2012ApJ...744...28P

The large number of high-J lines of C¹⁸O available via the Herschel Space Observatory provide an unprecedented ability to model the total CO column density in hot cores. Using the emission from all the observed lines (up to J = 15-14), we sum the column densities in each individual level to obtain the total column after correcting for the population in the unobserved states. With additional knowledge of source size, V_LSR, and line width, and both local thermodynamic equilibrium (LTE) and non-LTE modeling, we have determined the total C¹⁸O column densities in the Extended Ridge, Outflow/Plateau, Compact Ridge, and Hot Core components of Orion KL to be 1.4x10¹⁶/cm2, 3.5x10¹⁶/cm2, 2.2x10¹⁶/cm2, and 6.2x10¹⁶/cm2, respectively. We also find that the C¹⁸O/C¹⁷O abundance ratio varies from 1.7 in the Outflow/Plateau, 2.3 in the Extended Ridge, 3.0 in the Hot Core, and to 4.1 in the Compact Ridge. This is in agreement with models in which regions with higher ultraviolet radiation fields selectively dissociate C¹⁷O, although care must be taken when interpreting these numbers due to the size of the uncertainties in the C¹⁸O/C¹⁷O abundance ratio.