The Dynamics of Accretion Disks around Compact Stars with Complex Magnetic Fields

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Addis Ababa University


Strongly magnetized stellar objects have magnetospheres characterized by such activities that define the geometry of the inner edge of the disk as well as control the inflow of matter to the NS surface itself. Of all possible components of the surface magnetic field of the central object (neutron star-NS), we have only considered the quadrupole term to investigate what ever is going on nearer to the surface of the NS. There is a highly important difference of the accretion flow in a quadrupolar and dipolar magnetic fields. The dipolar magnetic field will in the end always present a barrier to the accretion flow since the field lines are perpendicular to the plane of the disk, but the quadrupolar magnetic field will in the simplest case lie in the plane of the disk, and thus it will rather channel the accretion flow all the way down to the stellar equator. This work involves a mathematical treatment of an accretion disk around a magnetized star. In order to define the disk structure magnetohydrodynamic (MHD) equations are solved in cylindrical coordinates. For the detailed results an ordinary differential equation (ODE) derived from the angular momentum equation is numerically solved. So, both Keplerian and non-Keplerian cases of thin accretion disk are solved. Further, introductory work on slim disk is included as a part of this work. The results of our analysis indicate the existence of two different regions: a super-Keplerian innermost region and a broader sub-Keplerian outer region. The effects of stellar and toroidal magnetic fields on the variations of viscosity, temperature and density have also been studied. We have identified the nature of the inner portion of an accretion disk. The velocity of the transition varies from corotating magnetospheric boundary to super-Keplerian for low density inner most portion of accretion disk, that extends from 0:5RM to the peak and then to sub-Keplerian. Our results are applicable to accreting astrophysical systems such viii ix as neutron stars (NSs) and white dwarfs (WDs). It can also explain observational results not yet fully backed with theories



The Dynamics of Accretion Disks