  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 1.35
Msun). It is proposed that the low-mass, low-kick neutron stars are formed by
the electron capture of degenerate O-Ne-Mg cores of stars with initial masses
below about 12-14 Msun, 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.
