Dense molecular clumps in the Orion Bar photon-dominated region

We present high angular resolution observations of the Orion Bar photon-dominated region (PDR) in optically thin H(13)CN and H(13)CO(+) (1-0) lines, obtained using the IRAM Plateau de Bure interferometer. At least 10 spatially resolved molecular condensations are identified in the H(13)CN image with virial masses in the range 0.5-1.5 M(circle dot). The median value of their H(2) volume density, similar to6x10(6) cm(-3), is a factor of similar to4 higher than the estimate based on previous PDR modeling of the main isotopomers of HCN and HCO(+). Since optically thin H(13)CN emission is likely to trace the densest gas in the clump interiors, as compared to the main isotopomer, the H(13)CN clumps appear to be close to virial equilibrium. The H(13)CN fractional abundance is a factor of similar to8 lower than that in the Orion ridge, well shielded from the far-ultraviolet (FUV) photons (similar to1x10(-10)). The H(13)CN condensations can be described in the framework of models of photoevaporating clumps exposed to an intense flux of FUV photons. The derived clump parameters are consistent with models of clumps of turbulent origin that evolve, so that their column densities are equal to the critical value determined by the incident FUV field. In this case, the column densities of the H(13)CN clumps seem high enough so that gravitational collapse can be triggered by the FUV-driven shock wave compression. The clumps may thus be collapsing to form low-mass stars. The observed H(13)CN clump parameters are also consistent with pressure-confined clump models. However, in this case the clumps would not be virialized and susceptible to gravitational collapse.