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Most of the mass of the Inter-Stellar Medium (ISM) is in the form of cold
neutral molecular clouds. These clouds are the sites of star formation. The
UV radiation from hot stars heat the nearby molecular gas establishing a
region called Photon Dominated Regions (PDRs), where the physical and the
chemical processes are dominated by the UV field. The UV radiation not only
heats the molecular gas but also regulates the star formation process
itself.
Moreover in low metallicity systems, such as LMC, SMC and dwarf galaxies, a
reduced number of dust grains and heavy elements per hydrogen atom, plays a
significant role on the chemistry and heating of the gas in PDRs.
The UV radiation penetrates much deeper into the medium dissociating CO and
producing a large layer of CI and CII. Observations show that the ratio of
[CII]-158 micron line emission to the CO rotational line emission is much
higher for dwarf galaxies than the Milky Way. Thus making CII as the tracer
of molecular hydrogen in these systems rather than CO. We study the effects
of reduced metallicity on the temperature and chemical
structure of PDRs.
Additionally, the models of PDRs were traditionally treated in a
semi-infinite plane parallel geometry where the medium is heated by the UV
field from one direction. However, many observational evidences suggest that
molecular clouds are clumpy. The clump spectrum analysis of the molecular
clouds show a power law spectrum of mass. We present here a clumpy (PDR)
model which takes into account the observed clump mass spectrum. We use
this model to study the dependence of the important cooling lines of CII, CI
and CO on metallicity. We also compare our results with the results
obtained using a semi-analytical approach by Bolatto et al., (1999).
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