Radiation hydrodynamic simulations of massive star formation via gravitationally trapped H II regions - spherically symmetric ionized accretion flows

This paper investigates the gravitational trapping of H II regions predicted by steady-state analysis using radiation hydrodynamical simulations. We present idealized spherically symmetric radiation hydrodynamical simulations of the early evolution of H II regions including the gravity of the central source. As with analytic steady-state solutions of spherically symmetric ionized Bondi accretion flows, we find gravitationally trapped H II regions with accretion through the ionization front on to the source. We found that, for a constant ionizing luminosity, fluctuations in the ionization front are unstable. This instability only occurs in this spherically symmetric accretion geometry. In the context of massive star formation, the ionizing luminosity increases with time as the source accretes mass. The maximum radius of the recurring H II region increases on the accretion time-scale until it reaches the sonic radius, where the infall velocity equals the sound speed of the ionized gas, after which it enters a pressure-driven expansion phase. This expansion prevents accretion of gas through the ionization front, the accretion rate on to the star decreases to zero, and it stops growing from accretion. Because of the time required for any significant change in stellar mass and luminosity through accretion our simulations keep both mass and luminosity constant and follow the evolution from trapped to expanding in a piecewise manner. Implications of this evolution of H II regions include a continuation of accretion of material on to forming stars for a period after the star starts to emit ionizing radiation, and an extension of the lifetime of ultracompact H II regions.