The Dynamics of Accretion Disks around Compact Stars with Complex Magnetic Fields
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Date
2014-04
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Addis Ababa University
Abstract
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
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as neutron stars (NSs) and white dwarfs (WDs). It can also explain observational results
not yet fully backed with theories
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The Dynamics of Accretion Disks