2000A&A...353..641N

We have proposed a core-envelope model with stiffest equation of state [speed of sound equal to that of light] in the core and a polytropic equation with constant adiabatic index Γ₁=[dlnP/dln ρ] in the envelope and obtained a stable configuration with a maximum value of u≃0.3574 when the ratio of pressure to density at the core-envelope boundary reaches about 0.014. The maximum mass of neutron star based upon this model comes out to be 7.944M_☉, if the (average) density of the configuration is constrained by fastest rotating pulsar, with rotation period, P_rot≃1.558ms, known to date. The average density of the configuration turns out to be 1.072x10¹⁴g/cm³. The model gives dynamically stable configurations with compaction parameter u [=(M/R), where M = mass and R = radius of the structure] > (1/3) which are important to study Ultra-Compact Objects [UCOs]. The theoretically obtained maximum value of u is also important regarding millisecond oscillations seen during X-Ray burst (if they are produced due to spin modulation) from a rotating neutron star, because the maximum modulation amplitude depends only upon the compaction parameter and the observed value of this amplitude provides a tool for testing theoretical models of neutron stars. The M(envelope)/M(star) ratio corresponds to a value ∼10^–2 which may be relevant in explaining the rotational irregularities in pulsars known as the timing noise and glitches.