DENSITY DISCONTINUITIES AND THE G-MODE OSCILLATION SPECTRA OF NEUTRON-STARS

Density discontinuities resulting from sudden composition variations, as we as smooth variations in entropy, both provide sources of buoyancy which together determine the global g-mode oscillation spectra of neutron stars. Previous calculations by Finn and McDermott investigated the g-modes induced by density discontinuities at zero temperature. That is, they neglected the contributions to buoyancy due to smooth variations in entropy at finite temperature. They also investigated only purely fluid models. We have extended these previous calculations by computing the quadrupole, spheroidal oscillation spectra, both with and without a single density discontinuity of two finite-temperature, three-component (core, crust, ocean) neutron star models. We have inserted in each model a single density discontinuity associated with the Fe-56 to Ni-62 transition predicted by the BPS equation of state (Baym, Pethick, & Sutherland). For one model this discontinuity occurs in the solid crust of the star, while for the other, hotter (younger) model it resides in the fluid ocean overlying the crust. When the discontinuity resides in the solid crust we find no significant differences between the low-order, spheroidal oscillation spectra computed with and without the discontinuity. However, when the discontinuity lies in the fluid ocean the surface g-mode spectrum is substantially altered. In particular, for a 5% density jump, the period of the fundamental surface g-mode of this model drops from about 50 ms to 37 ms. The periods of the higher overtones are also perturbed away from their values without a discontinuity.