Neutron Star Crustal Interface Waves

The eigenfrequencies of nonradial oscillations are a powerful probe of astar's interior structure. This is especially true when there existdiscontinuities such as at the neutron star (NS) ocean/crust boundary, as firstnoted by McDermott, Van Horn & Hansen. The interface mode associated with thisboundary has subsequently been neglected in studies of stellar nonradialoscillations. We revisit this mode, investigating its properties bothanalytically and numerically for a simple NS envelope model. We find that itacts like a shallow surface ocean wave, but with a large radial displacement atthe ocean/crust boundary due to flexing of the crust with shear modulus $\mu\llP$, the pressure. This displacement lowers the mode's frequency by a factor of$\sim(\mu/P)^{1/2}\sim0.1$ in comparison to a shallow surface wave frequency ona hard surface. The interface mode may be excited on accreting or bursting NSsand future work on nonradial oscillations should consider this mode. Our workalso implies an additional mode on massive and/or cold white dwarfs withcrystalline cores, which may have a frequency between the f-mode and g-modes,an otherwise empty part of the frequency domain.Comment: To appear in Astrophysical Journal, 10 pages, 5 figure