The Nutations of Neutron Stars and Core-Crust Coupling

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Neutron stars, like the earth, are rotating fluid-filled ellipsoids. Poincaré (Bull. Astron. 27, 321, 1910), Hough (Phil. Trans. R. Soc. A186, 469, 1895) and others have discussed the nutations of such objects through a simple model, which treats the crust as rigid and the core as an ideal fluid of uniform density and vorticity. The core and crust are coupled by inertial coupling: the forces which constrain the fluid to its cavity within the crust can produce a net torque, since the cavity is ellipsoidal. Additional torques, and the effects of the elasticity in the crust and density stratification in the core, may be accomodated in such models as well (Sasao et al., Proc. IAU Symposium 78, p. 165, 1980, and references therein).