Detection of Dust Condensations in the Orion Bar Photon-dominated Region
  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 < 0.01pc)
dust condensations located within a distance of ~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
$73\pm58$ K. A non-thermal velocity dispersion of ~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 HII 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.
