A numerical study of the rotational behavior of the white dwarf in DQ Herculis, II: the turn-over scenario |
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Authors: | V.S. Geroyannis P.J. Papasotiriou |
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Affiliation: | (1) Astronomy Laboratory, Department of Physics, University of Patras, Greece |
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Abstract: | The optical pulsations in DQ Her are universally believed to be due to emission from the magnetic poles of the white dwarf. However, there is no way for a pulsation to be seen if the magnetic axis and the spin axis are aligned; whether the optical pulsation is seen directly from the magnetic poles or as a result of re-processing this beam from sides in an accretion disc, the magnetic and spin axes must be offset. This fact explains why the `oblique rotator' model has been adopted as a standard model for the DQ Her primary. In a recent paper, we have computed several axisymmetric models simulating the DQ Her white dwarf before its `turn-over' (where the term `turn-over' describes the process by which the magnetic axis gets inclining relative to the spin axis at a gradually increasing angle, the so-called `turn-over angle'). For such models, we have found that the moment of inertia along the rotation axis, I 33, is less than the moment(s) of inertia along the two other principal axes, I 11=I 22. The situation I 11>I 33 is known as `dynamical asymmetry', and can cause a spontaneous turn-over of the magnetic axis with respect to the rotation axis . Consequently, the DQ Her white dwarf is either an oblique rotator undergoing its turn-over phase, or it is already qalmostequal a `perpendicular rotator', i.e., its turn-over angle is almost equal to 90°. Assuming the first case, we study numerically the so-called `turn-over scenario', that is, a scenario on rotational evolution in which the turn-over phase is taken into account. We give emphasis on computations concerning the spin-down time rate of the DQ Her white dwarf due to turn-over (not to be confused with its spin-uptime rate due to accretion) for several possible values of the magnetic field. This revised version was published online in July 2006 with corrections to the Cover Date. |
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Keywords: | stars: individual (DQ Her) stars: magnetic fields stars:rotation white dwarfs |
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