DOUBLE CORE EVOLUTION .7. THE INFALL OF A NEUTRON-STAR THROUGH THE ENVELOPE OF ITS MASSIVE STAR COMPANION

Binary systems with properties similar to those of high-mass X-ray bin aries are evolved through the common envelope phase. Three-dimensional simulations show that the timescale of the infall phase of the neutro n star depends upon the evolutionary state of its massive companion. W e find that tidal torques more effectively accelerate common envelope evolution for companions in their late core helium-burning stage and t hat the infall phase is rapid (similar to several initial orbital peri ods). For less evolved companions the decay of the orbit is longer; ho wever, once the neutron star is deeply embedded within the companion's envelope the timescale for orbital decay decreases rapidly. As the ne utron star encounters the high-density region surrounding the helium c ore of its massive companion, the rate of energy loss from the orbit i ncreases dramatically leading to either partial or nearly total envelo pe ejection. The outcome of the common envelope phase depends upon the structure of the evolved companion. In particular, it is found that t he entire common envelope can be ejected by the interaction of the neu tron star with a red supergiant companion in binaries with orbital per iods similar to those of long-period Be X-ray binaries. For orbital pe riods greater than or similar to 0.8-2 yr (for companions of mass 12-2 4 M.) it is likely that a binary will survive the common envelope phas e. For these systems, the structure of the progenitor star is characte rized by a steep density gradient above the helium core, and the commo n envelope phase ends with a spin up of the envelope to within 50%-60% of corotation and with a slow mass outflow. The efficiency of mass ej ection is found to be similar to 30%-40%. For less evolved companions, there is insufficient energy in the orbit to unbind the common envelo pe and only a fraction of it is ejected. Since the timescale for orbit al decay is always shorter than the mass-loss timescale from the commo n envelope, the two cores will likely merge to form a Thorne-Zytkow ob ject. Implications for the origin of Cyg X-3, an X-ray source consisti ng of a Wolf-Rayet star and a compact companion, and for the fate of t he remnant binary consisting of a helium star and a neutron star are b riefly discussed.