  A systematic investigation on the evolution of a prolate cloud at an Hii
boundary is conducted using Smoothed Particle Hydrodynamics (SPH) in order to
understand the mechanism for a variety of irregular morphological structures
found at the boundaries of various Hii regions. The prolate molecular clouds in
this investigation are set with their semi-major axes at inclinations between 0
and 90 degrees to a plane parallel ionizing radiation flux. A set of 4
parameters, the number density n, the ratio of major to minor axis gamma, the
inclination angle phi and the incident flux F_EUV, are used to define the
initial state of the simulated clouds. The dependence of the evolution of a
prolate cloud under Radiation Driven Implosion (RDI) on each of the four
parameters is investigated. It is found that: i) in addition to the well
studied standard type A, B or C Bright Rimmed Clouds (BRCs), many other types
such as asymmetrical BRCs, filamentary structures and irregular horse-head
structures could also be developed at Hii boundaries with only simple initial
conditions; ii) the final morphological structures are very sensitive to the 4
initial parameters, especially to the initial density and the inclination; iii)
The previously defined ionizing radiation penetration depth can still be used
as a good indicator of the final morphology. Based on the simulation results,
the efficiency of the RDI triggered star formation from clouds of different
initial conditions is also estimated. Finally a unified mechanism for the
various morphological structures found in many different Hii boundaries is
suggested.
