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

¹ INAF – Istituto di Astrofisica e Planetologia Spaziali, via del Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: gemma.busquet@iaps.inaf.it
² UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
³ Centro de Astrobiologia, CSIC-INTA, Carretera de Torrejón a Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁵ Observatoire de Paris, LERMA, UMR 8112 du CNRS, ENS, UPMC, UCP, 61 Av. de l’Observatoire, 75014 Paris, France
⁶ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁷ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁸ LERMA, UMR 8112 du CNRS, Observatoire de Paris, École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France

Received: 23 July 2013
Accepted: 12 November 2013

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 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 (~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 ≃ 200–300 K) and dense (n(H₂) ≃ (1–3) × 10⁶ cm^-3) component with an assumed extent of 10′′, and a compact (~2′′–5′′) and hot, tenuous (T_kin ≃ 900–1400 K, n(H₂) ≃ 10³⁻⁴ cm^-3) gas component that is needed to account for the line fluxes of high E_u transitions. The fractional abundance of the warm and hot H₂O 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 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.