DMCA
Line Emission from Clumpy Photodissociation Regions
BibTeX
@MISC{Burton_lineemission,
author = {Michael G. Burton and Michael Burton Hollenbach and A. G. G. M. Tielens},
title = {Line Emission from Clumpy Photodissociation Regions},
year = {}
}
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Abstract
We have studied the molecular and fine structure line emission from dense photodissociation regions (PDRs). In particular, we have concentrated on predicting the line emission from CO and H 2 molecules. We find that, when the gas density is sufficiently high relative to the FUV flux (n=G 0 10 2 cm \Gamma3 ), self-shielding of the molecules can move the C + /CO and H/H 2 atomic-molecular transitions close to the surfaces of the molecular cloud (A v ! 1), where they can feel the full effect of heating by the FUV radiation field. For n 10 5 cm \Gamma3 collisional deexcitation of the FUV-pumped H 2 can move the lower levels towards LTE (e.g., 1--0 S(1)/2--1 S(1) 10), producing line ratios 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 (e.g., 14--13) originates in this warm molecular gas. [OI] 6300 A as well as [FeII] 1.257 m and 1.644 m line emission may also form good diagnostics of this warm neutral zone. The [SI] 25 m line may be observable in absorption in dense (n 10 6 cm \Gamma3 ) gas and large FUV fields (G 0 10 5 ) when the grain temperature can rise considerably in excess of the gas temperature at large depths (A v 5) into the PDR. Comparison with observation suggests that a small volume filling factor of high density (n 10 6 --10 7 cm \Gamma3 ) clumps embedded within a moderate density (n 10 3 --10 5 cm \Gamma3 ) interclump medium are a common phenomena in photodissociation regions. In general, fine-structure line emissions from O and C + are well explained by the models. The strong Si + line emission observed in some sources may indicate high gas phase silicon abundances and/or favorable geometr...
Keyphrases
line emission cm gamma3 clumpy photodissociation region high relative fine-structure line emission atomic-molecular transition high transition gas temperature strong si line emission low level fuv radiation field fuv flux favorable geometr grain temperature good diagnostics gas density moderate density cm gamma3 collisional deexcitation level towards lte interclump medium full effect warm molecular gas warm neutral zone shocked region high level line ratio molecular cloud appreciable emission high gas phase silicon abundance photodissociation region common phenomenon line ratio fine structure line emission large depth dense photodissociation region large fuv field high density small volume
