Wolf-Rayet stars and relativistic objects: Distinctions between the mass distributions in close binary systems

Abstract

The observed properties of Wolf-Rayet stars and relativistic objects in close binary systems are analyzed. The final masses MCOf for the carbon-oxygen cores of WR stars in WR + O binaries are calculated taking into account the radial loss of matter via stellar wind, which depends on the mass of the star. The analysis includes new data on the clumpy structure of WR winds, which appreciably decreases the required mass-loss rates \(\dot M_{WR}\) for the WR stars. The masses MCOf lie in the range (1–2)M⊙–(20–44)M⊙ and have a continuous distribution. The masses of the relativistic objects Mx are 1–20M⊙ and have a bimodal distribution: the mean masses for neutron stars and black holes are 1.35 ± 0.15M⊙ and 8–10M⊙, respectively, with a gap from 2–4M⊙ in which no neutron stars or black holes are observed in close binaries. The mean final CO-core mass is \(\overline M _{CO}^f = 7.4 - 10.3M_ \odot\), close to the mean mass for the black holes. This suggests that it is not only the mass of the progenitor that determines the nature of the relativistic object, but other parameters as well-rotation, magnetic field, etc. One SB1R Wolf-Rayet binary and 11 suspected WR + C binaries that may have low-mass companions (main-sequence or subgiant M-A stars) are identified; these could be the progenitors of low-mass X-ray binaries with neutron stars and black holes.