THE IMPACT OF PULSATION-DRIVEN MASS LOSS AND ROTATION ON THE EVOLUTION OF PRIMORDIAL VERY MASSIVE STARS

My Thesis focuses on the evolution and final fates of primordial very massive stars. I begin with an introduction to the stellar structure equations and the methods used in the PAdova and tRieste Stellar Evolution Code (PARSEC). Then, I concentrate on pulsations in massive and very massive stars, and the resulting pulsation-driven mass loss. To achieve this, I implement the recipe for pulsation-driven mass loss developed by Nakauchi et al. (2020) in the PARSEC code. Using these models, I study the evolution and final fates of primordial very massive stars with initial masses from 100 M⊙ to 1000 M⊙ for two values of the initial metallicity Z = 0 and Z = 0.0002. These models form black holes within a very broad mass range: from ∼ 40 M⊙ to ∼ 1000 M⊙. On top of this, the 100 M⊙ zero-metallicity models could form black holes consistent with the primary black hole of the GW190521 merger event. Then, I investigate the effect of rotation on the evolution of very massive stars, particularly how it affects their final fates. To this end, I present the main implementation of stellar rotation in the PARSEC code and study the possible jet-driven events powered by an accretion disk within the collapsar scenario by Woosley (1993). This scenario demands a central black hole formed from the collapse of the star, enough angular momentum to sustain a disk, and the lack of an extended envelope for the jet propagation through the stellar progenitor. I recompute the models with initial masses 100 − 150 − 200 M⊙ with 4 different initial rotational velocities (20%, 30%, 40%, and 50% of the critical value). The models that undergo pulsational-pair instability supernovæ produce successful gamma- ray bursts, while those that collapse directly to a black hole are progenitors of jet-driven supernova events. Due to these jet-driven supernovæ, several models are expected to produce black holes within the pair-instability black-hole mass gap. Furthermore, the predicted successful gamma-ray burst events could be observable with the Swift-BAT X-ray detector up to redshift ∼ 20, while the corresponding afterglows are within the capabilities of the JWST. To facilitate the computation of stellar isochrones from all these tracks, I developed a Python script that finds the so-called critical points along stellar evolution tracks. These critical points are then crucial for the computation of stellar isochrones. The critical points code, along with the algorithm for calculating stellar isochrones, is thoroughly described in the appendixes at the end of this Thesis.