PRIMORDIAL GIANT MOLECULAR CLOUD CLUMPS CAN BE PROBED-OPTICALLY

Integrated CO emission from the Monoceros OB2 association shows the presence of a giant molecular cloud (GMC) whose major extension is SE of the Rosette Nebula (NGC 2237-NGC 2246). In this Letter we show that it is possible to probe primordial GMC clump-interclump structure optically. Specialized processing at the ESO photolabs of deep IIIaF Schmidt plates reveals the presence of a hitherto unexplored chain of clearly defined, clumpy globules well outside the central stellar-wind-evacuated cavity of the Rosette. The globule network lies along the Monoceros Ridge in the SE. The globules cannot be the result of energy-driven stellar-wind-blown bubbles subject to Rayleigh-Taylor instabilities. It is argued that the chain optically "etches" the clumpy structure of the GMC itself as ionization fronts driven by the OB stars in the cluster NGC 2244 interact with the CO cloud. Measured scale sizes of the etching by photoionization are in excellent accord with theoretical estimates and suggest that the GMC clumps are primordial rather than a result of localized star formation. The tenuous interclump molecular substrate which is all pervasive through the GMC betrays itself as a zone of low-density extended optical emission caused by diffuse UV radiation from the OB stars. Unsharp masked and image-enhanced photographs show that the well-known feature at the center of the Ear is not disjoint but rather embedded in the ionized substrate molecular material.