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 H ii region, especially in lateral directions in the plane of the accretion disk. The ionizing luminosity increases as Kelvin–Helmholz contraction proceeds, and the H ii region is formed when the stellar mass reaches ∼10–$20M_{\odot }$ depending on the initial cloud core properties. Although some part of the 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 10³–${10}^{4}yr$ after initial H ii region formation. Having calculated the extent and temperature structure of the H ii 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) $\times {\left(\nu /10GHz\right)}^{p}mJy$ for a source at a distance of 1 kpc with a spectral index p ≃ 0.4–0.7, and the apparent size is typically ∼500 AU at 10 GHz. The $H40\alpha$ line profile has a width of about $100km{s}^{-1}$. These properties of our model are consistent with observed radio winds and jets around forming massive protostars.