CO and H₂O vibrational emission toward Orion Peak 1 and Peak 2

¹ CSIC, IEM, Dpto. Física Molecular, Serrano 123, 28006 Madrid, Spain
² Universidad de Alcalá de Henares, Departamento de Física, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
³ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ School of Physics, University College, Australian Defence Force Academy, University of New South Wales, Canberra ACT 2600, Australia
⁵ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85741 Garching, Germany

Corresponding author: E. González-Alfonso, gonzalez@isis.iem.csic.es

Received: 4 December 2001
Accepted: 4 March 2002

Abstract

ISO/SWS observations of Orion Peak 1 and Peak 2 show strong emission in the ro-vibrational lines of CO at 4.45–4.95 μm and of H₂O at 6.3–7.0 μm. Toward Peak 1 the total flux in both bands is, assuming isotropic emission, ≈2.4 and ≈0.53 , respectively. This corresponds to ≈14 and ≈3% of the total  luminosity in the same beam. Two temperature components are found to contribute to the CO emission from Peak 1/2: a warm component, with –400 K, and a hot component with  K. At Peak 2 the CO flux from the warm component is similar to that observed at Peak 1, but the hot component is a factor of ≈2 weaker. The  band is ≈25% stronger toward Peak 2, and seems to arise only in the warm component. The P-branch emission of both bands from the warm component is significantly stronger than the R-branch, indicating that the line emission is optically thick. Neither thermal collisions with  nor with H I seem capable of explaining the strong emission from the warm component. Although the emission arises in the postshock gas, radiation from the most prominent mid-infrared sources in Orion BN/KL is most likely pumping the excited vibrational states of CO and . CO column densities along the line of sight of – are required to explain the band shape, the flux, and the , and beam-filling is invoked to reconcile this high N(CO) with the upper limit inferred from the  emission. CO is more abundant than  by a factor of at least 2. The density of the warm component is estimated from the  emission to be ~. The CO emission from the hot component is neither satisfactorily explained in terms of non-thermal (streaming) collisions, nor by resonant scattering. Vibrational excitation through collisions with  for densities of ~ or, alternatively, with atomic hydrogen, with a density of at least , are invoked to explain simultaneously the emission from the hot component and that from the high excitation  lines in the same beam. A jump shock is most probably responsible for this emission. The emission from the warm component could in principle be explained in terms of a C-shock. The underabundance of  relative to CO could be the consequence of  photodissociation, but may also indicate some contribution from a jump shock to the CO warm emission.