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U. Gorti, D. Hollenbach (NASA Ames Research Center)
We present the results of an investigation of the effects of Far Ultraviolet (FUV) radiation (6.0 eV < h\nu < 13.6 eV) from OB stars on clumps in star-forming molecular clouds (with densities n ~105-6 cm-3, and sizes r ~0.01-0.1 pc). Clumps in photodissociation regions (PDRs) undergo external heating as they are exposed to the FUV field. This heating can cause destruction of the clumpy structures as the dense gas expands to maintain pressure equilibrium. In some cases, FUV heating drives photoevaporative mass flows off the clump surface, and may drive shocks into the clumps, compressing them to high densities. The clump loses mass on relatively short timescales. We study the evolution of the clump by simple analytical calculations and 1-D numerical hydrodynamical models. The evolution of a clump is found to be sensitive to three dimensionless parameters, the ratio of the initial column density of the clump to the column N0 ~1021 cm-2 of a warm PDR surface region, \etac; the ratio of the sound speeds in the heated surface and the cold clump material, \nu; and the ratio of the ``turn-on time'' tFUV of the heating flux on a clump to its sound crossing-time tc. Clumps that are confined by an interclump medium may either get completely photoevaporated, or may preserve a shielded core with a warm, protective PDR shell. For clumps which are completely photoevaporated, the mass loss timescales are found to be quite small, ~ 104-5 years for a 1 M\odot clump in a typical star forming region(\etac ~10, \nu ~10).
This indicates a rapid destruction or photoevaporation of clumps in PDRs by FUV radiation, compared with other relevant timescales such as the lifetime of an OB star (~106 years) and the crossing timescale for a clump in a PDR with an advancing ionization front(~105 years). The FUV field is found to be very efficient in heating up a considerable amount of the cold molecular material and may directly influence star formation in the cloud by destruction of the parent clumps/cores. Compression of (magnetically supercritical) clumps or cores in strong radiation fields, may on the other hand, trigger collapse in stable clumps and increase the star formation rate in the cloud.
U.G. acknowledges the support of the National Research Council for the award of a Research Associateship to the NASA Ames Research Center.