A new class of g-modes in neutron stars

Abstract

Because a neutron star is born hot, its internal composition is close to chemical equilibrium. In the fluid core, this implies that the ratio of the number densities of charged particles (protons and electrons) to neutrons, x ≡ n_c/n_n, is an increasing function of the mass density. This composition gradient stably stratifies the matter giving rise to a Brunt-Väisälä frequency N ~ (xg/2H)^(1/2) ~ 500 s^(-1), where g is the gravitational acceleration, and H is the density scale height. Consequently, a neutron star core provides a cavity that supports gravity modes (g-modes). These g-modes are distinct from those previously identified with the thermal stratification of the surface layers and the chemical stratification of the crust. We compute the lowest-order, quadrupolar, g-modes for cold, Newtonian, neutron star models with M/M_☉ = 0.581 and M/M_☉ = 1.405 and show that the crustal and core g-modes have similar periods. We also discuss damping mechanisms and estimate damping rates for the core g-modes. Particular attention is paid to damping due to the emission of gravitational radiation.

Files

Additional details