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The first version of the Scenario Machine was aimed especially at the simulation of massive binary star evolution. We relate a binary star to the class of massive binaries if the primary component mass is large enough for the star to collapse at least to a NS without receiving any additional matter form the companion star. This limiting mass was assumed to be equal to 10 solar masses, which is the mean value of the stellar masses from 8 to in different calculations of single star evolution. In fact, the calculations we performed for different lower boundary masses showed only weak dependence on its value.
This, however, does not mean that all NS can be formed only in such systems. First, there is a large class of intermediate mass binaries (say, consisting initially of 8 + 5 solar masses) that can also produce NS and thus lead to the formation of a bright X-ray source. A possible example is Her X-1. Thus we consider a full range of stellar masses from 0.8 (a lower mass limit for the normal star to evolve out of the main sequence during the Hubble time) to . However, already early calculations have revealed a number of important properties of the joint evolution of normal and NS, which we discuss briefly in this section.
Until recently, only accreting NS had been observed in binaries (apart from binary pulsars), that is, the stage of an accreting X-ray pulsar in a pair with a supergiant (II+A) and/or with a Be star (I+A). But even first inspection of the observational situation (Table 8) revealed a large diversity of NS stages in massive binaries.
As expected, loop-like tracks of NS evolution (see, e.g. Figure 11, 10 in Section 4) were most frequently encountered. As a rule, evolutionary stages of a NS with magnetic moment follow the sequence
But other sequences may also appear. Since the initial distribution of mass ratios peaks toward , it is quite possible that the NS may appear when the secondary star is at stage II, III, or even IV instead of stage I. Generally, no characteristic loop is found in this case.
The following features of the evolution of the NS were discovered (or confirmed):
Figure 27: Relative fractions of NS at different evolutionary
states in massive binary systems (Lipunov and Prokhorov, 1987).
As the majority of NS in massive binaries prove to be at the ejector stage, it worth discussing such systems in more detail.