High space velocities of single radio pulsars versus low orbital eccentricities and masses of double neutron stars: Evidence for two different neutron star formation mechanisms

Radio pulsars tend to be high-velocity objects, which implies that the majority of them received a velocity kick of several hundreds of km s(-1) at birth. However, six of the eight known double neutron stars in the galactic disk have quite low orbital eccentricities (0.085-0.27), indicating - taking into account the eccentricity induced by the mass-loss effects in the second supernova - that their second-born neutron stars received hardly any velocity kick at birth. The second-born neutron stars in these systems tend to have low masses (1.25 +/- 0.07 M-circle dot) and the same is true for the neutron star in the close white dwarf-neutron star system PSRJ 1145-6545, which also is the second-born star in a system of low eccentricity (e = 0.17). Such a low mass would fit well with the formation of the second-born neutron stars by the electron-capture collapse of a Chandrasekhar-mass degenerate O-Ne-Mg core of stars of initial main-sequence mass between 8 and about 12 M-circle dot. Podsiadlowski et al. (2004) have pointed out that neutron-star formation in this mass range is possible only if the star is in a close binary, following loss of the hydrogen envelope, while single stars can leave neutron stars only if they started out with masses above about 12 M; in the latter case a neutron star forms by the final collapse of an iron core. We point out here that the low masses of the second-born neutron stars in the low-eccentricity double neutron stars, together with the high space velocities of the single radio pulsars find a consistent explanation if one postulates, following (Van den Heuvel, 2004; Podsiadlowski et al., 2004; Dewi et al., 2005) that neutron stars formed by electron-capture collapse of a degenerate O-Ne-Mg core receive hardly any kick velocity at birth, whereas neutron stars formed by iron-core collapse receive a large space velocity at birth. The existence of a population of low-kick neutron stars in binaries has important implications for understanding the formation of the neutron star population of globular clusters and for the origin of low-mass X-ray binaries and their descendants, the wide circular-orbit binary radio pulsars.