| Abstract
| - Context. In the framework of the Herschel-WISH key program, several ortho-H 2O and para-H 2O 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, which hosts what is considered to be the prototypical chemically-rich outflow. Aims. Our 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. Methods. For this purpose, we ran the non-LTE RADEX model for a plane-parallel geometry to constrain the physical parameters ( Tkin, NH 2O and nH 2) of the water emission lines detected. Results. A total of 5 ortho- and para-H 216O plus one o−H 218O transitions were observed in B2 and R with a wide range of excitation energies (27 K ≤ Eu ≤ 215 K). The H 2O spectra, observed in the two shocked regions, show that the H 2O profiles differ markedly 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 NH 2O ≤ 5 × 10 13 cm -2, a density range of 10 5 ≤ nH 2 ≤ 10 7 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 (~100 K), NH 2O ≃ 5 × 10 12-5 × 10 13 cm -2 and densities nH 2 ≃ 10 6-10 8 cm -3. Lower velocities are associated with higher excitation conditions with Tkin ≥ 300 K, very dense gas ( nH 2 ~ 10 8 cm -3) and low column density ( NH 2O < 5 × 10 13 cm -2). Conclusions. The overall analysis suggests that the emission in B2 comes from an extended ( ≥ 15″) region, whilst we cannot rule out the possibility that the emission in R arises from a smaller ( > 3″) region. In this context, H 2O seems to be important in tracing different gas components with respect to other molecules, e.g. such as SiO, a classical jet tracer. We compare a grid of C- and J-type shocks spanning different velocities (10 to 40 km s -1) and two pre-shock densities (2 × 10 4 and 2 × 10 5 cm -3), with the observed intensities. Although none of these models seem to be able to reproduce the absolute intensities of the water emissions observed, it appears that the occurrence of J-shocks, which can compress the gas to very high densities, cannot be ruled out in these environments.
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