2006A&A...453..979W

The very high main beam efficiency (90%) of the telescope on the sub-millimetre wave satellite Odin, in combination with the small calibration errors in the absence of atmospheric attenuation, assures that observed line brightness temperatures are very accurately determined. Based on this, we attempt to determine the column density distribution of H₂, and the ortho-water abundance, in the Orion KL region. We have, for the first time, mapped the ¹²CO J=5-4 emission in a 7' x 7' region covering Orion KL, observed simultaneously with a ¹³CO J=5-4 map. Also presented are C¹⁸O J=5-4 emission data at four different positions and a C¹⁷O J=5-4 emission spectrum detected towards the Orion KL position. The Odin mapping was performed at 1' spacing (beam full width at half maximum 126'' at 557 GHz). The CO J=5-4 narrow line emission from this region mainly arises in the warm, dense gas at the interface (the photon-dominated region) between the M42 HII region and the Orion A molecular cloud, the Orion PDR. The ¹²CO and ¹³CO J=5-4 emission maps have been used to determine the column density distribution of H₂ gas across the Orion KL region. The results have been verified by comparing to column densities obtained using the decidedly optically thin C¹⁸O emission as input to the RADEX radiative transfer code. We find H₂ column densities ranging from 5x10²¹cm^–2 at map edges to 7x10²²cm^–2 at the molecular ridge. The mass of the gas in the mapped region is estimated to be 480M_☉, of which 320M_☉ is situated towards the molecular ridge. We estimate that about half of this mass belongs to the warm Orion PDR interface layer. Finally, based on data from the positions where C¹⁸O J=5-4 has been observed, we estimate the ortho-water abundance in the Orion PDR layer to be ≥8x10^–8, higher than previously estimated.