Publication Abstracts

Thaddeus 1972

Thaddeus, P., 1972: Interstellar formaldehyde. I. The collisional pumping mechanism for anomalous 6-centimeter absorption. Astrophys. Lett., 173, 317-342, doi:10.1086/151423.

"Anomalous" absorption by formaldehyde in diffuse dust clouds — 6-cm line absorption in the absence of a continuum source — is generally conceded to result from a pump which maintains the 6-cm excitation temperature below 2 K, producing thereby absorption of the microwave background. A quantum-mechanical treatment is given here of the collisional-pumping process which Townes and Cheung suggest will cool the 6-cm levels by the required amount. It is shown that quantum effects probably limit the proposed process to regions where the kinetic temperature Tk ≳ 45 K, and may even suppress it altogether. (Hydrogen molecules are taken for the colliding neutral particles, but essentially the same results are expected from H or He collisions.) Electron impact, moreover, is shown to heat the 6-cm transition at essentially all Tk, and would suppress the proposed collisional cooling in regions having ne/nH2 ≳ 10-5 even if it were to occur in less ionized regions. Since radio observations of OH, H I, and CO Tk to be well below 45 K in a typical dark nebula where anomalous absorption is observed (Heiles's Cloud 2), and since the required combination of high Tk and low ne/nH2 is difficult to reconcile with thermal and ionization equilibrium, it is concluded that collisions are probably not the cause of anomalous absorption.

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BibTeX Citation

@article{th07200b,
  author={Thaddeus, P.},
  title={Interstellar formaldehyde. I. The collisional pumping mechanism for anomalous 6-centimeter absorption},
  year={1972},
  journal={Astrophysical Letters},
  volume={173},
  pages={317--342},
  doi={10.1086/151423},
}

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RIS Citation

TY  - JOUR
ID  - th07200b
AU  - Thaddeus, P.
PY  - 1972
TI  - Interstellar formaldehyde. I. The collisional pumping mechanism for anomalous 6-centimeter absorption
JA  - Astrophys. Lett.
JO  - Astrophysical Letters
VL  - 173
SP  - 317
EP  - 342
DO  - 10.1086/151423
ER  -

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