We present the results of an investigation of the effects of Far Ultraviolet (FUV) radiation from hot early type OB stars on clumps in star-forming molecular clouds. Clumps in Photodissociation regions (PDRs) undergo external heating which, if rapid, creates strong photoevaporative mass flows off the clump surfaces, and drives shocks into the clumps, compressing them to high densities. The clumps lose mass on relatively short timescales. The evolution of an individual clump is found to be sensitive to its initial colunm density, the temperature of the heated surface and the ratio of the ``turn-on time $t_{FUV}$ of the heating flux on a clump to its initial sound crossing-time $t_{c}$. In this paper, we use spherical 1-D numerical hydrodynamic models as well as approximate analytical models to study the evolution of turbulence-generated and pressure-confined clumps in PDRs. Turbulent clumps evolve so that their column densities are equal to a critical value determined by the local FUV field, and typically have short photoevaporation timescales, $sim 10^{4-5}$ years for a 1 M$_{odot}$ clump in a typical star-forming region. Clumps that are confined by an interclump medium may either get completely photoevaporated, or may preserve a shielded core with a warm, dissociated, protective shell that absorbs the incident FUV flux. We compare our results with observations of some well-studied PDRs: the Orion Bar, M17SW, NGC 2023 and the Rosette Nebula. The data are consistent with both interpretations of clump origin, with a slight indication for favouring the turbulent model for clumps over pressure-confined clumps.
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