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摘要
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 similar to 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 (CS)-S-34, (CS)-S-33 (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 similar to 0.25-0.50 km s(-1) is derived from the high spectral resolution (CS)-S-34 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-circle dot, 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.
- 出版日期2018-3-1
- 单位南京大学
