Moments of inertia for neutron and strange stars: limits derived for the Crab pulsar

Abstract

Recent estimates of the properties of the Crab nebula are used to derive constraints on the moment of inertia, mass and radius of the pulsar. To this purpose, we employ an approximate formula combining these three parameters. Our "empirical formula" I = a(x)MR^2, where x = (M/M_sun)(km/R), is based on numerical results obtained for thirty theoretical equations of state of dense matter. The functions a(x) for neutron stars and strange stars are qualitatively different. For neutron stars a_NS(x) = x/(0.1+2x) for x <= 0.1 (valid for M > 0.2 M_sun) and a_NS(x) = (2/9)(1+5x) for x > 0.1. For strange stars a_SS(x) = (2/5)(1+x) (not valid for strange stars with crust and M < 0.1 M_sun). We obtain also an approximate expression for the maximum moment of inertia I_max,45 = (-0.37 + 7.12x_max) (M_max/M_sun)(R_(M_max)/(10 km))^2, valid for both neutron stars and strange stars. Applying our formulae to the evaluated values of I_Crab, we derive constraints on the mass and radius of the pulsar. A very conservative evaluation of the expanding nebula mass, M_neb = 2 M_sun, yields M_Crab > 1.2 M_sun and R_Crab = 10-14 km. Setting the most recent evaluation ("central value") M_neb = 4.6 M_sun rules out most of the existing equations of state, leaving only the stiffest ones: M_Crab > 1.9 M_sun, R_Crab = 14-15 km.