Hydromagnetic structure of a neutron star accreting at its polar caps

The hydromagnetic structure of a neutron star accreting symmetrically at both magnetic poles is calculated as a function of accreted mass, M-a, starting from a polytropic sphere plus central magnetic dipole (M-a = 0) and evolving the configuration through a quasistatic sequence of two-dimensional, Grad-Shafranov equilibria as M-a increases. It is found that the accreted material spreads equatorward under its own weight, compressing the magnetic field into a thin boundary layer and burying it everywhere except in a narrow, equatorial belt. The magnetic dipole moment of the star is given by mu = 5.2 x 10(24) ( B-0 / 10(12.5) G)(1.3) and the fractional difference between its principal moments of inertia is given by is an element of = 2.1 x 10(-5) (B-0 / 10(12.5) G)(0.27) (M-a / 10(-8) M-circle dot yr(-1))(0.18) (M-a / M-circle dot)(1.7) for M-a in the range 10(-5) less than or similar to M-a / M-circle dot less than or similar to 10(-1),where B-0 is the pre-accretion magnetic field strength, and M-a is the accretion rate.