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In this series of papers, we model the formation and evolution of thephotoionized region and its observational signatures during massive starformation. Here we focus on the early break out of the photoionized region intothe outflow cavity. Using results of 3-D magnetohydrodynamic-outflowsimulations and protostellar evolution calculations, we perform post-processingradiative-transfer. The photoionized region first appears at a protostellarmass of 10Msun in our fiducial model, and is confined to within 10-100AU by thedense inner outflow, similar to some observed very small hypercompact HIIregions. Since the ionizing luminosity of the massive protostar increasesdramatically as Kelvin-Helmholz (KH) contraction proceeds, the photoionizedregion breaks out to the entire outflow region in <10,000yr. Accordingly, theradio free-free emission brightens significantly in this stage. In our fiducialmodel, the radio luminosity at 10 GHz changes from 0.1 mJy kpc2 at m=11Msun to100 mJy kpc2 at 16Msun, while the infrared luminosity increases by less than afactor of two. The radio spectral index also changes in the break-out phasefrom the optically thick value of 2 to the partially optically thin value of0.6. Additionally, we demonstrate that short-timescale variation in free-freeflux would be induced by an accretion burst. The outflow density is enhanced inthe accretion burst phase, which leads to a smaller ionized region and weakerfree-free emission. The radio luminosity may decrease by one order of magnitudeduring such bursts, while the infrared luminosity is much less affected, sinceinternal protostellar luminosity dominates over accretion luminosity after KHcontraction starts. Such variability may be observable on timescales as short10-100 yr, if accretion bursts are driven by disk instabilities.

Outflow-Confined HII regions. II. The Early Break-Out Phase