Deformation and crustal rigidity of rotating neutron stars

Abstract

We calculate parameters A and B of the Baym-Pines model of the hydro-elastic equilibrium of rotating neutron stars. Parameter A determines the energy increase of a non-rotating star due to a quadrupolar deformation of its shape. Parameter B determines residual quadrupolar deformation due to the crustal shear strain, in a neutron star that spun-down to a non-rotating state. The calculations of A are based on precise numerical 2-D calculations for rotating neutron stars with realistic equations of state (EOSs) of dense matter. An approximate, but quite precise, formula for B is used, which allows us to separate the contribution of the crust from the dependence on the stellar mass M and radius R. The elastic shear strain distribution within the crust is modeled following Cutler et al. (2003). A(M) and B(M) are calculated for 0.2Msun < M < 0.9M_max for seven EOSs of neutron star core, combined with several crust models. A standard formula based on the incompressible fluid model is shown to severely underestimate the value of A. For M<0.7Msun A(M) is nearly EOS independent and are given (within a few percent) by a universal formula A=3.87(M/Msun)^7/3 [10^53 erg]. We derive the scaling of B with respect to R and M, valid also for a thick crust. We show that B for accreted crust strongly depends on pycnonuclear fusions at rho>10^12 g/cm^3.