X-Ray Binaries and Their Descendants: Binary Radio Pulsars; Evidence for Three Classes of Neutron Stars?
An important recent discovery by Pfahl et al.(2002) is that there are two classes of Be X-ray binaries: one with orbits of small eccentricity (< 0.25), in which the neutron stars received hardly any kick velocity at birth and a class with substantial orbital eccentricities, in which the neutron stars received a kick velocity of order 100 km/s at birth. Also many of the double neutron stars (5 out of 7) have a low orbital eccentricity (0.09 to 0.27), which indicates that their second-born neutron stars received hardly any velocity kick at birth. These second-born neutron stars tend to have low masses (1.25 to 1.30 M ) . It is proposed that the low-mass, low-kick neutron stars formed by the electron-capture collapse of degenerate O- Ne-Mg cores of stars with initial masses below about 12- 14 M , while the high-kick neutron stars originated from the photo-disintegration collapse of the iron cores of stars which started out with masses larger than this limit. The latter group may be further subdivided into two classes of different mass.