Water emission from the chemically rich outflow L1157

Abstract

Context. 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, which hosts what is considered to be the prototypical chemically-rich outflow.
Aims: Our 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.
Methods: For this purpose, we ran the non-LTE RADEX model for a plane-parallel geometry to constrain the physical parameters (T_kin, N_H2O and n_H2) of the water emission lines detected.
Results: 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 ≤ E_u ≤ 215 K). The H₂O spectra, observed in the two shocked regions, show that the H₂O profiles differ markedly 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 N_H2O ≤ 5 × 10¹³ cm^-2, a density range of 10⁵ ≤ n_H2 ≤ 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 (~100 K), N_H2O ≃ 5 × 10¹²-5 × 10¹³ cm^-2 and densities n_H2 ≃ 10⁶-10⁸ cm^-3. Lower velocities are associated with higher excitation conditions with T_kin ≥ 300 K, very dense gas (n_H2 ~ 10⁸ cm^-3) and low column density (N_H2O < 5 × 10¹³ cm^-2).
Conclusions: The overall analysis suggests that the emission in B2 comes from an extended ( ≥ 15″) region, whilst we cannot rule out the possibility that the emission in R arises from a smaller ( > 3″) region. In this context, H₂O seems to be important in tracing different gas components with respect to other molecules, e.g. such as SiO, a classical jet tracer. We compare a grid of C- and J-type shocks spanning different velocities (10 to 40 km s^-1) and two pre-shock densities (2 × 10⁴ and 2 × 10⁵ cm^-3), with the observed intensities. Although none of these models seem to be able to reproduce the absolute intensities of the water emissions observed, it appears that the occurrence of J-shocks, which can compress the gas to very high densities, cannot be ruled out in these environments.