Outflow-Confined HII Regions and the Formation of Massive Stars by Accretion

If massive stars form by disk accretion, then bipolar outflows should be generated as in the case of low-mass star formation. High accretion rates lead to high outflow rates and make the wind density very large near the protostar. We therefore predict that massive protostars have very small, jet-like HII regions confined by their outflows; we identify these HII regions with "hypercompact" (<0.01 pc) HII regions. Their lifetime is approximately equal to the accretion timescale (~10^5 yr), much longer than the sound-crossing and dynamical timescales of the ionized region. We present an analytic description of the density distribution of the outflow, relate the overall mass loss rate to the accretion rate, and normalize to values appropriate to massive protostars. For a given ionizing luminosity, we calculate the extent of the HII region and its radio spectrum. A detailed comparison is made with observations of radio source "I" in the Orion Hot Core. The spectra and morphologies of many other sources are broadly consistent with this model. We argue that confinement of ionization allows disk and equatorial accretion to continue up to high masses, potentially overcoming a major difficulty of standard accretion models.