Fluids And Magnetohydrodynamics In Astrophysics

Hydrodynamics and magnetohydrodynamics have wide applications in astrophysics. Accretion processes and jets in supermassvie black hole in AGNs, density waves in spiral arms of spiral galaxies, accretion processes and outflows in star and planet formation, stellar oscillations as well as coronal mass ejections are all related to hydrodynamics and magnetohydrodynamics. This paper focuses on the application of hydrodynamics and magnetohydrodynamics to solar oscillations and accretion process and outflow in star formation.Searching g-mode oscillation has generated great interests among helioseismologists. If g-mode oscillations do exist, they would give rise to periodic variation of solar interior temperatures, pressures and densities. Accordingly, periodic variations of these thermodynamic quantities would lead to the periodic variations of solar sound speed. Thus the periodic behavior of sound speed would cause the frequencies of solar p-mode oscillation to change periodically on the time scale of g-mode oscillations. Frequency modulations (FMs) of solar p-modes of shorter periods may occur owing to the simultaneous presence of solar interior g-modes of longer periods and with sufficient strengths. Besides effects of differential rotation, stochastic excitation and magnetic fields, FMs of p-modes by interior g-modes would give rise to organized fine spectral fine structures in the form of frequency splitting intervals of the order of g-mode frequencies. We discuss such an approach to use frequency modulations of solar p-mode by g-modes to search solar g-modes. Analysis shows that it is possible for us to use high frequency resolution p-mode power spectrum data to search solar g-modes.In another work in this thesis, we investigate self-similar magnetohydrodynamics processes in an isothermal self-gravitating fluid with quasi-sphercial symmetry and extend the envelope expansion with core collapse solutions of Lou & Shen by incorporating magnetic fields. Magnetized expansion wave collapse solutions can be constructed as a special case. This inside-out MHD collapse occurs at the magnetosonic speed. Magnetized EECC solutions are also found. These similarity solutions can show various physical processes, such as radial inflow, core collapse, oscillations, outflow as well as shocks. We carefully address the question concerning magnetosonic critical line and topological property of magnetosonic critical points. When the parameter X is larger, the core accretion rate appropriate for magnetized EWCS is larger. In the frozen-in approximation, the magnetic fields in envelope expansion portion would behave as B ∝ r^-1, while in core collapse portion they would behave as B ∝ r^-1/2. We note the potential astrophysical application of EECC similarity solutions to star and planet formation processes, to the formation process of proto-planetary nebulae connecting the AGB phase and the planetary nebula phase, to supernova remnants, and to certain evolution stage of galaxy clusters.