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