Global, time-dependent flow of a neutron star superfluid

Andrew Melatos

University of Melbourne


The global time-dependent flow of superfluid inside a neutron star has received little theoretical attention in the past, partly thanks to numerical challenges (including visualization), and partly because local processes (e.g. vortex pinning), which set the boundary conditions for the global flow, have taken precedence. In this talk, we describe several new phenomena that emerge when the global problem is considered, driven by differential rotation in the star. (i) A Kelvin-Helmholtz instability occurs at the boundary between the (anisotropic) 3P2 core superfluid and the (isotropic) 1S0 crust superfluid, suddenly transferring circulation (carried by clusters of vortices) to the crust in amounts comparable to those observed during glitches (and in accord with high-resolution laboratory experiments). (ii) Quasiperiodic torque noise ("oscillations") occurs as a natural consequence of the frictionally coupled (Hall-Vinen-Bekharevich-Khalatnikov) flow of normal fluid and superfl! uid inside a sheared spherical shell, as in the stellar inner crust. We present the results of high-resolution, 3-dim hydrodynamic simulations of this flow, which displays a rich structure of Taylor-like vortices, Ekman layers, and meridional currents. (iii) The rectilinear vortex array in the core quickly breaks up into a reconnecting vortex tangle when the meridional flow exceeds a threshold. We present the results of line vortex simulations which show that the mutual friction and tension forces in the HVBK theory decrease suddenly when this happens, exciting jumps and damped oscillations in the torque. These phenomena are related to radio observations of timing irregularities in rotation-powered pulsars.

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