Asteroidscomets

COBE/DIRBE Observations of the Orion Constellation from the Near- to Far-Infrared

January 1996 • 1996ApJ...456..566W

Authors • Wall, W. F. • Reach, W. T. • Hauser, M. G. • Arendt, R. G. • Weiland, J. L. • Berriman, G. B. • Bennett, C. L. • Dwek, Eli • Leisawitz, D. • Mitra, P. M. • Odenwald, Sten F. • Sodroski, T. J. • Toller, G. N.

Abstract • Observations by the DIRBE instrument aboard the COBE spacecraft, collected in 10 wavelength bands spanning the near-infrared to the far-infrared in a 0°.7 beam, are presented for a region covering much of the Orion constellation. For an adopted distance of 450 pc, the total luminosity from dust (from 12 to 240 μm) throughout the Orion A, Orion B, and λ Ori fields, covering 16,900 pc2, is ∼106 Lsun. About 24%-36% of this dust luminosity is the result of dust heating by a general interstellar radiation field, with the rest resulting from heating by the Orion OB1 and λ Ori OB associations. Given that the luminosity of the Orion OB1 and λ Ori OB associations is 2.5 × 106 Lsun, and also given that up to ∼76% of dust luminosity is caused by dust heated primarily by the Orion stars, ≲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 μm) spectral distributions of the Orion Nebula and NGC 2024 indicate the presence of hot (T ∼few × 102 K) dust, both because of large Iv(4.9 μm)/Iv(1.25 μm) ratios and because of a substantial excess in the 3.5 μm band relative to the intensities in the adjacent bands, some of which (≳30%) is caused by the 3.28 μm emission line, commonly attributed to polycyclic aromatic hydrocarbons (PAHs).

In the far-IR, the 100, 140, and 240 μm intensities are consistent with a cool (usually 18-20 K, for emissivity index = 2) single-temperature component. The Iv(60 μm)Iv(100 μm) color temperature is ∼5-6 K higher than that from the cool component, suggesting that an additional warmer component or stochastically heated dust is contributing appreciably to the 60 μm emission. Consequently, dust column densities derived from the 60 and 100 μm intensities, assuming grains in thermal equilibrium, underestimate the dust-to-gas ratio by factors of 5-10. In contrast, the 140 and 240 μm intensities yield dust column densities consistent with reasonable dust-to-gas mass ratios (i.e, 0.01) to within a factor of 2. However, within this factor of 2, there appears to be a temperature-dependent systematic error in the dust column density derivation.

The results of this paper may apply to external galaxies, since the region studied is more than 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 nearby galaxy (∼1 Mpc) and observed in the DIRBE wavelength bands in an ∼1' beam (provided the signal-to-noise ratio was unaffected). Hence, observations of the interstellar medium (ISM) in external galaxies that have resolutions of ∼100 pc can still yield meaningful results. Further, if the stars and clouds in a spiral galaxy's arms can be represented by a series of Orion star and cloud complexes, one would expect the surface luminosity in the arms (for λ = 12-240 μm) to be 2-4 times that in the interarm regions, averaged over 100 pc scales.

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Bruce Berriman

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