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Water emission from the chemically rich outflow L1157

M. Vasta ; C. Codella ; A. Lorenzani ; G. Santangelo ; B. Nisini ; T. Giannini ; M. Tafalla ; René Liseau (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; E. F. van Dishoeck ; L. Kristensen
Astronomy & Astrophysics (0004-6361). Vol. 537 (2012), p. Article Number: A98 .
[Artikel, refereegranskad vetenskaplig]

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 nH(2)) of the water emission lines detected. Results. A total of 5 ortho- and para-(H2O)-O-16 plus one o-(H2O)-O-18 transitions were observed in B2 and R with a wide range of excitation energies (27K <= 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 (similar to 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 (similar to 6 km s(-1)), peaking at the systemic velocity. The excitation analysis suggests, for B2, low values of column density N-H2O <= 5 x 10(13) cm(-2), a density range of 10(5) <= nH(2) <= 10(7) 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 (similar to 100 K), N-H2O similar or equal to 5 x 10(12)-5 x 10(13) cm(-2) and densities nH(2) similar or equal to 10(6)-10(8) cm(-3). Lower velocities are associated with higher excitation conditions with T-kin >= 300 K, very dense gas (nH(2) similar to 10(8) cm(-3)) and low column density (N-H2O < 5 x 10(13) 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 x 10(4) and 2 x 10(5) 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.

Nyckelord: ISM: molecules, stars: formation, ISM: jets and outflows, stars:, low-mass, stars: individual: L1157, star-forming regions, magnetohydrodynamic shock-waves, chess spectral, survey, molecular outflow, sio, excitation, h-2, program, ammonia, clouds

Denna post skapades 2012-04-19.
CPL Pubid: 156813


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