The CHESS survey of the L1157-B1 bow-shock: high and low excitation
  water vapor
  Molecular outflows powered by young protostars strongly affect the kinematics
and chemistry of the natal molecular cloud through strong shocks resulting in
substantial modifications of the abundance of several species. As part of the
"Chemical Herschel Surveys of Star forming regions" guaranteed time key
program, we aim at investigating the physical and chemical conditions of H20 in
the brightest shock region B1 of the L1157 molecular outflow. We observed
several ortho- and para-H2O transitions using HIFI and PACS instruments on
board Herschel, providing a detailed picture of the kinematics and spatial
distribution of the gas. We performed a LVG analysis to derive the physical
conditions of H2O shocked material, and ultimately obtain its abundance. We
detected 13 H2O lines probing a wide range of excitation conditions. 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 (Tkin~200-300 K) and dense (n(H2)~(1-3)x10^6 cm-3) component
with an assumed extent of 10" and a compact (~2"-5") and hot, tenuous
(Tkin~900-1400 K, n(H2)~10^3-10^4 cm-3) gas component, which is needed to
account for the line fluxes of high Eu transitions. The fractional abundance of
the warm and hot H2O gas components is estimated to be (0.7-2)x10^{-6} and
(1-3)x10^{-4}, respectively. Finally, we identified an additional component in
absorption in the HIFI spectra of H2O lines connecting with the ground state
level, probably arising from the photodesorption of icy mantles of a
water-enriched layer at the edges of the cloud.
