Universality in oscillation modes of superfluid neutron stars?
arXiv:0812.3708 · doi:10.1088/0004-637X/699/2/1809
Abstract
It has been well established that the -mode of relativistic ordinary-fluid neutron stars displays a universal scaling behavior. Here we study whether the "ordinary" - and "superfluid" -modes of superfluid neutron stars also show similar universal behavior. We first consider a simple case where the neutron superfluid and normal fluid are decoupled, and with each fluid modeled by a polytropic equation of state. We find that the -mode obeys the same scaling laws as established for the -mode of orindary-fluid stars. However, the oscillation frequency of the -mode obeys a different scaling law, which can be derived analytically from a homogenous two-fluid stellar model in Newtonian gravity. Next the coupling effect between the two fluids is studied via a parameterized model of entrainment. We find that the coupling in general breaks the universal behavior seen in the case of decoupled fluids. Based on a relativistic variational principle, an approximated expression is derived for the first-order shift of the -mode squared frequency due to the entrainment.
arXiv comment32 pages, 14 figures; accepted for publication in ApJ
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