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Issue A&A
Volume 399, Number 3, March I 2003
Page(s) 1047 - 1061
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021799



A&A 399, 1047-1061 (2003)
DOI: 10.1051/0004-6361:20021799

Gas-phase CO $_\mathsf{2}$, C $_\mathsf{2}$H $_\mathsf{2}$, and HCN toward Orion-KL

A. M. S. Boonman1, E. F. van Dishoeck1, F. Lahuis1, 2, S. D. Doty3, C. M. Wright4 and D. Rosenthal5

1  Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
2  SRON National Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
3  Department of Physics and Astronomy, Denison University, Granville, Ohio 43023, USA
4  School of Physics, University College, ADFA, UNSW, Canberra ACT 2600, Australia
5  Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85741 Garching, Germany

(Received 10 June 2002 / Accepted 22 November 2002 )

Abstract
The infrared spectra toward Orion-IRc2, Peak 1 and Peak 2 in the 13.5-15.5  $\mu$m wavelength range are presented, obtained with the Short Wavelength Spectrometer on board the Infrared Space Observatory. The spectra show absorption and emission features of the vibration-rotation bands of gas-phase CO 2, HCN, and C 2H 2, respectively. Toward the deeply embedded massive young stellar object IRc2 all three bands appear in absorption, while toward the shocked region Peak 2 CO 2, HCN, and C 2H 2 are seen in emission. Toward Peak 1 only CO 2 has been detected in emission. Analysis of these bands shows that the absorption features toward IRc2 are characterized by excitation temperatures of ~175-275 K, which can be explained by an origin in the shocked plateau gas. HCN and C 2H 2 are only seen in absorption in the direction of IRc2, whereas the CO 2 absorption is probably more widespread. The CO 2 emission toward Peak 1 and 2 is best explained with excitation by infrared radiation from dust mixed with the gas in the warm component of the shock. The similarity of the CO 2 emission and absorption line shapes toward IRc2, Peak 1 and Peak 2 suggests that the CO 2 is located in the warm component of the shock ( $T \sim 200$ K) toward all three positions. The CO 2 abundances of ~10 -8 for Peak 1 and 2, and of a few times 10 -7 toward IRc2 can be explained by grain mantle evaporation and/or reformation in the gas-phase after destruction by the shock. The HCN and C 2H 2 emission detected toward Peak 2 is narrower ( $T \sim 50$-150 K) and originates either in the warm component of the shock or in the extended ridge. In the case of an origin in the warm component of the shock, the low HCN and C 2H 2 abundances of ~10 -9 suggest that they are destroyed by the shock or have only been in the warm gas for a short time ( $t \la10^4$ yr). In the case of an origin in the extended ridge, the inferred abundances are much higher and do not agree with predictions from current chemical models at low temperatures.


Key words: stars: formation -- ISM: individual objects: Orion IRc2, Peak 1, Peak 2 -- ISM: abundances -- ISM: molecules -- ISM: lines and bands -- molecular processes

Offprint request: A. M. S. Boonman, boonman@strw.leidenuniv.nl

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