Inertial modes of slowly rotating isentropic stars

We investigate inertial mode oscillations of slowly and uniformly rotating, isentropic, Newtonian stars. Inertial mode oscillations are induced by the Coriolis force due to the star's rotation, and their characteristic frequencies are comparable with the rotation frequency Ω of the star. So-called r-mode oscillations form a subclass of the inertial modes. In this paper, we use the term "r-modes" to denote the inertial modes for which the toroidal motion dominates the spheroidal motion, and the term "inertial modes" to denote the inertial modes for which the toroidal and spheroidal motions have comparable amplitudes. Using the slow rotation approximation consistent up to the order of Ω³, we study the properties of the inertial modes and r-modes by taking account of the effect of the rotational deformation of the equilibrium on the eigenfrequencies and eigenfunctions. The eigenfrequencies of the r-modes and inertial modes calculated in this paper are in excellent agreement with those obtained by Lindblom et al. and Lockitch & Friedman. We also estimate the dissipation timescales due to the gravitational radiation and several viscous processes for polytropic neutron star models. We find that for the inertial modes the mass multipole gravitational radiation dominates the current multipole radiation, which dominates in the case of the r-modes. It is also found that the inertial mode instability is more unstable than previously reported by Lockitch & Friedman and survives the viscous damping processes relevant in neutron stars.