Radiative Regulation of Population III Star Formation

We explore the impact of ultraviolet (UV) radiation from massive Population III (Pop III) stars of 25, 40, 80, and 120 M⊙ on the subsequent Pop III star formation. In this paper, particular attention is paid to the dependence of radiative feedback on the mass of source Pop III star. UV radiation from the source star can work to impede the secondary star formation through the photoheating and photodissociation processes. Recently, Susa & Umemura (2006) have shown that the ionizing radiation alleviates the negative effect by H2-dissociating radiation from 120M⊙ PopIII star, since an H2 shell formed ahead of an ionizing front can effectively shield H2-dissociating radiation. On the other hand, it is expected that the negative feedback by H2-dissociating radiation can be predominant if a source star is less massive, since a ratio of the H2-dissociating photon number to the ionizing photon number becomes higher. In order to investigate the radiative feedback effects from such less massive stars, we perform three-dimensional radiation hydrodynamic simulations, incorporating the radiative transfer effect of ionizing and H2-dissociating radiation. As a result, we find that if a source star is less massive than ≈ 25M⊙, the ionizing radiation cannot suppress the negative feedback of H2-dissociating radiation. Therefore, the fate of the neighboring clouds around such less massive stars is determined solely by the flux of H2-dissociating radiation from source stars. With making analytic estimates of H2 shell formation and its shielding effect, we derive the criteria for radiation hydrodynamic feedback depending on the source star mass.