A far-infrared molecular and atomic line survey of the Orion KL region

Abstract

We have carried out a high spectral resolution (λ/Δλ∼ 6800–9700) line survey towards the Orion Kleinmann–Low (KL) cluster from 44 to 188 μm. The observations were taken with the Long Wavelength Spectrometer (LWS) in Fabry–Pérot mode, on board the Infrared Space Observatory (ISO). A total of 152 lines are clearly detected and a further 34 features are present as possible detections. The spectrum is dominated by the molecular species H2O, OH and CO, along with [O i] and [C ii] lines from photodissociation region (PDR) or shocked gas and [O iii] and [N iii] lines from the foreground M42 H ii region. Several isotopic species, as well as NH3, are also detected. HDO and H3O+ are tentatively detected for the first time in the far-infrared (FIR) range towards Orion KL. A basic analysis of the line observations is carried out, by comparing with previous measurements and published models and deriving rotational temperatures and column densities in the case of the molecular species. Analysis of the [O i] and [C ii] fine structure lines indicates that although a shock model can reproduce the observed [O i] surface brightness levels, it falls short of the observed [C ii] level by more than a factor of 30. A PDR model can reproduce the [O i] 63.2 μm and [C ii] surface brightness levels within 35 per cent, although overpredicting the LWS [O i] 145.5 μm-emission by a factor of 2.7. The 70 water lines and 22 OH lines detected by the survey appear with mainly P Cygni profiles at the shortest survey wavelengths and with mainly pure emission profiles at the longest survey wavelengths. The emission and absorption velocity peaks of the water and OH lines indicate that they are associated with gas expanding in the outflow from the KL cluster. The estimated column densities are (2–5) × 1014 cm−2 for H2O and (2.5–5.1) × 1016 cm−2 for OH. The 26 detected CO lines confirm the presence of three distinct components, with temperature and column density combinations ranging from 660 K, 6 × 1017 cm−2 to 360 K, 2 × 1019 cm−2. Comparison of the surface brightnesses and integrated fluxes of the CO lines measured in the 80-arcsec LWS beam with those measured previously by the Kuiper Airborne Observatory (KAO) in a 44-arcsec beam shows similar surface brightnesses in the different beams for the lowest-J CO lines and similar integrated fluxes in the different beams for the highest-J CO lines, indicating that emission from the former lines has a uniform surface brightness within the LWS beam, while the latter lines originate from a region less than 44 arcsec in diameter. The complexity of the region requires more sophisticated models for the interpretation of all the line observations.