THE FORMATION OF METAL-FREE POPULATION III STARS IN X-RAY AND LYMAN-WERNER RADIATION BACKGROUNDS

Abstract

Metal-free Population III (or Pop III) stars are instrumental in shaping the early universe, influencing the formation of the first galaxies. The formation of Pop III stars depends on the fraction of molecular hydrogen (H2), which is regulated by X-ray and Lyman-Werner (LW) radiation backgrounds. Therefore, gaining insight into the impact of these radiation backgrounds is essential for unraveling the mysteries surrounding Pop III star formation and their impacts on the first galaxies. In this dissertation, I investigate the interaction between X-ray/LW backgrounds and the formation of Pop III stars.

To conduct this investigation, I employed the radiative hydrodynamics code RAMSES - RT. I implemented various physical processes governing Pop III star formation, such as primordial chemistry, radiation background, secondary ionization/heating, and self-shielding. Performing a grid of simulations covering a large parameter space of X-ray/LW intensity, I systematically explored the effects of radiation backgrounds on Pop III stars. I found that a moderate X-ray background boosts the H2 fraction in dark matter halos, facilitating Pop III star formation in low-mass halos. In contrast, a LW background dissociates H2 and prevents star formation in low-mass halos. This result suggests that the number of Pop III supernovae detected by the JWST is enhanced by an X-ray background. Furthermore, I discovered that an X-ray background reduces the characteristic mass and multiplicity of Pop III stars. This leads to a top-heavier initial mass function and may have a potential impact on galaxy formation.

Moreover, I made further improvements to the simulations by incorporating radiative feedback from Pop III protostars. This study confirmed previous works that radiation from protostars suppresses their growth, thereby playing a significant role in determining the mass of Pop III stars theoretically. I also found that hierarchical binaries (binaries of binaries), eccentric orbits, and outward migration are common occurrences in Pop III star formation. Eccentric orbits induce variability of Pop III protostars and this is observable by the JWST when light is magnified through gravitational lensing. In a follow-up study, I investigated the origin of outward migration and found that the gas disks around the protostars accrete gas with high angular momentum and transfer the angular momentum to the binary stars through torques. This finding paves the wayfor studies of migration behaviors across different stellar populations.

Finally, I explored the X-ray effects on the number of Pop III stars using cosmological simulations. Developing methods to calculate the intensity of the radiation background on the fly and realistically accounting for the X-ray feedback loop, I found that a weak X-ray background develops and this background ionizes the intergalactic medium, thereby moderately increasing the number density of Pop III stars (by a factor of ∼ 2). This rise in the number of Pop III stars due to X-ray radiation lowers the star formation rate of metal-enriched Pop II stars, highlighting the significance of the X-ray background in galaxy formation.This thesis covers various aspects of Pop III star formation and the effect of X-ray radiation backgrounds which has been overlooked by previous studies. It lays a foundation for future research aimed at connecting the theoretical understanding of Pop III star formation and observations targeting Pop III stars and the first galaxies.