COBE DIRBE OBSERVATIONS OF THE ORION CONSTELLATION FROM THE NEAR-INFRARED TO FAR-INFRARED/

Observations by the DIRBE instrument aboard the COBE spacecraft, colle cted in 10 wavelength bands spanning the near-infrared to the far-infr ared in a 0 degrees 7 beam, are presented for a region covering much o f the Orion constellation. For an adopted distance of 450 pc, the tota l luminosity from dust (from 12 to 240 mu m) throughout the Orion A, O rion B, and lambda Ori fields, covering 16,900 pc(2), is similar to 10 (6) Lo.. About 24%-36% of this dust luminosity is the result of dust h eating by a general interstellar radiation field, with the rest result ing from heating by the Orion OB1 and lambda Ori OB associations. Give n that the luminosity of the Orion OB1 and lambda Ori OB associations is 2.5 x 10(6) L., and also given that up to similar to 76% of dust lu minosity is caused by dust heated primarily by the Orion stars, less t han or similar to 30% of the stellar luminosity is trapped within the clouds and intercloud medium of Orion and reradiated at mid- to far-IR wavelengths. The near-IR (1.25, 2.2, 3.5, and 4.9 mu m) spectral dist ributions of the Orion Nebula and NGC 2024 indicate the presence of ho t (T similar to few x 10(2) K) dust, both because of large I-V(4.9 mu m)/I-V(1.25 mu m) ratios and because of a substantial excess in the 3. 5 mu m band relative to the intensities in the adjacent bands, some of which (greater than or similar to 30%) is caused by the 3.28 mu m emi ssion line, commonly attributed to polycyclic aromatic hydrocarbons (P AHs). In the far-IR, the 100, 140, and 240 mu m intensities are consis tent with a cool (usually 18-20 K, for emissivity index = 2) single-te mperature component. The I-V(60 mu m)/I-V(100 mu m) color temperature is similar to 5-6 K higher than that from the cool component, suggesti ng that an additional warmer component or stochastically heated dust i s contributing appreciably to the 60 mu m emission. Consequently, dust column densities derived from the 60 and 100 mu m intensities, assumi ng grains in thermal equilibrium, underestimate the dust-to-gas ratio by factors of 5-10. In contrast, the 140 and 240 mu m intensities yiel d dust column densities consistent with reasonable dust-to-gas mass ra tios (i.e, similar to 0.01) to within a factor of 2. However, within t his factor of 2, there appears to be a temperature-dependent systemati c error in the dust column density derivation. The results of this pap er may apply to external galaxies, since the region studied is more th an 200 pc in size. All the above conclusions would have been obtained if the stars and clouds of Orion were placed at the distance of a near by galaxy (similar to 1 Mpc) and observed in the DIRBE wavelength band s in an similar to 1' beam (provided the signal-to-noise ratio was una ffected). Hence, observations of the interstellar medium (ISM) in exte rnal galaxies that have resolutions of similar to 100 pc can still yie ld meaningful results. Further, if the stars and clouds in a spiral ga laxy's arms can be represented by a series of Orion star and cloud com plexes, one would expect the surface luminosity in the arms (for lambd a=12-240 mu m) to be 2-4 times that in the interarm regions, averaged over 100 pc scales.