Volume 537, January 2012
|Number of page(s)||9|
|Section||Interstellar and circumstellar matter|
|Published online||16 January 2012|
Water emission from the chemically rich outflow L1157
1 Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Florence, Italy
2 INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Italy
3 Observatorio Astronomico Nacional (IGN), Calle Alfonso XII 3, 28014 Madrid, Spain
4 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
5 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
6 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
Received: 4 October 2011
Accepted: 30 October 2011
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 (Tkin, NH2O and nH2) 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 ≤ Eu ≤ 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 NH2O ≤ 5 × 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 (~100 K), NH2O ≃ 5 × 1012–5 × 1013 cm-2 and densities nH2 ≃ 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 × 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.
Key words: ISM: molecules / stars: formation / ISM: jets and outflows / stars: low-mass / stars: individual: L1157
© ESO, 2012
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