Line Emission from Clumpy Photodissociation Regions
Abstract
A theoretical parameter study of dense photodissociation regions is presented. It is found that when the gas density is sufficiently high relative to the FUV flux, self-shielding of the molecules can move the C(+)/CO and H/H2 atomic-molecular transitions close to the surfaces of the molecular cloud, where they can feel the full effect of heating by the FUV radiation field. For n of 100,000/cu cm, collisional deexcitation of the FUV-pumped H2 can move the lower levels toward the LTE, producing line ratio resembling those of shocked regions for these low-v levels, while the high-v level line ratios retain a 'fluorescent' value. Appreciable emission in high-J transitions of CO originates in this warm molecular gas. Comparison with observations suggests that a small volume filling factor of high density clumps embedded within a moderate density interclump medium are a common phenomenon in photodissociation regions.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 1990
- DOI:
- 10.1086/169516
- Bibcode:
- 1990ApJ...365..620B
- Keywords:
-
- H Ii Regions;
- Infrared Spectra;
- Interstellar Gas;
- Molecular Clouds;
- Photodissociation;
- Carbon Monoxide;
- Emission Spectra;
- Fine Structure;
- Hydrogen;
- Line Spectra;
- Ultraviolet Radiation;
- Astrophysics;
- INFRARED: SPECTRA;
- INTERSTELLAR: MOLECULES;
- MOLECULAR PROCESSES;
- NEBULAE: H II REGIONS