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Cataclysmic Variables spin rates

  1. Jul 1, 2006 #1

    I am have been looking into magnetic CV's and am unsure of something i hope some one can help. Considering only the specific angular momentum of accreting matter, would a polar (Magnetic CV) have a faster spin rate than a disk accreting CV?

    I know that in magnetic polars the accretion of matter onto the WD is via an accretion column onto the pole regions of the WD rather than an accretion disk. I think i am right in thinking that this then means that the infalling material would have a higher specific angular momentum (because in the accretion disk torque would affect its angular momentum).

    I have two questions

    1) Does the material in the accretion column have a higher angular momentum than an accretion disk?

    2) And because of the conservation of angular momentum does that mean that a magnetic CV without an accretion disk would spin faster than an accretion disk CV?

    Thanks for your input.
  2. jcsd
  3. Jul 1, 2006 #2
    Far away from the accreting object, you'll have an accretion disk irrespective of the magnetic field. As material advects down the accretion disk, it looses angular momentum (though it is rotating faster and with the shorter period). So without a magnetic field, the specific angular momentum carried by the accreting mass is the lowest.

    If you have a magnetic field, it effectively makes the accreting object larger. The mass loses angular momentum in the accretion disk down to a radius where the magnetic field is strong enough to inhibit any "free motion" of the mass. The material is then forced to follow the magnetic field lines to the magnetic polar regions.

    As the mass follows the magnetic field lines it loses angular momentum, not to the accretion disk (as it did in the accretion disk) but to the magnetic field lines which are hooked to the star. So this angular momentum goes to the object, causing it to rotate faster.

    Thus, with the magnetic field, there is larger accretion of angular momentum per unit accreted mass (even though that by the time the mass reaches the star, it has lost almost all of its angular momentum to the magnetic field).

    Because however the fastest orbits in the magnetic system are slower than without a magnetic field (compare the Keplerian orbit near the object when there is no B field to the further away inner edge of the disk if there is a field), a system with a B-field will accelerate faster but to a lower maximum angular velocity.

    – NJS

    www.sciencebits.com - a random walk in science
  4. Jul 3, 2006 #3
    Hi Bugon

    Thanks for your comments, i think i understand what you mean.

    So in brief the magnetic field of a CV (polar) causes the primary to spin-up faster than a non-magnetic CV (i.e intermediate), however because the orbital periods of polars is longer than intermediates then the final angular velocity is longer as well?

    Is this because...Magnetic CV's have a magnetospheric radius similar to that of the seperation distance between the two stars. As a result does this tidally lock the primary star to the secondary star? resulting in the orbital period to be the same as the spin period of the primary?

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