Spheroidal magnetic stars rotating in vacuum

Gravity shapes stars to become almost spherical because of the isotropic nature of gravitational attraction in Newton's theory. However, several mechanisms break this isotropy like for instance their rotation generating a centrifugal force, magnetic pressure or anisotropic equations of state. The stellar surface therefore deviates slightly or significantly from a sphere depending on the strength of these anisotropic perturbations. In this paper, we compute analytical and numerical solutions of the electromagnetic field produced by a rotating spheroidal star of oblate or prolate nature. This study is particularly relevant for millisecond pulsars for which strong deformations are produced by the rotation or a strong magnetic field, leading to indirect observational signatures of the polar cap thermal X-ray emission. First we solve the time harmonic Maxwell equations in vacuum by using oblate and prolate spheroidal coordinates adapted to the stellar boundary conditions. The solutions are expanded in series of radial and angular spheroidal wave functions. Particular emphasize is put on the magnetic dipole radiation. Second, we compute approximate solutions by integrating numerically the time-dependent Maxwell equations in spheroidal coordinates. We show that the spin down luminosity corrections compared to a perfect sphere are to leading order given by terms involving $(a/r_L)^2$ and $(a/R)^2$ where $a$ is the stellar oblateness or prolateness, $R$ the smallest star radius and $r_L$ the light-cylinder radius. The corresponding perturbations in the electromagnetic field are only perceptible close to the surface, deforming the polar cap rims. At large distances $r\gg a$, the solution tends asymptotically to the perfect spherical case of a rotating dipole.

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