Smoothed particle hydrodynamics simulations of expanding HII regions

Context. Ionizing radiation plays a crucial role in star formation at all epochs. In contemporary star formation, ionization abruptly raises the pressure by more than three orders of magnitude; the temperature increases from ~ to ~ , and the mean molecular weight decreases by a factor of more than 3. This may result in positive feedback, either by compressing pre-existing clouds and rendering them unstable, or by sweeping up gravitationally unstable shells. It may also result in negative feedback (by dispersing residual dense gas). Ionizing radiation from OB stars is also routinely invoked as a means of injecting kinetic energy into the interstellar medium and as a driver of sequential self-propagating star formation in galaxies. Aims. We describe a new algorithm for including the dynamical effects of ionizing radiation in SPH simulations, and we present several examples of how the algorithm can be applied to problems in star formation. Methods. We use the HEALPix software to tessellate the sky and to solve the equation of ionization equilibrium along a ray towards each of the resulting tesserae. We exploit the hierarchical nature of HEALPix to make the algorithm adaptive, so that fine angular resolution is invoked only where it is needed, and the computational cost is kept low. Results. We present simulations of (i) the spherically symmetric expansion of an HII region inside a uniform-density, non-self-gravitating cloud; (ii) the spherically symmetric expansion of an HII region inside a uniform-density, self-gravitating cloud; (iii) the expansion of an off-centre HII region inside a uniform-density, non-self-gravitating cloud, resulting in rocket acceleration and dispersal of the cloud; and (iv) radiatively driven compression and ablation of a core overrun by an HII region. Conclusions. The new algorithm provides the means to explore and evaluate the role of ionizing radiation in regulating the efficiency and statistics of star formation