Triggered star formation in bright-rimmed clouds: the Eagle nebula revisited

A three dimensional Smoothed Particle Hydrodynamical (SPH) model has been extended to study the radiative driven implosion effect of massive stars on the dynamical evolutions of surrounding molecular clouds. The new elements in the upgraded code are the inclusion of Lyman continuum in the incident radiation flux and the trea tment of hydrogen ionisation process; introducing ionisation heating & recombination cooling effects; and adding a proper description of the magnetic and turbulent pressures to the i nternal pressure of the molecular cloud. This extended code provides a realistic model to trace not only the dynamical evolution of a molecular cloud, but also can be used to model the kinematics of the ionisation & shock fronts and the photo-evaporating gas surrounding the molecular cloud, which the previous code is unable to deal with. The application of this newly developed model to the structure of the middle Eagle Nebula finger suggests that the shock induced by the ionising rad iation at the front side of the head precedes an ionisation front moving towards the center of the core, and that the core at the fingertip is at transition stage evolving toward a state of indu ced star formation. The dynamical evolution of the velocity field of the simulated cloud struct ure is discussed to illustrate the role of the self-gravity and the different cloud morphologies which appear at different stages in the evolutionary process of the cloud. The motion of the ionisation front and the evaporating gas are also investigated. The modelled gas evaporation rate is consistent with that of current other models and the density, temperature and chemical profiles ar e agreement with the observed values. The relative lifetimes of different simulated cloud morphologies suggests a possible answer to the question of why more bright-rimmed clouds are observed to possess a flat-core than an elongated-core morphology.

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