Publication Abstracts

White 1971

White, R.E., 1971: Microwave Radiative Transfer in Interstellar Formaldehyde. Ph.D. thesis. Columbia University.

The importance of finite optical depth in producing the "anomalous absorption" in interstellar formaldehyde is explored. The excitation of interstellar formaldehyde by isotropic microwave background radiation and collisions is treated theoretically for projected densities of ortho-formaldehyde ranging from 3×1012 to 3×1015 cm-2. The cloud is treated as a homogeneous plane-parallel slab with uniform kinetic temperature, illuminated by isotropic 2.7°K blackbody radiation. The equations of radiative transfer for the 6cm, 2cm, and two 2mm lines of ortho-formaldehyde are solved simultaneously with the equations of statistical equilibrium by the complete linearization technique developed by Auer and Mihalas (1969) for non-LTE radiative transfer problems in stellar atmospheres. The Eddington approximation is adopted, and pure Doppler broadening and completely non-coherent scattering are assumed. Collision rates for excitation by electrons and neutral particles (designated H2) are taken from the work of Thaddeus and Solomon (1971) and Thaddeus (1971) respectively.

The calculations cover the range of parameters 10° ≤ TK ≤ 75° 10-7 ≤ ne/nH2 ≤ 10-4; 10 cm-3 ≤ nH2 ≤ 3×105 cm-3; representative results are presented and discussed.

It is found that the two 2mm lines are "effectively thin" under almost all the conditions considered. Thus, these lines remain essentially in equilibrium with the background radiation (i.e., at an excitation temperature of 2.7°) throughout the cloud. As a conxsequence, it is found that the radiation in the 6cm and 2cm lines can be calculated with moderate accuracy on the assumption that the excitation temperature given in the optically thin case prevails throughout the cloud. Specifically, the objections to the collisional "cooling" mechanism proposed by Townes and Cheung (1969), which were raised by Thadeus (1971) on the basis of calculations made in the optically thin case, are sustained in cases of finite optical depth.

For cases of high projected density (N ≳ 1015 cm-2) and large neutral particle density (nH2 ≳ 105 cm-3), trapping of radiation in the 2mm lines becomes important, and the "self-cooling" effect suggested by Thaddeus and Solomon (1971) is found to occur. This effect can drastically affect the radiation at 6cm, but it is noted that the required conditions do not commonly occur in interstellar clouds. It is also noted that the "effectively thin" behavior of the 2mm lines, which should be much more common, is favorable to the mechanism of Thaddeus and Solomon (1971), in which deviations from a blackbody spectrum in the background radiation at about 2mm is responsible for the anomalous cooling of formaldehyde.

The origins of the self-cooling effect and the "effective thinness" of the clouds to the 2mm lines are discussed. And the results are applied to three specific sources: Heiles Cloud 2, W51, and Orion A.

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BibTeX Citation

  author={White, R. E.},
  title={Microwave Radiative Transfer in Interstellar Formaldehyde},
  school={Columbia University},
  address={New York, N.Y.},

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RIS Citation

ID  - wh08000d
AU  - White, R. E.
PY  - 1971
BT  - Microwave Radiative Transfer in Interstellar Formaldehyde
PB  - Columbia University
CY  - New York, N.Y.
ER  -

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