the exact combination of density and temperature values, the gas thermal pressure nT is constrained within less than a factor of 2. The typical nT over the sample is 4 { imes} 10$^{9}$ cm$^{-3}$K, which represents an increase of 10$^{4}$ with respect to the ambient value. The data also constrain the water column density within a factor of 2 and indicate values in the sample between 2 { imes} 10$^{12}$ and 10$^{14}$ cm$^{-2}$. When these values are combined with estimates of the H$_{2}$ column density, the typical water abundance is only 3 { imes} 10$^{-7}$, with an uncertainty of a factor of 3. Conclusions: Our data challenge current C-shock models of water production through the combination of wing-line profiles, high gas compressions, and low abundances. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Show less
