2018ApJ...855...48Q

We report Submillimeter Array dust continuum and molecular spectral line observations toward the Orion Bar photon-dominated region (PDR). The 1.2 mm continuum map reveals, for the first time, a total of nine compact (r < 0.01 pc) dust condensations located within a distance of ∼0.03 pc from the dissociation front of the PDR. Part of the dust condensations are also seen in spectral line emissions of CS (5-4) and H₂CS (7_1,7-6_1,6), though the CS map also reveals dense gas further away from the dissociation front. We also detect compact emissions in H₂CS (6_0,6-5_0,5), (6_2,4-5_2,3) and C³⁴S, C³³S (4-3) toward bright dust condensations. The line ratio of H₂CS (6_0,6-5_0,5)/(6_2,4-5_2,3) suggests a temperature of 73 ± 58 K. A nonthermal velocity dispersion of ∼0.25-0.50 km s^–1 is derived from the high spectral resolution C³⁴S data and indicates a subsonic to transonic turbulence in the condensations. The masses of the condensations are estimated from the dust emission, and range from 0.03 to 0.3 M_☉, all significantly lower than any critical mass that is required for self-gravity to play a crucial role. Thus the condensations are not gravitationally bound, and could not collapse to form stars. In cooperating with recent high-resolution observations of the compressed surface layers of the molecular cloud in the Bar, we speculate that the condensations are produced as a high-pressure wave induced by the expansion of the H II region compresses and enters the cloud. A velocity gradient along a direction perpendicular to the major axis of the Bar is seen in H₂CS (7_1,7-6_1,6), and is consistent with the scenario that the molecular gas behind the dissociation front is being compressed.