Photoevaporation of Clumps in Photodissociation Regions

Abstract

We present the results of an investigation of the effects of far-ultraviolet (FUV) radiation (6.0eV <13.6 eV) from hot early-type OB stars on clumps in star-forming molecular clouds. Clumps in FUV-illuminated regions (or photodissociation regions [PDRs]) undergo external heating and photodissociation as they are exposed to the FUV field, resulting in a loss of cold, molecular clump mass as it is converted to warm atomic gas. The heating, 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 three dimensionless parameters: ηc0, the ratio of the initial column density of the clump to the column N0~1021 cm-2 of a warm FUV-heated surface region; ν, the ratio of the sound speed in the heated surface to that in the cold clump material; and tFUV/tc, the ratio of the ``turn-on time'' tFUV of the heating flux on a clump to its initial sound crossing time tc. The evolution also depends on whether a confining interclump medium exists or whether the interclump region has negligible pressure, as is the case for turbulence-generated clumps. In this paper, we use spherical one-dimensional numerical hydrodynamic models as well as approximate analytical models to study the dependence of clump photoevaporation on the physical parameters of the clump and to derive the dynamical evolution, mass-loss rates, and photoevaporative timescales of a clump for a variety of astrophysical situations. 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, ~104-105 yr for a 1 Msolar clump in a typical star-forming region (ηc0=10, ν=10). Clumps with insufficient magnetic pressure support and in strong FUV fields may be driven to collapse by the compressional effect of converging shock waves. We also estimate the rocket effect on photoevaporating clumps and find that it is significant only for the smallest clumps, with sizes much less than the extent of the PDR itself. Clumps that are confined by an interclump medium may either get completely photoevaporated or 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, M17 SW, NGC 2023, and the Rosette Nebula. The data are consistent with both interpretations of clump origin, turbulence, and pressure confinement, with a slight indication for favoring the turbulent model for clumps over pressure-confined clumps.