Time variability in simulated ultracompact and hypercompact H ii regions

Ultracompact and hypercompact H II regions appear when a star with a mass larger than about 15 Mstarts to ionize its own environment. Recent observations of time variability in these objects are one of the pieces of evidence that suggest that at least some of them harbour stars that are still accreting from an infalling neutral accretion flow that becomes ionized in its innermost part. We present an analysis of the properties of the H II regions formed in the three-dimensional radiation-hydrodynamic simulations presented by Peters et al. as a function of time. Flickering of the H II regions is a natural outcome of this model. The radio-continuum fluxes of the simulated H II regions as well as their flux and size variations are in agreement with the available observations. From the simulations, we estimate that a small but non-negligible fraction (∼10 per cent) of observed H II regions should have detectable flux variations (larger than 10 per cent) on time-scales of ∼10 yr, with positive variations being more likely to happen than negative variations. A novel result of these simulations is that negative flux changes do happen, in contrast to the simple expectation of ever growing H II regions. We also explore the temporal correlations between properties that are directly observed (flux and size) and other quantities like density and ionization rates.