Evolution of prolate molecular clouds at HII boundaries - I. formation of fragment-core structures

The evolution of a prolate cloud at an HII boundary is investigated using smoothed particle hydrodynamics. The prolate molecular clouds in our investigation are setwith their semi-major axis perpendicular to the radiative direction of a plane-parallel ionizing extreme ultraviolet (EUV) flux. Simulations on three high-mass prolate clouds reveal that EUV radiation can trigger distinctive high-density core formation embedded in a final linear structure. This contrasts with results of the previous work in which only an isotropic far-ultraviolet interstellar background flux was applied. A systematic investigation on a group of prolate clouds of equal mass but different initial densities and geometric shapes finds that the distribution of the cores over the final linear structure changes with the initial conditions of the prolate cloud and the strength of the EUV radiation flux. These highly condensed cores may either scatter over the full length of the final linear structure or form two groups of high-density cores at two foci, depending on the value of the ionizing radiation penetration depth dEUV, the ratio of the physical ionizing radiation penetration depth to the minor axis of the cloud. Data analysis on the total mass of the high-density cores and the core formation time finds that the potential for EUV radiation triggered star formation efficiency is higher in prolate clouds with shallow ionization penetration depth and intermediate major-to-minor axial ratio, for the physical environments investigated. Finally, it is suggested that the various fragment-core structures observed at HII boundaries may result from the interaction between ionizing radiation and pre-existing prolate clouds of different initial geometrical and physical conditions.