Herschel-PACS observations of shocked gas associated with the jets of L1448 and L1157

Santangelo, G.; Nisini, B.; Antoniucci, S.; Codella, C.; Cabrit, S.; Giannini, T.; Herczeg, G.; Liseau, R.; Tafalla, M.; Dishoeck, E.F. van

doi:doi:10.1051/0004-6361/201321674

Aims: In the framework of the Water In Star-forming regions with Herschel (WISH) key program, several H$_{2}$O (E$_u$ {gt} 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 H$_{2}$O 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 H$_{2}$O and CO emitting gas. Complementary Spitzer mid-IR H$_{2}$ data were used to derive the H$_{2}$O abundance.
Results: Consistent with other studies, at all selected shock spots a close spatial association between H$_{2}$O, 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...

Aims: In the framework of the Water In Star-forming regions with Herschel (WISH) key program, several H$_{2}$O (E$_u$ {gt} 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 H$_{2}$O 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 H$_{2}$O and CO emitting gas. Complementary Spitzer mid-IR H$_{2}$ data were used to derive the H$_{2}$O abundance.
Results: Consistent with other studies, at all selected shock spots a close spatial association between H$_{2}$O, 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 H$_{2}$O, CO, and mid-IR H$_{2}$ lines: an extended warm component (T ~{} 450 K) is traced by the H$_{2}$O 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 H$_{2}$O emission with E$_u$ {gt} 190 K and by the higher-J CO transitions. At L1448-B2 we obtain an H$_{2}$O abundance (3-4) { imes} 10$^{-6}$ for the warm component and (0.3-1.3) { imes} 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 H$_{2}$O and CO emission. The other examined shock positions show an H$_{2}$O excitation similar to L1448-B2, but a slightly higher H$_{2}$O 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 H$_{2}$O abundance variations. 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

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Herschel-PACS observations of shocked gas associated with the jets of L1448 and L1157