Adiabatic oscillations of non–rotating superfluid neutron stars

We present results concerning the linear (radial and non–radial) oscillations of non–rotating superfluid neutron stars in Newtonian physics. We use a simple two–fluid model to describe the superfluid neutron star, where one fluid consists of the superfluid neutrons, while the second fluid contains all the comoving constituents (protons, electrons). The two fluids are assumed to be “free” in the sense of absence of vortex–mediated forces like mutual friction or pinning, but they can be coupled by the equation of state, in particular by entrainment. We calculate numerically the eigen-frequencies and -modes of adiabatic oscillations, neglecting beta–reactions that would lead to dissipation. We find a doubling of all acoustic–type modes (f–modes, p–modes), and confirm the absence of g–modes in these superfluid models. We show analytically and numerically that only in the case of non–stratified background models (i.e. with no composition gradient) can these doublets of acoustic modes be separated into two distinct families, which are characterised by either co– or counter–moving fluids respectively, and which are sometimes referred to as “ordinary”and “superfluid” modes. In the general, stratified case, however, this separation is not possible, and these acoustic modes can not be classified as being either purely “ordinary” or “superfluid”. We show how the properties of the two–fluid modes change as functions of the coupling by entrainment. We find avoided mode-crossings for the stratified models, while the crossings are not avoided in the non–stratified, separable case. The oscillations of normal-fluid neutron stars are recovered as a special case simply by locking the two fluids together. In this effective one–fluid case we find the usual singlet f– and p–modes, and we also find the expected g–modes of stratified neutron star models.