Monday, June 24, 2013

1306.4658 (J. G. Coelho et al.)

Dynamical instability of white dwarfs and breaking of spherical symmetry
under the presence of extreme magnetic fields

J. G. Coelho, R. M. Marinho Jr., M. Malheiro, R. Negreiros, J. A. Rueda, R. Ruffini
In this letter we discuss some basic properties of the equilibrium of magnetized white dwarfs, in particular the condition for dynamical instability of the star in the presence of an extremely large magnetic field. This will be done in the context of the virial theorem extended to include a magnetic term. We show, following the work of Chandrasekhar & Fermi of 1953, that when the star magnetic energy $W_B$ exceeds its gravitational potential energy $\lvert W_G\lvert$ ($W_B>\lvert W_G\lvert$), the system becomes dynamically unstable. In that seminal work it was shown that for extreme magnetic fields, a sphere is not the equilibrium configuration, and the star will become an oblate spheroid contracted along the symmetry axis. In light of this, the new mass limit for very magnetized and spherical white dwarf of 2.58$M_\odot$, recently calculated, should be considered carefully, since these objects are unstable and unbound, and also because the extreme magnetic fields violate the spherical symmetry assumed to obtain the aforementioned limit.
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