r-mode oscillations in rotating magnetic neutron stars

We show that r-mode oscillations distort the magnetic fields of neutron stars and that their occurrence is likely to be limited by this interaction. If the field is ≥ 10¹⁶(Ω/Ω_Β) G, where Ω and Ω_Β are the angular velocities of the star and at which mass shedding occurs, r-mode oscillations cannot occur. Much weaker fields will prevent gravitational radiation from exciting r-mode oscillations or will damp them on a relatively short timescale by extracting energy from the modes faster than gravitational-wave emission can pump energy into them. For example, a 10¹⁰ G poloidal magnetic field that threads the star's superconducting core is likely to prevent the l = 2 mode from being excited unless Ω exceeds 0.35Ω_Β. If Ω is larger than 0.35Ω_Β initially, the l = 2 mode may be excited but is likely to decay rapidly once Ω falls below 0.35Ω_Β, which happens in ≤15 days if the saturation amplitude is ≥0.1. The r-mode oscillations may play an important role in determining the structure of neutron star magnetic fields.