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Abstract: Six of the eight double neutron stars known in the Galactic disk have loworbital eccentricities < 0.27 indicating that their second-born neutron starsreceived only very small velocity kicks at birth. This is similar to the caseof the B-emission X-ray binaries, where a sizable fraction of the neutron starsreceived hardly any velocity kick at birth Pfahl et al. 2002. The masses ofthe second-born neutron stars in five of the six low-eccentricity doubleneutron stars are remarkably low between 1.18 and 1.30 Msun. It is arguedthat these low-mass, low-kick neutron stars were formed by the electron-capturecollapse of the degenerate O-Ne-Mg cores of helium stars less massive thanabout 3.5 Msun, whereas the higher-mass, higher kick-velocity neutron starswere formed by the collapses of the iron cores of higher initial mass. Theabsence of low-velocity single young radio pulsars Hobbs et al. 2005 isconsistent with the model proposed by Podsiadlowski et al. 2004, in which theelectron-capture collapse of degenerate O-Ne-Mg cores can only occur in binarysystems, and not in single stars.



Autor: E.P.J. van den Heuvel University of Amsterdam, University of California

Fuente: https://arxiv.org/







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