Aug 2, 2000
R-Modes in Superfluid Neutron Stars
Gregory Mendell (LIGO Hanford Observatory)
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Scattering off quantized vortices causes an unique form of dissipation to occur
in superfluids known as mutual friction. In superfluid neutron stars mutual
friction is due to the scattering of normal-fluid electrons off quantized
magnetic-flux carrying neutron vortices. The purpose of this talk is to explain
exactly what mutual friction is, how one calculates the mutual friction damping
rate, and what effects this has on the onset of the gravitational-radiation
instability of the r-modes in superfluid neutron stars. The talk begins by
reviewing the salient features of superfluidity in neutron stars. It then
discusses mutual friction in detail. It ends with the results for the stability
analysis of the
r-modes. The last overhead is an estimate of the effects of mutual friction in
a viscous
boundary layer.
The results shown here are published in L. Lindblom and G. Mendell, Phys. Rev.
D61, 104003 (2000), and can be found online at gr-qc/9909084. The abstract from
that paper follows.
The analogs of r-modes in superfluid neutron stars are studied here. These
modes, which are governed primarily by the Coriolis force, are identical to
their ordinary-fluid counterparts at the lowest order in the small
angular-velocity expansion used here. The equations that determine the next
order terms are derived and solved numerically for fairly realistic superfluid
neutron-star models. The damping of these modes by superfluid ``mutual
friction'' (which vanishes at the lowest order in this expansion) is found to
have a characteristic time-scale of about 10^4 s for the m=2 r-mode in a
``typical'' superfluid neutron-star model. This time-scale is far too long to
allow mutual friction to suppress the recently discovered gravitational
radiation driven instability in the r-modes. However, the strength of the mutual
friction damping depends very sensitively on the details of the neutron-star
core superfluid. A small fraction of the presently acceptable range of
superfluid models have characteristic mutual friction damping times that are
short enough (i.e. shorter than about 5 s) to suppress the gravitational
radiation driven instability completely. Links to this, and related papers,
follow.
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gr-qc/9909084, gr-qc/9902052, astro-ph/9702032
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