Far Infrared Emission of Molecular Clouds

Abstract

Data presently available on far infrared (10 – 300 µm) thermal emission and carbon monoxide millimeter radiation are compiled in order to generate a sample of objects where the thermal radiation of the dust mixed with the molecular hydrogen can be quantitatively studied. A list of 10 massive molecular clouds is obtained. A luminosity, L ^H_IR , normalized per hydrogen atom, is introduced and found to range from 0.5 to 5 x 10^-30 W (H-atom)^-1, confirmed by a value L ^H_IR = 2.4 x 10^-30 W (H-atom^-1), which can be deduced from a measurement of the diffuse galactic far infrared emission (Pipher 1973).

The quantity L ^H_IR is more than one order of magnitude above the value expected from heating of the dust by the average starlight density, u ≈ 0.5 eV cm^-3. The clouds are clearly heated from the interior, by newly born stars still unobservable. But any attempt to infer the power released in the clouds using published star formation rates deduced from star count in the solar neighborhood falls short by a factor ~ 10 or larger.

Then it was assumed that the dust is made of a mixture of “silicate” and ice grains matching the abundances deduced from spectral (3.1 and 10 µm) observations; however, the “silicate” is supposed to be in an amorphous state (recently observed), in which the far infrared emissivity is substantially enhanced. The temperature deduced from the observed value of L ^H_IR and the grain model is found to be in very good agreement with the temperatures actually obtained from multicolor photometry.