School of Astronomy (SoA) - Weekly Seminar

A  r-mode in a magnetic rotating spherical layer: application to neutron stars

 By: Shahram Abbassi ( Damghan Univ. & IPM)

Wednesday / 2-November-2011 / 11-Aban-1390/ 2:00 PM

abstract:

The impact of the combination of rotation and magnetic fields on oscillations of stellar fluids is still not well known theoretically. It mixes Alfvéand inertial waves. Neutron stars are a place where both effects may be at work. We wish to decipher the solution of this problem in the context of r-modes instability in neutron stars, as it appears when these modes are coupled to gravitational radiation.
We consider a rotating spherical shell filled with a viscous fluid but of infinite electrical conductivity and analyze propagation of modal perturbations when a dipolar magnetic field is bathing the fluid layer. We perform an extensive numerical analysis and find that the m=2r-mode oscillation is influenced by the magnetic field when the Lehnert number (ratio of Alfvéspeed to rotation speed) exceeds a value proportional to the one-fourth power of the Ekman number (non-dimensional measure of viscosity).  This scaling is interpreted as the coincidence of
the width of internal shear layers of inertial modes and the wavelength of the Alfvéwaves.  Applied to the case of rotating magnetic neutron stars, we find that dipolar magnetic fields less than 10^{14} G may weakly perturb the r-modes instability.

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