PARSEC-SCALE PENETRATION OF ULTRAVIOLET PHOTONS INTO MOLECULAR CLOUDS - [C-II] 158-MICRON MAPPING OF W3, NGC-1977, AND NGC-2023

We have mapped the spatial distribution of the 158-mu-m [C II] fine-structure line in the Galactic sources W3, NGC 1977, and NGC 2023. The emission arises from warm (100-300 K), dense (n(H) > 10(4) cm-3) photo-dissociation regions at the surface of molecular gas. In all three sources the emission extends over parsec scales or greater. For W3 and NGC 1977, where the UV source-molecular cloud geometry presents us with an edge-on view of the variation of [C II] intensity into the molecular gas, we have constructed two-dimensional models of the [C II] emission which include the effects of gas clumping and scattering by dust on the transport of UV photons. The observed [C II] distribution and intensity is well modeled by a clumpy or filamentary distribution to the molecular gas, with a clump-interclump gas density ratio of 10(2) or more, which allows deep penetration of carbon-ionizing UV photons into the clouds. The penetration of UV into clumped molecular clouds may also explain the extended far-IR continuum emission from these sources. The total luminosity of [C II] emission from a clumpy molecular cloud with adjacent or embedded OB stars can be as much as an order of magnitude higher than the [C II] luminosity of a uniform cloud. In addition, the extended penetration of UV into molecular clouds will affect the abundances of atomic and molecular species and increase the fractional ionization of interclump gas and UV-illuminated clump surfaces.