Water emission from the chemically rich outflow L1157

TL;DR

This study uses Herschel observations of water emission lines in the L1157 outflow to analyze physical conditions in shock regions, revealing distinct excitation and velocity characteristics.

Contribution

It provides detailed excitation analysis of water emission in two shock regions, distinguishing their physical conditions and modeling multiple components with different velocities and temperatures.

Findings

Different water emission profiles in B2 and R regions.

B2 has low column density and warm temperatures (~300 K).

R shows broad wings with high velocities and multiple excitation components.

Abstract

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. The 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. 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 (27 K<=Eu<=215 K). The H2O spectra, observed in the two shocked regions, show that the H2O profiles are markedly different in the two regions. In particular, at the bow-shock R, we observed broad (~30 km s-1 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 (~6 km s-1), peaking at the systemic velocity. The excitation analysis suggests, for B2, low values of column density NH2O <=5{\times}1013 cm-2, a density range of 105 <=nH2 <=107 cm-3, and warm temperatures (>=300 K). 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),NH2O{\simeq}5{\times}1012-5{\times}1013 cm-2 and densities nH2{\simeq}106-108 cm-3.Lower velocities are associated with higher excitation conditions with Tkin>=300 K, very dense gas (nH2 ~108 cm-3) and low column density (NH2O<5{\times}1013 cm-2).