2017ApJ...849..133T

In this series of papers, we model the formation and evolution of the photoionized region and its observational signatures during massive star formation. Here, we focus on the early breakout of the photoionized region into the outflow cavity. Using results of 3D magnetohydrodynamic-outflow simulations and protostellar evolution calculations, we perform a post-processing radiative transfer. The photoionized region first appears at a protostellar mass of m_*=10 M_☉ in our fiducial model and is confined to within 10-100 au by the dense inner outflow, which is similar to some of the observed very small hypercompact H II regions. Since the ionizing luminosity of the massive protostar increases dramatically as the Kelvin-Helmholtz (KH) contraction proceeds, the photoionized region breaks out to the entire outflow region in <=10,000 year. Accordingly, the radio free-free emission brightens significantly in this stage. In our fiducial model, the radio luminosity at 10 GHz changes from 0.1 mJy kpc² at m_*=11 M_☉ to 100 mJy kpc² at m_*=16 M_☉, while the infrared luminosity increases by less than a factor of two. The radio spectral index also changes in the break-out phase from the optically thick value of ∼2 to the partially optically thin value of ∼0.6. Additionally, we demonstrate that short-timescale variation in the free-free flux would be induced by an accretion burst. The outflow density is enhanced in the accretion burst phase, which leads to a smaller ionized region and weaker free-free emission. The radio luminosity may decrease by one order of magnitude during such bursts, while the infrared luminosity is much less affected because internal protostellar luminosity dominates over accretion luminosity after the KH contraction starts. Such a variability may be observable on timescales as short 10-100 year if accretion bursts are driven by disk instabilities.