Generation of Seed Magnetic Field Around First Stars: Effects of Radiation Force

We investigate seed magnetic field generation in the early universe by the radiation force of first stars. In a previous study with the steady assumption, large amplitudes (∼10 −15 G for first stars, ∼10 −11 G for QSOs) are predicted. In this study, we formulate this issue in an unsteady framework. Then, we consider a specific model of magnetic field generation around a very massive first star. Consequently, we (1) find that the steady assumption is not valid in realistic situations and (2) obtain a much smaller magnetic field strength than that predicted by Langer et al. In addition, we find that the momentum transfer process during photoionization is more important than Thomson scattering. The resultant magnetic flux density around the first star is ≲10 −19 G. This seed magnetic field will not affect subsequent star formation in the neighborhood of first stars.