This thesis presents an investigation into the formation and evolution of the universe's earliest and most massive stars, emphasizing Population III stars, including Supermassive Stars (SMS) with masses exceeding 100,000 solar masses. Utilising the enhanced Geneva Stellar Evolution Code (GENEC), the study offers insights into the chemical impacts, and ultimate fates of these stars.
The research commences with an analysis of massive stars with solar and near-solar metallicity. It uncovers how rotational modifications in GENEC impact the evolutionary paths of massive stars, particularly those between 15 and 60 solar masses. Various approaches to incorporating rotation in stellar models have been examined. In non-magnetic models, the level of chemical mixing varies based on the chosen diffusion coefficients, yet the angular velocity distribution remains consistent. Conversely, magnetic models yield much flatter internal angular velocity profiles. At the end of their evolutionary cycle, the magnetic models predict a core angular momentum significantly lower than the predictions by non-magnetic models.
The thesis then shifts to exploring massive, fast-rotating stars at low and zero metallicities. These stars are identified as key contributors to the chemical enrichment of the interstellar medium, especially in nitrogen enrichment of high-redshift galaxies like GN-z11 and CEERS-1019. The research demonstrates that fast-rotating Population III stars closely reproduce observed abundance ratios, such as nitrogen to oxygen, which are not replicated by stars with higher metallicities. Further, the study investigates accreting, non-rotating extremely massive stars, delving into their chemical influence and revealing the significant role of accretion rates in their evolution. A critical accretion rate is identified, marking the transition of these stars into the red supergiant phase and influencing their lifespan, surface helium enrichment, and physical properties.
A major focus is on SMS within the atomically cooled halo regime. The research explores their evolution, internal structures, and potential to form black holes. The thesis reveals that SMS at specific accretion rates can reach the general relativistic instability during core hydrogen burning, leading to direct collapse into black holes.
In addition to these findings, the thesis outlines future research directions, including the study of Population III and very metal-poor stars with different rotational mixing physics, the implementation of rotation and accretion in SMS models, and the exploration of internal magnetic fields instabilities and WIMP annihilation in SMS evolution. The potential observational traces of SMS, particularly through the James Webb Space Telescope data, are also identified as a key area for future study.
