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 H₂ 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 H₂, and its intensity follows the H₂ intensity derived from IRAC images. This suggests that water traces the outflow gas at hundreds of kelvins that is responsible for the H₂ 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 × 10⁹ cm^-3K, which represents an increase of 10⁴ 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 × 10¹² and 10¹⁴ cm^-2. When these values are combined with estimates of the H₂ column density, the typical water abundance is only 3 × 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.