An upper limit on the mass of a primordial star due to the formation of an H II region: The effect of ionizing radiation force

We investigate analytically the formation of an H II region in the accreting envelope of a newborn star. Special care is taken to examine the role of ionizing radiation force. This effect modifies velocity and density distributions, and thereby affects the expansion of the Hn region. As a result, the upper limit of the stellar mass imposed by the growth of an H II region around a forming star is increased by a larger factor than the previous estimate. In particular, for a star forming out of metal-free gas. this mechanism does not impose a firm upper limit on its mass.