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The Evolutionary Status of Radiopulsars in Massive Binary


As explained in detail above, the evolution of a NS, by its nature, must be studied jointly with the evolution of normal stars. This problem was discussed qualitatively by Bisnovatyi-Kogan and Komberg (1974)[15], van den Heuvel (1977)[202] and Lipunov (1982a)[98].

In Figure 10 we show possible evolutionary tracks of NS. As a rule, the NS is generated at the instant when the companion lies on the main sequence (track b). During the first tex2html_wrap_inline9939 - tex2html_wrap_inline9690 years, the star is in the ejector state usually unobservable due to absorption in the stellar wind  from the normal star. The period of the NS increases in accordance with the magnetic dipole losses. After that, plasma penetrates inside the light cylinder and the NS passes first to the propeller  stage and then to the accretor stage. By this time, the normal star leaves the main sequence and the stellar wind is intensified. This results in the formation of a bright X-ray pulsar. The period of the NS is stabilized close to its equilibrium value.  Finally, the normal star fills the Roche lobe  and the accretion rate  suddenly increases; the NS moves first to the right and then vertically down on the tex2html_wrap_inline9773 diagram. In other words, the NS moves into the supercritical stage SA (superaccretor).  Its period tends to a new equilibrium value (see Lipunov, 1992[107]):


After the mass exchange, only the helium core of the normal star is left (WR star), a separate system is formed (in some systems the NS has spiraled  completely into the center of its companion, and a Thorne-Zytkow object  is formed), and the NS again becomes a propeller  or an ejector.  Accretion is hampered by the rapid rotation. This is probably a reason for the observed lack of X-ray pulsars  in pairs with Wolf-Rayet stars (Lipunov, 1982e[102]). The helium star  evolves on a short time-scale ( tex2html_wrap_inline9044 tex2html_wrap_inline9939 yr), so the companion NS does not have time to spindown  considerably: after the explosion of the normal star, the system is disrupted and the NS becomes an ejector  again, that is, it may be observed as a radiopulsar.

The ``loop-shaped'' track discussed in Section 4.1. shows another evolutionary behavior of a NS formed in the process of mass exchange in the binary system:


The total lifetime of a NS in a binary system depends on the lifetime of the normal star and the parameters of the binary system. However, the rate of transition from one stage to another is proportional to the NS magnetic field.

next up previous contents index
Next: The Number of Ejectors Up: Ejectors in Massive Binary Previous: Ejectors in Massive Binary

Mike E. Prokhorov
Sat Feb 22 18:38:13 MSK 1997