1995A&A...299..179W

The spatial distributions of the ³P₁-³P₀ atomic fine structure of carbon (CI), the CO J=43,-the CO J=21-transitions of CO, ¹³CO, C¹⁸O and C¹⁷O and the 790µm continuum emission have been mapped towards the central region of the Orion molecular cloud (OMC1 cloud), and the Bright Bar ionisation front. The CO data are analysed in a consistent way, allowing the inter isotopomeric abundance ratios to be studied over a wide range of extinction values. The ¹³CO lines are optically thick; the ¹³CO abundance being enhanced because of strong isotopic fractionation near the Bright Bar, but less convincingly in the OMC1 cloud. The fractionation occurs mostly in the less opaque regions where the ¹³CO column density N(¹³CO) may be enhanced by up to one order of magnitude, relative to the more shielded parts. No isotope selective enhancement of the other CO isotopomers was seen; C¹⁸O may in fact show a slight depletion in more exposed material. The C¹⁸O and C¹⁷O lines are optically thin, and correlated with the 790µm dust continuum emission. The C I emission comes from hot optically thin gas; the abundance ratios of [CI]/[CO] are typically 0.05-0.3, with the larger ratios towards the northern section of the Orion ridge. The CI abundance ratios remain high along the edge of the Bright Bar which is adjacent to the HII region (and the Trapezium cluster which excites it), but decrease in the dense shielded material behind the Bar.