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

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Stable ultracompact objects and an upper bound on neutron star masses

P.S. Negi and M.C. Durgapal

Department of Physics, Kumaun University, Nainital - 263002, India

Received 27 April 1999 / Accepted 7 October 1999


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 [FORMULA] = [d[FORMULA]d[FORMULA] in the envelope and obtained a stable configuration with a maximum value of [FORMULA] 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 [FORMULA], if the (average) density of the configuration is constrained by fastest rotating pulsar, with rotation period, [FORMULA] ms, known to date. The average density of the configuration turns out to be [FORMULA] g cm-3. The model gives dynamically stable configurations with compaction parameter [FORMULA], where [FORMULA] mass and [FORMULA] radius of the structure] [FORMULA] 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 [FORMULA] which may be relevant in explaining the rotational irregularities in pulsars known as the timing noise and glitches.

Key words: dense matter – equation of state – stars: neutron

Send offprint requests to: P.S. Negi (psn@kumaon.ernet.in)

© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999