Odin CO and

Aims.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. Methods.We have, for the first time, mapped the 12CO$J=5{-}4$ emission in a $7'\times7'$ 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). Results.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 M 42 $\ion{H}{ii}$ 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 $5\times 10^{21}$ cm-2 at map edges to $7\times 10^{22}$ cm-2 at the molecular ridge. The mass of the gas in the mapped region is estimated to be 480 $M_{\odot}$, of which 320 $M_{\odot}$ 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 ≥$8\times 10^{-8}$, higher than previously estimated.

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