  The Orion Bar is the archetypal edge-on molecular cloud surface illuminated
by strong ultraviolet radiation from nearby massive stars. Owing to the close
distance to Orion (about 1,350 light-year), the effects of stellar feedback on
the parental cloud can be studied in detail. Visible-light observations of the
Bar(1) show that the transition between the hot ionised gas and the warm
neutral atomic gas (the ionisation front) is spatially well separated from the
transition from atomic to molecular gas (the dissociation front): about 15
arcseconds or 6,200 astronomical units (one astronomical unit is the Earth-Sun
distance). Static equilibrium models(2,3) used to interpret previous
far-infrared and radio observations of the neutral gas in the Bar(4,5,6)
(typically at 10-20 arcsecond resolution) predict an inhomogeneous cloud
structure consisting of dense clumps embedded in a lower density extended gas
component. Here we report one-arcsecond-resolution millimetre-wave images that
allow us to resolve the molecular cloud surface. In contrast to stationary
model predictions(7,8,9), there is no appreciable offset between the peak of
the H2 vibrational emission (delineating the H/H2 transition) and the edge of
the observed CO and HCO+ emission. This implies that the H/H2 and C+/C/CO
transition zones are very close. These observations reveal a fragmented ridge
of high-density substructures, photoablative gas flows and instabilities at the
molecular cloud surface. The results suggest that the cloud edge has been
compressed by a high-pressure wave that currently moves into the molecular
cloud. The images demonstrate that dynamical and nonequilibrium effects are
important for the cloud evolution.
