The Stability of Core-Envelope Models for the Crab and the Vela pulsars

The core-envelope analytic models of Negi et al. (1989) have been found to be consistent with both the mechanisms of glitch generation in pulsars, namely - (i) the starquake model and (ii) the vortex unpinning model. In Negi (2019), the author has been able to reproduce the observed values of glitch healing parameter, $G_h (= I_{\rm core}/I_{\rm total}; G_h$ represents the fractional moment of inertia of the core component in the starquake mechanism of glitch generation) for the Crab as well as for the Vela pulsar. In another study, by using the models discussed in Negi et al. (1989), the author has obtained the minimum value of fractional moment of inertia of the crust about 7.4\% and larger for all the values of masses in the range - $1M_\odot - 1.96M_\odot$ considered for the Vela pulsar (Negi 2020a). The latter study of the author is found to be consistent with the recent requirement (on the basis of vortex unpinning model of the glitch generation) which refers: The minimum fractional crustal moment of inertia of the Vela pulsar should be about 7\% for a mass higher than about 1$M_\odot$. However, an important study which requires investigation of pulsational stability and gravitational binding of the models of Negi et al. (1989) has not been carried out so far. The present paper deals with such a study of the models (Negi et al. 1989) for all permissible values of the compactness parameter $u (\equiv M/a$, total mass to size ratio in geometrized units) and compressibility factor $Q$ (defined in Tolman's VII solution as: $x = r^2/K^2 = r^2/a^2Q)$. It is seen that the configurations remain pulsationally stable and gravitationally bound for all permissible values of $u (\leq 0.25)$ and $Q$ ($0 < Q \leq 1.2$).