In this paper we investigate the statistical evolution of magnetic neutron stars, recycled in binary systems, simulating synthetic populations. To bracket uncertainties, we consider a soft (FP) and a stiff (PS) equation of state (EoS) for nuclear matter and explore the hypothesis that the magnetic field is confined in the stellar crust. We follow the magnetorotational evolution within a simple recycling scenario. The decay of the magnetic field is modeled imposing at the crust-core boundary either complete field expulsion by the superconducting core or advection and freezing in a very highly conducting transition shell. Irrespective of the details of the physical models, we find the presence of a tail in the period distribution of the synthetic populations at periods shorter than 1.558 ms, the minimum detected so far. For the soft EoS, and independent of the details of the magnetic field evolution, the recycling gives rise to a spin distribution that is increasing monotonically toward short periods, and a clear "barrier" forms at the minimum period for the onset of mass shedding (similar or equal to 0.7 ms). For the stiff EoS, the distribution is flatter, displaying a broad maximum about 2-4 ms. On the other hand, if in low-mass binaries the neutron stars experience a progressive decrease of the mass accretion rate (due to transient behavior and/or the quenching of accretion), the magnetospheric propeller produces (together with the magnetic dipole losses) an overall depletion of neutron stars in the millisecond region of the mu-P plane. The estimated fraction of neutron stars spinning close to their shedding limit over the millisecond pulsar population is found to be significant. Crustal magnetic field decay models also predict the existence of massive rapidly spinning neutron stars with very low magnetic moment mu < 10(25.8) G cm(3).
Recycling neutron stars to ultrashort periods: A statistical analysis of their evolution in the mu-P plane
BURDERI, LUCIANO;D'AMICO, NICOLO'
1999-01-01
Abstract
In this paper we investigate the statistical evolution of magnetic neutron stars, recycled in binary systems, simulating synthetic populations. To bracket uncertainties, we consider a soft (FP) and a stiff (PS) equation of state (EoS) for nuclear matter and explore the hypothesis that the magnetic field is confined in the stellar crust. We follow the magnetorotational evolution within a simple recycling scenario. The decay of the magnetic field is modeled imposing at the crust-core boundary either complete field expulsion by the superconducting core or advection and freezing in a very highly conducting transition shell. Irrespective of the details of the physical models, we find the presence of a tail in the period distribution of the synthetic populations at periods shorter than 1.558 ms, the minimum detected so far. For the soft EoS, and independent of the details of the magnetic field evolution, the recycling gives rise to a spin distribution that is increasing monotonically toward short periods, and a clear "barrier" forms at the minimum period for the onset of mass shedding (similar or equal to 0.7 ms). For the stiff EoS, the distribution is flatter, displaying a broad maximum about 2-4 ms. On the other hand, if in low-mass binaries the neutron stars experience a progressive decrease of the mass accretion rate (due to transient behavior and/or the quenching of accretion), the magnetospheric propeller produces (together with the magnetic dipole losses) an overall depletion of neutron stars in the millisecond region of the mu-P plane. The estimated fraction of neutron stars spinning close to their shedding limit over the millisecond pulsar population is found to be significant. Crustal magnetic field decay models also predict the existence of massive rapidly spinning neutron stars with very low magnetic moment mu < 10(25.8) G cm(3).
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