Advection in photodissociation regions and its effects on the intensities of rotational lines of H(_2)O

A parameter based study has been carried out in order to investigate advection in photodissociation regions (PDRs) by using plane-parallel, semi-infinite assumption. The PDR material has been assumed to be advected from the molecular cloud towards the cloud surface with an initial flow velocity of 1 km s(^-1). The flow velocity, number density and mass density of the gas have been computed self-consistently as a function of visual extinction A(_u). The total cloud size has been assumed to be A(_u) = 10 mag. The models have been constructed for the radiation and density parameters, x = 10, 10(^3), 10(^5) with respect to Draine field and n(_H) = 10(^4), 10(^5), 10(^6) cm(^-3) respectively. In addition to 9 models characterized by these parameters, another model with X = 10(^2) and n (_H) = 10(^4) cm (^-3) has been studied in order to compare the results to the previously obtained by Bergin et al. (2003). The rotational line intensities of 0-H(_2)O and p-H(_2)O have been computed. The effects of advection in PDR structure, abundance profiles and line intensities of H(_2)O have been investigated. The lower density, lower radiation models among the models studied have been found to be effected by advection, significantly. H(_2)O abundance profile has been found to be effected by advection even in the models in which the PDR structure and the location of H/H(_2) transition zone are the least effected compared to the other models. The intensity of o-H(_2) O have been found to be sensitive to the order of magnitude of the flow velocity. It is concluded that the comparison of these results to the data from future observations with Herschel HIFI can provide information about the characteristics of the flow in nonequilibrium PDRs. This thesis includes observational work of depletion of NO in pre-protostellar cores, L1544 and L183. NO has been found to show depletion characteristics intermediate between the C-containing and N-containing species.