TOPICS IN THEORETICAL ASTROPHYSICS: PRECESSION OF WARPED DISKS, OSCILLATIONS OF PRESUPERNOVA STARS, AND THERMAL EVOLUTION AND NUCLEOSYNTHESIS OF YOUNG NEUTRON STARS
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This thesis consists of three parts. In the first part, we study the magnetically driven precession of warped disks. An accretion disk around a rotating magnetized star is subjected to the magnetic torques that induce warping and precession of the disk. We study the global hydrodynamical warping/precession modes of the disk under the combined influences of the magnetic torques, relativistic frame dragging, and the classical precession due to oblateness of the neutron star. We apply our analysis to two types of accreting systems: low-mass X-ray binaries (LMXBs) and accreting X-ray pulsars. We argue that some features of low-frequency quasi-periodic oscillations (QPOs) in LMXBs and milli-Hertz QPOs in accreting X-ray pulsars can be explained by the magnetically driven precession of warped disks. The second part is related to the hydrodynamically-driven mechanism for asymmetric supernova explosions / neutron star kicks. We explore the possibility that the gravity modes in the core of a presupernova star may be amplified in the silicon burning shell to produce the global asymmetric perturbations that lead to an asymmetric supernova explosion. By performing a linear analysis of the oscillations in the cores of presupernova stars, we estimate the growth/damping rates of the modes. We find that most of the modes are damping modes with a few exceptions. We also find that, even for a growing mode, the timescale of mode growth is much longer than the remaining time before the core collapse. We conclude that the gravity modes in a presupernova core cannot provide the global asymmetric perturbations that lead to an asymmetric supernova explosion. In the last part, we attempt to predict the innate chemical composition of a neutron star atmosphere. There has been great progress in X-ray observations and now thermal radiation from neutron stars is being studied in detail. There has also been significant progress in modeling thermal spectra from neutron stars. However, the unknown chemical composition of the neutron star surface is assumed in these models. Our goal is to predict the chemical composition of a neutron star atmosphere by evolving its thermal structure and the chemical composition from the earliest possible time. We study necessary steps to achieve this goal. We study models for a static atmosphere of a young neutron star, cooling of the bulk of a young neutron star, the nuclear statistical equilibrium abundances, nucleosynthesis, and the possible role of diffusion.