Simulation Studies of the Predicted Origins of SiC Grains from Stellar Yields and of Relativistic Jet Flow for Particle Acceleration in Long Gamma-Ray Bursts

Abstract

Numerical simulations serve as valuable tools in astrophysics, providing insights into complex phenomena that are difficult to observe or experiment with. In this study, two applications of simulations are introduced, tailored to specific astrophysical fields. In the first chapter, we examine the isotopic compositions of ruthenium (Ru) measured from presolar silicon carbide (SiC) grains through the stellar evolution simulation data. Under the classical scenario that SiC grains condense in the carbon-rich winds of asymptotic giant branch (AGB) stars and are later incorporated into the presolar molecular cloud, we analyze data from the Nucleosynthesis Grid (NuGrid) project. We focus on stellar wind models that satisfy the condition C > O, a requisite for SiC condensation. Analysis results confirm that massive stars do not satisfy this condition, while low-mass stars with low metallicities can reproduce most of the measured isotopic signatures of Ru. Moreover, we explore Ru isotope abundances across a variety of SiC-forming environments beyond AGB winds and find that Planetary Nebulae can influence the observed presolar Ru compositions. Also, the AGB and SN mixing models are considered for the SiC condensation environment. Second, we employ relativistic hydrodynamic simulations to investigate the internal dynamics of gamma-ray burst (GRB) jets, particularly focusing on the mechanisms of prompt emission. Using a three-dimensional simulation code, we set the long GRB jet models under energy injection duration and energy types. The resulting jet morphologies and cocoon structures vary depending on injection duration. Our analysis reveals that shear and turbulence are prominently developed in the long-duration model, favoring shear particle acceleration as a viable emission mechanism. In contrast, the short-duration model is dominated by shock structures, pointing to shock acceleration as the dominant process. By estimating collisional timescales, we further validate the potential role of shear acceleration in GRB jets.