Herschel-PACS observations of shocked gas associated with the jets of L1448 and L1157
Journal article, 2013

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 H-2 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 H-2, 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 H-2 lines: an extended warm component (T similar to 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) x 10(-6) for the warm component and (0.3-1.3) x 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 (HO)-O-2 and CO emission. The other examined shock positions show an H2O excitation similar to L1448-B2, but a slightly higher (HO)-O-2 abundance (a factor of similar to 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.

stars: formation

ISM: molecules

stars: low-mass

ISM: individual objects: L1157

ISM: jets and outflows

ISM: individual objects: L1448


G. Santangelo

Osservatorio Astronomico di Roma

B. Nisini

Osservatorio Astronomico di Roma

S. Antoniucci

Osservatorio Astronomico di Roma

C. Codella

Arcetri Astrophysical Observatory

S. Cabrit

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

T. Giannini

Osservatorio Astronomico di Roma

G. J. Herczeg

Beijing University of Technology

René Liseau

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

M. Tafalla

Spanish National Observatory (OAN)

E. F. van Dishoeck

Leiden University

Max Planck Society

Astronomy and Astrophysics

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

Vol. 557 A22

Subject Categories

Astronomy, Astrophysics and Cosmology



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