Water emission from the chemically rich outflow L1157

In the framework of the Herschel-WISH key program, several ortho-H2O and para-H2O emission lines, in the frequency range from 500 to 1700 GHz, were observed with the HIFI instrument in two bow-shock regions (B2 and R) of the L1157 cloud. The primary aim is to analyse water emission lines as a diagnostic of the physical conditions in the blue (B2) and red-shifted (R) lobes to compare the excitation conditions. A total of 5 ortho- and para-H216O plus one o-H218O transitions were observed in B2 and R with a wide range of excitation energies (27 K<=Eu<=215 K). The H2O spectra, observed in the two shocked regions, show that the H2O profiles are markedly different in the two regions. In particular, at the bow-shock R, we observed broad (~30 km s-1 with respect to the ambient velocity) red-shifted wings where lines at different excitation peak at different red-shifted velocities. The B2 spectra are associated with a narrower velocity range (~6 km s-1), peaking at the systemic velocity. The excitation analysis suggests, for B2, low values of column density NH2O <=5{\times}1013 cm-2, a density range of 105 <=nH2 <=107 cm-3, and warm temperatures (>=300 K). The presence of the broad red-shifted wings and multiple peaks in the spectra of the R region, prompted the modelling of two components. High velocities are associated with relatively low temperatures (~100K),NH2O{\simeq}5{\times}1012-5{\times}1013 cm-2 and densities nH2{\simeq}106-108 cm-3.Lower velocities are associated with higher excitation conditions with Tkin>=300 K, very dense gas (nH2 ~108 cm-3) and low column density (NH2O<5{\times}1013 cm-2).