CO Abundance Variations in the Orion Molecular Cloud
  Infrared stellar photometry from 2MASS and spectral line imaging observations
of 12CO and 13CO J = 1-0 line emission from the FCRAO 14m 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 Av regimes
are identified in which the ratio between the 13CO 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 13CO abundance varies
by 100% with a peak value located near regions of enhanced star formation
activity. The effect of CO depletion onto the ice mantles of dust grains is
limited to regions with AV > 10 mag and gas temperatures less than 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 Av and 13CO column densities with a constant 13CO-to-H2
abundance over various extinction ranges. Within the strongly self-shielded
interior of the cloud (Av > 3 mag), 13CO provides a reliable tracer of H2 mass
with the exception of the cold, dense volumes where depletion is important.
However, owing to its reduced abundance, 13CO does not trace the H2 mass that
resides in the extended cloud envelope, which comprises 40-50% 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 H2 column densities.
