| Abstract
| - 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 H 2O in the brightest shock region B1 of the L1157 molecular outflow. Methods. We observed several ortho- and para-H 2O 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 H 2O shocked material, and ultimately obtain its abundance. Results. We detected 13 H 2O 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 H 2O traces weak and extended emission associated with the outflow identified also with HIFI in the o-H 2O line at 556.9 GHz, and a compact (~10′′) bright, higher excitation region. The LVG analysis of H 2O lines in the bow-shock show the presence of two gas components with different excitation conditions: a warm ( Tkin ≃ 200-300 K) and dense ( n(H 2) ≃ (1-3) × 10 6 cm -3) component with an assumed extent of 10′′, and a compact (~2′′-5′′) and hot, tenuous ( Tkin ≃ 900-1400 K, n(H 2) ≃ 10 3−4 cm -3) gas component that is needed to account for the line fluxes of high Eu transitions. The fractional abundance of the warm and hot H 2O gas components is estimated to be (0.7-2) × 10 -6 and (1-3) × 10 -4, respectively. Finally, we identified an additional component in absorption in the HIFI spectra of H 2O 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.
|
