158-MICRON [C-II] MAPPING OF THE ORION MOLECULAR CLOUD

We have constructed a fully sampled, 1000 point, 1' resolution map of the inner 6.5 x 10'(alpha x delta) regions of the Orion Nebula in the 157.7409 mum [C II] fine-structure line. We have also obtained large-scale (approximately 1-degrees) strip maps in [C II] across the face of the Orion molecular cloud, and CO(17-16), (14-13), and (7-6) spectra at selected positions in the Orion H II region/molecular cloud interface. Within approximately 2.'5 of THETA1 Ori C (hereafter, the interface region) the [C II] line emission arises in warm (T approximately 300 K), dense (n(H) approximately 4 x 10(5) cm-3), photodissociated gas at the interface between the H II region and the molecular cloud. The photodissociated gas represents approximately 3% of the total gas mass toward the interface region of the Orion molecular cloud. The [C II] intensity distribution is similar to that of CO(1-0) and (7-6). Furthermore, for the interface region, the CO(7-6) and [C II] spectral profiles are similar, indicating that both submillimeter and millimeter CO lines originate from the UV-heated warm quiescent gas in the interface region. The distribution of CO intensity with rotational quantum number indicates that this CO-emitting gas is warm, T(gas) approximately 140 K, and dense, n(H-2) approximately 1.4 x 10(4) cm-3. This warm CO component is a large fraction (approximately 35%) of the total molecular gas mass in the interface region. Current photodissociation region models fail to explain the large column densities of warm CO. Strong [C II] line emission is observed across the face of the Orion molecular. We estimate the total [C II] luminosity from the Orion molecular cloud is approximately 1500 L. or 0.3% of the far-infrared luminosity. The extended [C II] emission probably arises from either the UV-exposed surface of the molecular cloud or from the surfaces of UV-exposed clumps within the molecular cloud. The source of the far-ultraviolet (FUV) radiation is likely the Trapezium cluster. A simple model is presented which matches the observed [C II] intensity distribution by invoking a clumpy rim of intervening gas between the Orion H ii region and the outer regions of the Orion molecular cloud. FUV flux from the Trapezium penetrates this rim and falls on the outer regions of the molecular cloud, thereby giving rise to the relatively flat distribution of [C II] emission we observe. Alternatively, the FUV source for the extended [C II] emission may be the ambient interstellar radiation field if that field is approximately 25 times the local interstellar radiation field, or embedded FUV sources in the molecular cloud. The mass contained in these extended photodissociation regions is -3% of the molecular mass of the Orion molecular cloud. The extended [C II] distribution is similar to that of the [C I] 610 mum fine-structure line. It is therefore likely that the [C I] emission also arises from these extended photodissociation regions in molecular clouds. The overall properties of the Orion molecular cloud deduced through our measurements including the photodissociated gas to molecular gas mass fraction, the [C II] line to far-infrared continuum intensity ratio, and the [C II]/12CO(1-0) line intensity ratio are the same for the Orion molecular cloud and the nuclei of nonstarburst galaxies. Hence the molecular medium in nonstarburst nuclei may be constructed through a superposition of Orion-like molecular clouds. These observations therefore provide an important link between [C II] emission from Galactic molecular clouds and the nuclei of external galaxies.