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 dd in the envelope. We obtain a stable configuration with a maximum value of , 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, , 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 , if the (average) density ( g cm-3) of the configuration is constrained by the fastest rotating pulsar (rotation period, ms), known to date.
The M(envelope)/M(star) ratio corresponds to a value 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.
© European Southern Observatory (ESO) 2000
Online publication: December 17, 1999