Water emission from the chemically rich outflow L1157
Journal article, 2012

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.


magnetohydrodynamic shock-waves



ISM: molecules




chess spectral

molecular outflow

stars: formation

star-forming regions

ISM: jets and outflows




stars: individual: L1157


M. Vasta

Arcetri Astrophysical Observatory

C. Codella

Arcetri Astrophysical Observatory

A. Lorenzani

Arcetri Astrophysical Observatory

G. Santangelo

Osservatorio Astronomico di Roma

B. Nisini

Osservatorio Astronomico di Roma

T. Giannini

Osservatorio Astronomico di Roma

M. Tafalla

Spanish National Observatory (OAN)

René Liseau

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

E. F. van Dishoeck

Max Planck Society

Leiden University

L. Kristensen

Leiden University

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 537 Article Number: A98 - A98

Subject Categories

Astronomy, Astrophysics and Cosmology



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