Photoevaporation of Clumps in Photodissociation Regions

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-formingmolecular clouds. Clumps in Photodissociation regions (PDRs) undergo externalheating which, if rapid, creates strong photoevaporative mass flows off theclump surfaces, and drives shocks into the clumps, compressing them to highdensities. The clumps lose mass on relatively short timescales. The evolutionof 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 asapproximate analytical models to study the evolution of turbulence-generatedand pressure-confined clumps in PDRs. Turbulent clumps evolve so that theircolumn densities are equal to a critical value determined by the local FUVfield, and typically have short photoevaporation timescales, $\sim 10^{4-5}$years for a 1 M$_{\odot}$ clump in a typical star-forming region. Clumps thatare confined by an interclump medium may either get completely photoevaporated,or may preserve a shielded core with a warm, dissociated, protective shell thatabsorbs the incident FUV flux. We compare our results with observations of somewell-studied PDRs: the Orion Bar, M17SW, NGC 2023 and the Rosette Nebula. Thedata are consistent with both interpretations of clump origin, with a slightindication for favouring the turbulent model for clumps over pressure-confinedclumps.Comment: To appear in the Astrophysical Journa