Studies of clump structure of photodissociation regions at millimeter and sub-millimeter wavelengths |
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| Authors: | Abdul Qaiyum Syed Salman Ali |
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| Institution: | (1) Department of Physics, Aligarh Muslim University, 202 002 Aligarh, India |
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| Abstract: | To interpret the millimeter and sub-millimeter line emissions of atomic and molecular species from galactic and extragalactic
photodissociation regions, warm gas components and molecular clouds, generally, escape probability formalism of Tielens &
Hollenbach (herein referred as TH) are employed which is based on the assumption of plane parallel geometry of infinite slab
allowing photons to escape only from the front. Contrary to the assumption observationally it is found that these lines are
optically thin except OI(63μ m) and low rotational transitions of CO and some other molecules. This observational evidence
led us to assume that emitting regions are finite parallel plane slab in which photons are allowed to escape from both the
surfaces (back and front). Therefore, in the present study escape of radiations from both sides of the homogeneous and also
clumpy PDR/molecular clouds are taken into consideration for calculating the line intensities at millimeter and sub-millimeter
wavelengths (hereinafter referred as QA). Results are compared with that of the TH model. It is found that thermal and chemical
structures of the regions are almost similar in both the formalisms. But line intensities are modified by differing factors.
Particularly at low density and low kinetic temperature and also for optically thin lines line intensities calculated from
TH and QA model differ substantially. But at density higher than the critical density and also for optically thick lines TH
and QA models converge to almost same values. An attempt has been made to study the physical conditions of the M17 region
employing the present formalism. |
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| Keywords: | Photodissociation region clump structure cooling lines fine structure transitions and rotational transitions |
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