Astronomy and Astrophysics, volume 537A, 98-98 (2012/1-1)
Water emission from the chemically rich outflow L1157.
VASTA M., CODELLA C., LORENZANI A., SANTANGELO G., NISINI B., GIANNINI T., TAFALLA M., LISEAU R., VAN DISHOECK E.F. and KRISTENSEN L.
Abstract (from CDS):
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. 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.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. 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 (27K≤Eu≤215K). 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 (∼30km/s 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 (∼6km/s), peaking at the systemic velocity. The excitation analysis suggests, for B2, low values of column density NH2O≤5x1013/cm2, a density range of 105≤nH2≤107cm–3, and warm temperatures (≥300K). 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≃5x1012-5x1013cm–2 and densities nH2≃106-108cm–3. Lower velocities are associated with higher excitation conditions with Tkin≥300K, very dense gas (nH2∼108cm–3) and low column density (NH2O<5x1013cm–2). 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 40km/s) and two pre-shock densities (2x104 and 2x105cm–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.
ISM: molecules - stars: formation - ISM: jets and outflows - stars: low-mass - stars: individual: L1157
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