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 9 compact ( r < 0.01 pc ) dust condensations located within a distance of \sim 0.03 pc from the dissociation front of the PDR .
Part of the dust condensations are seen in spectral line emissions of CS ( 5–4 ) and H _ { 2 } CS ( 7 _ { 1 , 7 } – 6 _ { 1 , 6 } ) , though the CS map also reveals dense gas further away from the dissociation front .
We detect compact emissions in H _ { 2 } CS ( 6 _ { 0 , 6 } – 5 _ { 0 , 5 } ) , ( 6 _ { 2 , 4 } – 5 _ { 2 , 3 } ) and C ^ { 34 } S , C ^ { 33 } S ( 4–3 ) toward bright dust condensations .
The line ratio of H _ { 2 } CS ( 6 _ { 0 , 6 } – 5 _ { 0 , 5 } ) / ( 6 _ { 2 , 4 } – 5 _ { 2 , 3 } ) suggests a temperature of 73 \pm 58 K. A non-thermal velocity dispersion of \sim 0.25–0.50 km s ^ { -1 } is derived from the high spectral resolution C ^ { 34 } S 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 _ { \odot } , 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 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 H _ { 2 } CS ( 7 _ { 1 , 7 } – 6 _ { 1 , 6 } ) , and is consistent with the scenario that the molecular gas behind the dissociation front is being compressed .