Wolf-Rayet Systems and the Origin of Massive X-ray Binaries

Abstract

AT present four strong (Px≥1036 erg s−1) X-ray sources have been identified with early type stars, probably all of binary nature; a fifth (Cen X-3) is also a massive binary, with a mass function ≥15 M○ (ref. 1). Basic parameters of these five systems, and references, are listed in Table 1. For Cen X-3 no optical identification is available as yet, but the secondary is known to have a mass of ≤0.84 M○ (refs. 2–4), hence the primary mass must be of the order of 15 M○ or larger, which means that it should also be an O- or early B-type star5. The three BO I and B 0.5 I supergiants in Table 1 must have originated from O-type main sequence stars, because during their evolution off the main sequence massive stars move towards later spectral types6. Therefore the masses of these supergiants are expected to be at least 15 M○ to 20 M⊙ (refs. 5–8). Because of the X-ray emission the secondaries in these systems are most likely to be neutron stars or black holes9,10. In the case of the Cen X-3 pulsar a neutron star seems most likely4,11. Systems of this type (consisting of an early-type star together with a collapsed star) are probably the products of the evolution of normal massive close binaries4. In such systems, the primary, after it has left the main sequence, transfers most of its outer layers (some 70% of its total mass) towards the secondary and becomes an almost pure helium star (in the helium burning core stage) close to the helium main sequence12. The secondary, enlarged in mass with the hydrogen rich outer layers of the primary, becomes a massive OB-type main sequence star and begins to evolve again practically at zero age. The helium star evolves much more rapidly than this rejuvenated secondary. If the mass of the helium star is ≥ 4 M○ (an initial primary mass ≥ 16 M⊙) it will finish its life as a Type II supernova within 1.7 × 106 yr of the first stage of mass exchange, leaving behind a neutron star or black hole4. The supernova explosion does not disrupt the binary system, because it is the less massive component which explodes13. Some ˜4 × 106 to ˜ 6 × 106 yr after the explosion the OB secondary leaves the main sequence, fills its Roche lobe and begins to transfer matter to the collapsed object, which then becomes an X-ray source. This is probably what is observed in the systems listed in Table 1, where indeed in three of the systems the visible component is definitely a post main sequence object.