Context :

Aims : Herschel -HIFI spectra of H _ { 2 } O 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 origin .

Methods : A velocity component with an offset of \sim 10 km s ^ { -1 } detected in spectra of the H _ { 2 } O 1 _ { 10 } –1 _ { 01 } 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 H _ { 2 } O lines .
The emission from this component is quantified and local 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 .

Results : The velocity component is detected in all six targeted H _ { 2 } O transitions ( E _ { up } \sim 50–250 K ) , as well as in CO 16–15 towards one source , Ser SMM1 .
Inferred excitation conditions imply that the emission arises in dense ( n \sim 5 \times 10 ^ { 6 } –10 ^ { 8 } cm ^ { -3 } ) and hot ( T \sim 750 K ) gas .
The H _ { 2 } O and CO column densities are \gtrsim 10 ^ { 16 } and 10 ^ { 18 } cm ^ { -2 } , respectively , implying a low H _ { 2 } O abundance of \sim 10 ^ { -2 } with respect to CO .
The high column densities of ions such as OH ^ { + } and CH ^ { + } ( both \gtrsim 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 100 AU .
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 T _ { rot } \sim 700 K is identified as arising in the same component and traces the part of the shock where H _ { 2 } reforms .
Thus , H _ { 2 } O reveals new dynamical components , even on small spatial scales in low-mass protostars .

Conclusions :