r-modes in superfluid neutron stars

The analogues 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 s for the m52 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.