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

We made a large area mapping of the CO (J = 2-1) emission toward the entire extent of the Orion A and B giant molecular clouds using the Tokyo-NRO 60 cm survey telescope, covering ∼35 square degrees with an angular resolution of 9′. The data were compared with the CO (J = 1-0) data taken by Maddalena et al. on the same observing grids with the same angular resolution. Although the J = 2-1 emission exhibits basically similar spatial distribution to the J = 1-0 emission, a closer comparison shows that the CO (J = 2-1)/CO (J = 1-0) intensity ratio systematically varies over the entire extent of the Orion A and B clouds. The ratio is approximately unity on the main ridges of the clouds and declines down to 0.5 in the peripheral regions. This variation of the intensity ratio is understood mainly in terms of the density variation of unresolved clumps (or filaments) that comprise the clouds. With the aid of large velocity gradient radiative transfer calculations, we conclude that the observed variation of the intensity ratio is explained if the gas density of CO emitting regions is more than ∼3 × 10³ cm^-3 on the main ridges and it declines down to ∼2 × 10² cm^-3 in the peripheral regions. Surface and volume filling factors of the clumps are derived to be ≳0.7 and ∼0.1, respectively, in the peripheral regions. This suggests the existence of small-scale (≲2 pc) substructures even in the cloud peripheries, allowing penetration of external UV radiation. The CO (J = 2-1)/CO (J = 1-0) luminosity ratios derived by integrating intensities over the mapping area are 0.77 and 0.66 for the Orion A and B clouds, respectively. Correcting for the unobserved extended low-level emission with small intensity ratios, we estimate the luminosity ratios of the entire GMCs to be 0.75 and 0.62 for the respective clouds. These values are consistent with those often observed in disks of external galaxies, but are significantly less than those observed toward active star-forming regions in galaxies.