Herschel–PACS observations of shocked gas associated with the jets of L1448 and L1157^⋆,⋆⋆

¹ Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Italy
e-mail: gina.santangelo@oa-roma.inaf.it
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

Abstract

Aims. In the framework of the Water In Star-forming regions with Herschel (WISH) key program, several H₂O (E_u > 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 (E_u = 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 E_u = 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 E_u > 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.