Volume 557, September 2013
|Number of page(s)||24|
|Section||Interstellar and circumstellar matter|
|Published online||20 August 2013|
1 Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Italy
2 Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Florence, Italy
3 LERMA, Observatoire de Paris, UMR 8112 of the CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
4 Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Hai Dian Qu, 100871 Beijing, PR China
5 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
6 Observatorio Astronómico Nacional (IGN), Alfonso XII 3, 28014 Madrid, Spain
7 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
8 Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
Received: 9 April 2013
Accepted: 21 June 2013
Aims. In the framework of the Water In Star-forming regions with Herschel (WISH) key program, several H2O (Eu > 190 K), high-J CO, [Oi], and OH transitions are mapped with Herschel-PACS in two shock positions along two prototypical outflows around the low-luminosity sources L1448 and L1157. Previous Herschel-HIFI H2O observations (Eu = 53−249 K) are also used. The aim is to derive a complete picture of the excitation conditions at the selected shock positions.
Methods. We adopted a large velocity gradient analysis (LVG) to derive the physical parameters of the H2O and CO emitting gas. Complementary Spitzer mid-IR H2 data were used to derive the H2O abundance.
Results. Consistent with other studies, at all selected shock spots a close spatial association between H2O, mid-IR H2, and high-J CO emission is found, whereas the low-J CO emission traces either entrained ambient gas or a remnant of an older shock. The excitation analysis, conducted in detail at the L1448-B2 position, suggests that a two-component model is needed to reproduce the H2O, CO, and mid-IR H2 lines: an extended warm component (T ~ 450 K) is traced by the H2O emission with Eu = 53−137 K and by the CO lines up to J = 22−21, and a compact hot component (T = 1100 K) is traced by the H2O emission with Eu > 190 K and by the higher-J CO transitions. At L1448-B2 we obtain an H2O abundance (3−4) × 10-6 for the warm component and (0.3−1.3) × 10-5 for the hot component and a CO abundance of a few 10-5 in both components. In L1448-B2 we also detect OH and blue-shifted [Oi] emission, spatially coincident with the other molecular lines and with [Feii] emission. This suggests a dissociative shock for these species, related to the embedded atomic jet. On the other hand, a non-dissociative shock at the point of impact of the jet on the cloud is responsible for the H2O and CO emission. The other examined shock positions show an H2O excitation similar to L1448-B2, but a slightly higher H2O abundance (a factor of ~4).
Conclusions. The two gas components may represent a gas stratification in the post-shock region. The extended and low-abundance warm component traces the post-shocked gas that has already cooled down to a few hundred Kelvin, whereas the compact and possibly higher-abundance hot component is associated with the gas that is currently undergoing a shock episode. This hot gas component is more affected by evolutionary effects on the timescales of the outflow propagation, which explains the observed H2O abundance variations.
Key words: stars: formation / stars: low-mass / ISM: jets and outflows / ISM: molecules / ISM: individual objects: L1448 / ISM: individual objects: L1157
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2013
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