Nuclear constraints on properties of neutron star crusts

Abstract

The transition density $\ddot{I}{}_t$ and pressure $P_t$ at the inner edge separating the liquid core from the solid crust of neutron stars are systematically studied using a modified Gogny (MDI) and 47 popular Skyrme interactions within well established dynamical and thermodynamical methods. It is shown that the widely used parabolic approximation to the full Equation of State (EOS) of isospin asymmetric nuclear matter may lead to huge errors in estimating the ρ_{t} and P_{t}, especially for stiffer symmetry energy functionals $E_{sym}(\ddot{I})$. The ρ_{t} and P_{t} decrease roughly linearly with the increasing slope parameter $L$ of the $E_{sym}(\ddot{I})$ using the full EOS within both methods. It is also shown that the thickness, fractional mass and moment of inertia of neutron star crust are all very sensitive to the parameter $L$ through the $\ddot{I}{}_t$. Moreover, it is shown that the $E_{sym}(\ddot{I})$ constrained in the same sub-saturation density range as the neutron star crust by the isospin diffusion data in heavy-ion collisions at intermediate energies limits the transition density and pressure to 0.040$ fm^-3}< ρ_{t} < 0.065$ fm^-3 and 0.01 MeV/fm^3 < P_{t} < 0.26$MeV/f{m}^3,respectively.Theseconstrainedvaluesforthetransitiondensityandpressurearesignificantlylowerthantheirfiducialvaluescurrentlyusedintheliterature.Furthermore,themass-radiusrelationandseveralotherpropertiescloselyrelatedtotheneutronstarcrustarestudiedbyusingtheMDIinteraction.Itisfoundthatthenewlyconstrained$ρ_t$and$P_t$togetherwiththeearlierestimateof$ΔI/I>0.014$forthecrustalfractionofthemomentofinertiaoftheVelapulsarimposeastringentconstraintofR>=4.7+4.0M/M_{s}unkmfortheradius$R$andmass$M$ of neutron stars.

55 pages, 20 figures, 2 tables, new results and discussions added, accepted version to appear in ApJ