CO abundance variations in the Orion Molecular Cloud

Infrared stellar photometry from the Two Micron All-Sky Survey (2MASS) and spectral line imaging observations of (CO)-C-12 and (CO)-C-13 J = 1-0 line emission from the Five College Radio Astronomy Observatory (FCRAO) 14-m telescope are analysed to assess the variation of the CO abundance with physical conditions throughout the Orion A and Orion B molecular clouds. Three distinct A(nu) regimes are identified in which the ratio between the (CO)-C-13 column density and visual extinction changes corresponding to the photon-dominated envelope, the strongly self-shielded interior, and the cold, dense volumes of the clouds. Within the strongly self-shielded interior of the Orion A cloud, the (CO)-C-13 abundance varies by 100 per cent with a peak value located near regions of enhanced star formation activity. The effect of CO depletion on to the ice mantles of dust grains is limited to regions with A(nu) > 10 mag and gas temperatures less than similar to 20 K as predicted by chemical models that consider thermal evaporation to desorb molecules from grain surfaces. Values of the molecular mass of each cloud are independently derived from the distributions of A(nu) and (CO)-C-13 column densities with a constant (CO)-C-13-to-H-2 abundance over various extinction ranges. Within the strongly self-shielded interior of the cloud (A(nu)> 3 mag), (CO)-C-13 provides a reliable tracer of H-2 mass with the exception of the cold, dense volumes where depletion is important. However, owing to its reduced abundance, (CO)-C-13 does not trace the H-2 mass that resides in the extended cloud envelope, which comprises 40-50 per cent of the molecular mass of each cloud. The implied CO luminosity to mass ratios, M/L-CO, are 3.2 and 2.9 for Orion A and Orion B, respectively, which are comparable to the value (2.9), derived from. gamma-ray observations of the Orion region. Our results emphasize the need to consider local conditions when applying CO observations to derive H-2 column densities.