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Herschel-HIFI observations of water in the low-mass star-forming objectL1448-MM, known for its prominent outflow, are presented, as obtained withinthe `Water in star-forming regions with Herschel' (WISH) key programme. SixH2-16O lines are targeted and detected (E_up/k_B ~ 50-250 K), as is CO J= 10-9(E_up/k_B ~ 305 K), and tentatively H2-18O 110-101 at 548 GHz. All lines showstrong emission in the "bullets" at |v| > 50 km/s from the source velocity, inaddition to a broad, central component and narrow absorption. The bullets areseen much more prominently in H$_2$O than in CO with respect to the centralcomponent, and show little variation with excitation in H2O profile shape.Excitation conditions in the bullets derived from CO lines imply a temperature>150 K and density >10^5 cm^-3, similar to that of the broad component. TheH2O/CO abundance ratio is similar in the "bullets" and the broad component, ~0.05-1.0, in spite of their different origins in the molecular jet and theinteraction between the outflow and the envelope. The high H2O abundanceindicates that the bullets are H2 rich. The H2O cooling in the "bullets" andthe broad component is similar and higher than the CO cooling in the samecomponents. These data illustrate the power of Herschel-HIFI to disentangledifferent dynamical components in low-mass star-forming objects and determinetheir excitation and chemical conditions.

Water in low-mass star-forming regions with Herschel (WISH-LM): High-velocity H2O bullets in L1448-MM observed with HIFI