2003ApJ...597L.145L

We present high angular resolution observations of the Orion Bar photon-dominated region (PDR) in optically thin H¹³CN and H¹³CO⁺ (1-0) lines, obtained using the IRAM Plateau de Bure interferometer. At least 10 spatially resolved molecular condensations are identified in the H¹³CN image with virial masses in the range 0.5-1.5 M_☉. The median value of their H₂volume density, ∼6x10⁶ cm^–3, is a factor of ∼4 higher than the estimate based on previous PDR modeling of the main isotopomers of HCN and HCO⁺. Since optically thin H¹³CN emission is likely to trace the densest gas in the clump interiors, as compared to the main isotopomer, the H¹³CN clumps appear to be close to virial equilibrium. The H¹³CN fractional abundance is a factor of ∼8 lower than that in the Orion ridge, well shielded from the far-ultraviolet (FUV) photons (∼1x10^–10). The H¹³CN 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 H¹³CN 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 H¹³CN 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.