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Extensive High-Resolution 12CO Imaging of L1641 in Orion

We obtained extensive 12CO J = 1-0 images along the minor axis of the L1641 cloud in Orion with a 15'' (0.03 pc) beam spaced by 34''. Besides bright ridgelike emission in the middle of the cloud, we detected CO emission almost everywhere within the outermost boundary of the cloud. There are at least two distinct populations of molecular gas: well-defined “clumps” with high brightness temperature (~25 K) and small line width (≃1.5 km s-1), and “extended components” with low brightness temperature (~2.5 K) and broad line width (≃2.5 km s-1). The 12CO J = 1-0 emission shows clumpy features even in the periphery of the cloud. The clumps dominate the CO luminosity from the central ridge region and are severely overlapped, while they are isolated in the periphery of the cloud. The extended component is widely distributed, especially in the eastern side of the cloud, and seems to be connected to the atomic gas layer in the Galactic plane. It lacks notable substructures at our spatial resolution and exhibits a surface filling factor very close to unity. The extended component does not fill the interclump space, at least in the velocity domain. The ridge component may be separating from the extended molecular material. To deduce physical conditions of molecular gas, we conducted simultaneous 12CO J = 2-1/J = 1-0 strip-scan observations along the minor axis of the cloud with higher sensitivity and denser sampling. Molecular gas density estimated from excitation analysis of the 12CO emission was ≳3 × 103 cm-3 in the clumps and ≃2 × 102 cm-3 in the extended component. The peripheral region of the cloud is fainter in CO brightness not because the surface filling factor of emitting gas is smaller, but because the emission is dominated by the tenuous extended component with low brightness temperature.