Gravitational acceleration and edge effects in molecular clouds
  Gravity plays important roles in the evolution of molecular clouds. We
present an acceleration mapping method to estimate the acceleration induced by
gravitational interactions in molecular clouds based on observational data. We
find that the geometry of a region has a significant impact on the behavior of
gravity. In the Pipe nebula which can be approximated as a gas filament, we
find that gravitational acceleration can effectively compress the end of this
filament, which may have triggered star formation. We identify this as the
"gravitational focusing" effect proposed by Burkert & Hartman (2004). In the
sheet-like IC348-B3 region, gravity can lead to collapse at its edge, while in
the centrally condensed NGC1333 cluster-forming region gravity can drive
accretion towards the center. In general, gravitational acceleration tends to
be enhanced in the localized regions around the ends of the filaments and the
edges of sheet-like structures. Neglecting magnetic fields, these
"gravitational focusing" and "edge collapse" effects can promote the formation
of dense gas in a timescale that is much shorter than the global dynamical
time. Since the interstellar medium is in general structured, these edge
effects should be prevalent.
