Investigation into Triggered Star Formation by Radiative Driven Implosion

When massive stars form, they emit strong, hydrogen ionising radiation fields into their molecular cloud environment, forming HII regions. This is believed to be capable of inducing effects which can trigger further star formation through a process known as Radiative Driven Implosion. Hydrodynamic shock fronts are generated at the interface between ionised and un-ionised material. These shocks propagate into the clouds, and their motion and increase in density can result in the conditions required for star formation.

Using the method of Smoothed Particle Hydrodynamics, the effect of varied initial geometrical and physical properties of a molecular cloud on the prospect of radiation triggered star formation is investigated over a large parameter space. The physical processes of the model include a detailed ray-tracing implementation of the ionising radiation, along with a thermodynamic model and chemical evolution for multiple species of atoms.

A parameter d_euv, defined as the ratio of the initial ionising penetration depth to the scale length of the cloud along the radiation axis, was found to be an effective indicator of the final evolutionary prospects of the molecular clouds investigated. Low d_euv clouds typically exhibit shock front motion which converges on a focus or foci, and form symmetric or asymmetric B or C type Bright Rimmed Clouds depending on orientation. At medium d_euv there is a mixture of focus/foci convergent and linear or filamentary structure formation with cores formed indirectly, after disruption of material by the shock fronts. At high d_euv only fragment-core and irregular structures form, with the clouds being increasingly dominated by photoevaporation. At extremely high d_euv cores cannot form and the cloud will photoevaporate.

In addition, qualitative impressions of the scope of structure morphologies, especially those for irregular morphologies, is compiled. Of note, it is found that the simple initial conditions of a uniform prolate cloud at inclinations to incident radiation are capable of producing a wide variety of the structures observed at HII boundaries.