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This paper investigates the gravitational trapping of HII regions predictedby steady-state analysis using radiation hydrodynamical simulations. We presentidealised spherically symmetric radiation hydrodynamical simulations of theearly evolution of HII regions including the gravity of the central source. Aswith analytic steady state solutions of spherically symmetric ionised Bondiaccretion flows, we find gravitationally trapped HII regions with accretionthrough the ionisation front onto the source. We found that, for a constantionising luminosity, fluctuations in the ionisation front are unstable. Thisinstability only occurs in this spherically symmetric accretion geometry. Inthe context of massive star formation, the ionising luminosity increases withtime as the source accretes mass. The maximum radius of the recurring HIIregion increases on the accretion timescale until it reaches the sonic radius,where the infall velocity equals the sound speed of the ionised gas, afterwhich it enters a pressure-driven expansion phase. This expansion preventsaccretion of gas through the ionisation front, the accretion rate onto the stardecreases to zero, and it stops growing from accretion. Because of the timerequired for any significant change in stellar mass and luminosity throughaccretion our simulations keep both mass and luminosity constant and follow theevolution from trapped to expanding in a piecewise manner. Implications of thisevolution of HII regions include a continuation of accretion of material ontoforming stars for a period after the star starts to emit ionising radiation,and an extension of the lifetime of ultracompact HII regions.

Radiation hydrodynamic simulations of massive star formation via gravitationally trapped HII regions - Spherically symmetric ionised accretion flows