The CHESS survey of the L1157-B1 bow-shock: high and low excitation water vapor

Context. Molecular outflows powered by young protostars strongly affect the kinematics and chemistry of the natal molecular cloud through strong shocks. This results in substantial modifications of the abundance of several species. In particular, water is a powerful tracer of shocked material because of its sensitivity to both physical conditions and chemical processes. Aims. As part of the Chemical HErschel Surveys of Star-forming regions (CHESS) guaranteed time key program, we aim at investigating the physical and chemical conditions of H2O in the brightest shock region B1 of the L1157 molecular outflow. Methods. We observed several ortho-and para-H2O transitions using the HIFI and PACS instruments on board Herschel toward L1157-B1, providing a detailed picture of the kinematics and spatial distribution of the gas. We performed a large velocity gradient (LVG) analysis to derive the physical conditions of H2O shocked material, and ultimately obtain its abundance. Results. We detected 13 H2O lines with both instruments probing a wide range of excitation conditions. This is the largest data set of water lines observed in a protostellar shock and it provides both the kinematics and the spatial information of the emitting gas. The PACS maps reveal that H2O traces weak and extended emission associated with the outflow identified also with HIFI in the o-H2O line at 556.9 GHz, and a compact (similar to 10 '') bright, higher excitation region. The LVG analysis of H2O lines in the bow-shock show the presence of two gas components with different excitation conditions: a warm (T-kin similar or equal to 200-300 K) and dense (n(H-2) similar or equal to (1-3) x 10(6) cm(-3)) component with an assumed extent of 10 '', and a compact (similar to 2 ''-5 '') and hot, tenuous (T-kin similar or equal to 900-1400 K, n(H-2) similar or equal to 10(3-4) cm(-3)) gas component that is needed to account for the line fluxes of high E-n transitions. The fractional abundance of the warm and hot H2O gas components is estimated to be (0.7-2) x 10(-6) and (1-3) x 10(-4), respectively. Finally, we identified an additional component in absorption in the HIFI spectra of H2O lines that connect with the ground state level. This absorption probably arises from the photodesorption of icy mantles of a water-enriched layer at the edges of the cloud, driven by the external UV illumination of the interstellar radiation field.