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Is the formation of massive stars significantly different than that of low mass stars? A review of observations across the mass spectrum from B through O suggests that all stars form by accretion, yet the structure of the accretion flows and accompanying outflows changes because of the intense ionizing radiation of early stars. Observations of accretion flows around O stars show a two-phased structure with the molecular inflow crossing an ionization boundary and continuing inward as an ionized accretion flow. Observations also show that the formation of O stars is associated with wide-angle ionized outflows with properties consistent with an origin in thermal pressure gradients rather than the highly collimated outflows characteristic of magnetic origin that are seen associated with the formation of B and later stars. A simple model in which a massive star ionizes the inner portion of scaled-up version of a low-mass star forming accretion flow explains how the increasing ionization first produces a small HII region that is gravitationally trapped around the star. Higher levels of ionization extend the HII region beyond the radius at which the escape velocity equals the sound speed resulting in a thermally driven outflow perpendicular to the rotational flattening.
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