Far-Infrared Spectroscopy of the Orion IRc2-BN/KL region

Figure 1. Colour composite image of the central part of the Orion Nebula, M42, also showing the BN/KL region, based on 81 images obtained with the infrared multi-mode ISAAC instrument on the ESO Very Large Telescope (VLT) at the Paranal Observatory (ESO PR photo 03a/01 2001).

The BN/KL nebula (Becklin and Neugebauer 1967, Kleinmann and Low 1967), at a distance of ~450pc, is known to be a complex star forming region which encompasses an extraordinary variety of physical conditions; ranging from quiescent cool gas to large outflows that alter the temperature and consequently the local chemistry. Molecular observations of star forming regions provide a tool for the study of their chemical and dynamical evolution. The transitions from many of the molecules responsible for cooling the gas, such as CO, OH and H₂O, arise at far-infrared wavelengths where ground-based observations are restricted due to the constraints imposed by the Earth's atmosphere.

We are analysing high spectral resolution (λ/Δλ ~ 6800-9700) 40-198μm far-infrared (FIR) observations of the Orion BN/KL region, taken using the Fabry-Perot (LO3) mode of the European Space Agency's Infrared Space Observatory (ISO) Long Wavelength Spectrometer (LWS). The complicated reduction process of these observations was performed at the Rutherford Appleton Laboratory taking advantage of the latest improved pipeline specifically developed for ISO data (LIA package version 10). We have carried out a spectral line survey in which more than 130 lines are detected from a total of 17 different species. The overall spectrum is dominated by H2O, OH and CO lines. Forbidden lines of [OIII] 52μm, 88μm, [NIII] 57μm from the foreground HII region, and the PDR [OI] 63μm and [CII] 158μm lines are also detected while water isotopes, CO isotopes, H3O+, NH3 and H2S are present as weak features.

Figure 2. Approximate LWS observation center at the positions of the BN object and IRc2, superimposed on a false color-image of the 350μm continuum emission toward the Orion A molecular cloud from Lis et al. (1998) observed with SHARC. (Courtesy of D. Lis).

The large LWS beam (~80″) includes contributions from several physically distinct regions that have been identified from previous molecular observations (in the millimeter and submillimeter ranges, Blake et al. 1987, Genzel et al. 1989 and KAO far infrared range, Bereiko et al. 1989) as a Hot Core, a Plateau and an Extended Ridge. These regions are indicated in Figure 4 (below).

Figure 3 (below) shows velocity plots for a number of detected H₂O and CO lines which exhibit profiles ranging from P Cygni to pure emission.

Figure 3. Selection of CO lines and water lines detected in our far-infrared survey toward Orion BN/KL with the ISO LWS in L03 Fabrey-Perot mode.

We are currently studying the contribution of each component to our molecular detections using chemical models that reproduce typical abundances in each component coupled with a radiative transfer model able to reproduce line intensities and profiles.

Figure 4. Diagram showing the structure of the Orion BN/KL region.