Outflow-Confined H II Regions. I. First Signposts of Massive Star Formation

We present an evolutionary sequence of models of the photoionized disk-wind outflow around forming massive stars based on the Core Accretion model. The outflow is expected to be the first structure to be ionized by the protostar and can confine the expansion of the HII region, especially in lateral directions in the plane of the accretion disk. The ionizing luminosity increases as Kelvin-Helmholz contraction proceeds, and the HII region is formed when the stellar mass reaches ~10-20Msun depending on the initial cloud core properties. Although some part of outer disk surface remains neutral due to shielding by the inner disk and the disk wind, almost the whole of the outflow is ionized in 1e3-1e4 yr after initial HII region formation. Having calculated the extent and temperature structure of the HII region within the immediate protostellar environment, we then make predictions for the strength of its free-free continuum and recombination line emission. The free-free radio emission from the ionized outflow has a flux density of ~(20-200)x(nu/10GHz)^p mJy for a source at a distance of 1 kpc with a spectral index p~0.4-0.7, and the apparent size is typically ~500AU at 10GHz. The H40alpha line profile has a width of about 100km/s. These properties of our model are consistent with observed radio winds and jets around forming massive protostars.