A large area CO(J=2−1) mapping of the Orion giant molecular clouds

A large‐area CO(J=2−1) map of the Orion A and B clouds is presented. The J=2−1/J=1−0 intensity ratio of CO varies systematically over the whole extent of these clouds, i.e., the ratio is ∼1 in their main ridges and declines to ∼0.5 in their peripheries. This variation of the intensity ratio is understood in terms of the variation of the surface gas density of clumps which is ≳3×103 cm−3 for those in the ridges and ∼1×102 cm−3 for those in the peripheries. The peripheral regions seen in low‐J transitions of 12CO is more surface‐filling (≳0.7) than expected.The J=2−1/J=1−0 luminosity ratio for the Orion A and B clouds is 0.75 and 0.62, respectively. These values are consistent with those observed typically along the Solar circle and are significantly lower than large values often observed in the inner Galaxy and the Galactic center.