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Astron. Astrophys. 353, 641-645 (2000)

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4. Results and conclusions

We have proposed a 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 [FORMULA]d[FORMULA]d[FORMULA] in the envelope. We obtain a stable configuration with a maximum value of [FORMULA], when the minimum ratio of pressure to density at the core-envelope boundary reaches about 0.014. In our model all the metric parameters including their first derivatives and the speed of sound are continuous at the core-envelope boundary and at the exterior boundary of the structure. The maximum u for this core-envelope model comes out to be nearly as large as that obtained by using the most stiff EOS [which is abnormal in the sense that the nuclear matter does not correspond to the state of self-bound matter] throughout the structure. The structures are dynamically stable and gravitationally bound even for the value of compaction parameter, [FORMULA], thus giving a suitable model for studying the Ultra-compact Objects [UCOs].

The maximum mass of neutron star based upon this model comes out to be [FORMULA], if the (average) density ([FORMULA] g cm-3) of the configuration is constrained by the fastest rotating pulsar (rotation period, [FORMULA] ms), known to date.

The M(envelope)/M(star) ratio corresponds to a value [FORMULA] which may be relevant in explaining the rotational irregularities in pulsars known as the timing noise and glitches.

The 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 (if the rotation is not enough rapid to modify the exterior Schwarzschild geometry), because the maximum modulation amplitude depends only upon the compaction parameter [see, e.g., Strohmayer et al. 1998; Miller & Lamb 1998; and references therein] and the observed value of this amplitude provides a powerful tool for testing theoretical models of neutron stars.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999