Centrifugal barriers in magnetospheric accretion

Abstract

ABSTRACT We reconsider the dynamics of accretion flows onto magnetized central star. For dipolar magnetically aligned case, the centrifugal barrier is at Rcb = (2/3)1/3Rc = 0.87Rc, where Rc = (GM/Ω2)1/3 is the corotation radius. For oblique dipole direct accretion from the corotation radius Rc is possible only for magnetic obliquity satisfying $\tan \theta _\mu \ge 1/(2 \sqrt{3})$ (θμ ≥ 16.1°). The accretion proceeds in a form of funnel flows – along two streams centred on the μ–Ω plane, with azimuthal opening angle $\cos (\Delta \phi) = { \cot ^ 2 {\theta _\mu } }/{12}$. For the magnetosphere distorted by the diamagnetic disc, the centrifugal barrier can be at as small radius as Rcb = 0.719Rc for the fully confined dipole, extending out to Rcb ∼ Rc for the magnetically balanced case. Type-II X-ray bursts in accreting neutron stars may be mediated by the centrifugal barrier; this requires nearly aligned configuration. Centrifugally barriered material trapped in the magnetosphere may lead to periodic obscuration (‘dips’) in the light curve of the host star, e.g. as observed in accreting young stellar objects and X-ray binaries.