High-pressure, low-abundance water in bipolar outflows Results from a Herschel-WISH survey

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 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 4x10(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 x 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 x 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