Hydrodynamics of neutron star interiors and laboratory superfluids

Abstract

This thesis contains the following papers: 1) A new class of g-modes in neutron stars (Astrophys. J., 395, 240 (1992)): In the fluid core of a neutron star, the ratio of the number densities of charged particles (protons and electrons) to neutrons is an increasing function of the mass density. This composition gradient stably stratifies the matter, giving rise to g-modes with periods ranging upward from a few milliseconds. Some of these modes are computed and their damping mechanisms are discussed. 2) Magnetic field decay in isolated neutron stars (Astrophys. J., 395, 250 (1992)): We investigate mechanisms that promote the loss of magnetic flux from an isolated neutron star. Ambipolar diffusion involves a drift of the magnetic field and charged particles relative to the neutrons, opposed by frictional drag. Variants of it include both the buoyant rise and the dragging by superfluid neutron vortices of magnetic flux tubes. The charged particle flux decomposes into a solenoidal and an irrotational component. The irrotational component perturbs the chemical equilibrium, generating pressure gradients that effectively choke it. The solenoidal component can transport magnetic flux from the outer core to the crust on a short timescale. Flux that threads the inner core is permanently trapped unless particle interactions can rapidly smooth departures from chemical equilibrium. We speculate that Hall drift may lead to a turbulent cascade of the magnetic field in the solid crust, terminated by ohmic dissipation at small scales. 3) The spin-up problem in helium II (To appear, J. Low Temp. Phys., 92 (1/2) (July 1993)): The laminar spin-up of helium II is studied by solving the linearized two-fluid equations in a simple case. No direct interactions between vortex lines and container walls are included. Two mechanisms are identified for the transfer of angular momentum from the container to the interior fluid. Both involve a poloidal secondary flow. An analytic expression for the spin-up time is found.