Magnetically Driven Warping, Precession and Resonances in Accretion Disks

Abstract

Interaction between an accretion disk and a magnetic star lies at the heart of the physics of a variety of astrophysical systems, including accreting neutron stars, white dwarfs and pre-main-sequence stars (e.g., Frank et al. 1992). The basic picture of disk—magnetosphere interaction was first outlined by Pringle & Rees (1972), following the discovery of accretion-powered X-ray pulsars. These are rotating, highly magnetized(B ^ti10¹²G) neutron stars (NSs) that accrete material from a companion star, either directly from a stellar wind, or in the form of an accretion disk. The strong magnetic field disrupts the accretion flow at the magnetospheric boundary (typically at a few hundreds NS radii), and channels the plasma onto the polar caps of the NS. The magnetosphere boundary is located where the magnetic and plasma stresses balance, r_rn_ qµ⁴/⁷(GMM²)^-1/⁷, whereMand tt are the mass and magnetic moment of the central star,Mis the mass accretion rate,_gis a dimensionless constant of order unity. In low-mass X-ray binaries containing weakly magnetized(B10⁸G) NSs, the magnetosphere lies close to the stellar surface. In this case, complex field topology and general relativity can affect the determination of the magnetosphere boundary (e.g., Lai 1998). Similar magnetosphere—disk interaction also occurs in T Tauri stars (e.g., Hartmann 1998), where the stellar magnetic field(B10³G) strongly affects the accretion flow, as well as in certain class (the so-called DQ Her stars or intermediate polars) of magnetic CVs.