Odin CO and CO J = 5–4 mapping of Orion KL – a step towards accurate water abundances

¹ Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden e-mail: eva@oso.chalmers.se
² European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile
³ Swedish Space Corporation, PO Box 4207, 17104 Solna, Sweden
⁴ Stockholm Observatory, SCFAB-AlbaNova, 10691 Stockholm, Sweden

Received: 1 July 2005
Accepted: 23 March 2006

Abstract

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 emission in a  region covering Orion KL, observed simultaneously with a ¹³CO  map. Also presented are C¹⁸O  emission data at four different positions and a C¹⁷O  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  narrow line emission from this region mainly arises in the warm, dense gas at the interface (the photon-dominated region) between the M 42  region and the Orion A molecular cloud, the Orion PDR. The ¹²CO and ¹³CO  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  cm^-2 at map edges to  cm^-2 at the molecular ridge. The mass of the gas in the mapped region is estimated to be 480 , of which 320  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  has been observed, we estimate the ortho-water abundance in the Orion PDR layer to be ≥, higher than previously estimated.