2012A&A...537A..98V

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, which hosts what is considered to be the prototypical chemically-rich outflow. 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.For this purpose, we ran the non-LTE RADEX model for a plane-parallel geometry to constrain the physical parameters (T_kin, N_H2_O and n_H2) of the water emission lines detected. 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 (27K≤E_u≤215K). The H₂O spectra, observed in the two shocked regions, show that the H₂O profiles differ markedly in the two regions. In particular, at the bow-shock R, we observed broad (∼30km/s 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 (∼6km/s), peaking at the systemic velocity. The excitation analysis suggests, for B2, low values of column density N_H2_O≤5x10¹³/cm2, a density range of 10⁵≤n_H2≤10⁷cm^–3, and warm temperatures (≥300K). 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), N_H2_O≃5x10¹²-5x10¹³cm^–2 and densities n_H2≃10⁶-10⁸cm^–3. Lower velocities are associated with higher excitation conditions with T_kin≥300K, very dense gas (n_H2∼10⁸cm^–3) and low column density (N_H2_O<5x10¹³cm^–2). 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, H₂O 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 40km/s) and two pre-shock densities (2x10⁴ and 2x10⁵cm^–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.