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Why must a radiopulsar coupled with an optical star have high

The radiopulsar PSRB1259-63 in a massive binary has an extremely high eccentricity tex2html_wrap_inline11322 (Johnston et al., 1994[79]). The probability of finding a pulsar in an orbit with this eccentricity is at least tex2html_wrap_inline89450.77. The natural questions arise - how such eccentricity could be explained within the framework of the theory of binary evolution and what is the probability of finding a radiopulsar in such an elongated orbit? To answer these questions we have numerically simulated the evolution of an ensemble of binaries with the aim to calculate the eccentricity distribution of PSR+I binaries. Three factors are known to affect the orbital motion of a pulsar in a binary after the supernova (SN) explosion: (a) the initial eccentricity of the supernova progenitor system; (b) the mass loss during SN explosion; and (c) the additional ``kick'' velocity  of a NS caused by anisotropy  of collapse. All these factors were taken into account in our simulations.

The first factor - the initial eccentricity - is of importance only in wide systems, because mass exchange leads to orbital circularization during of the common envelope  stage, so that such systems ``forget'' the initial eccentricity. Mass loss  associated with the SN explosion leads to a decrease of the gravitational binding energy of the binary, and the disruption of the binary occurs if the mass lost exceeds half of the total binary mass prior to the explosion. We have assumed that explosion occurs at a random moment on the orbit and all the mass thrown off by a star instantly leaves the system because the characteristic velocity of matter from the exploded star is much higher than the orbital velocity. The third factor - an additional ``kick'' - is the most uncertain parameter, this could dramatically change destiny of the system .

We have calculated the distribution of eccentricities of binary radiopulsars with normal stars in two cases - with an additional ``kick'' velocity 75 km stex2html_wrap_inline8853   and without it. The calculation has been done for all pulsars in pairs with normal stars and separately for visible pulsars (the optical depth tex2html_wrap_inline11328 for free-free absorption in the stellar wind is less than 1). These distributions are presented in Figure 32.


Figure 32: Distribution of binary NS with normal components over orbital eccentricities for ejectors (dashed line) and visible pulsars (solid line) without (left-hand panel) and with ``kick'' velocity 75 km stex2html_wrap_inline8853 (right-hand panel) (Lipunov et al., 1994a).  

In the case of the scenario without collapse anisotropy, the systems show a bimodal eccentricity distribution with two maxima - close to 0 and at 1.

The systems with highly eccentric orbit origin from essentially wide pairs. Almost all wide systems are disrupted after the first SN explosion (formal eccentricity is more than 1) and only a few (which could have weak mass exchange) remain in binaries. On the other hand, close binaries with originally high mass ratio (q > 3) remain on almost circular orbit - the primary star tex2html_wrap_inline11334 tex2html_wrap_inline1133610-20 tex2html_wrap_inline11338 loses a significant part of its mass during the mass exchange stage and before the SN explosion its mass is less than that of the secondary component. So the total loss due to explosion is less than a quarter of the total mass and the system remains in an almost circular orbit. If the mechanism of an additional ``kick'' is taken into account the distribution of eccentricities is almost uniform (Figure 32). However the distribution of systems with ``visible'' pulsars displays only one maximum at tex2html_wrap_inline11340 because the pulsars with high eccentricity are in systems with relatively large semi-major axes (so that tex2html_wrap_inline11342 ). Moreover, these pulsars spend almost all the time at the most distant part of the orbit, which increases the probability of their detection. The probability of finding a pulsar in an orbit with the eccentricity tex2html_wrap_inline11322 is at least tex2html_wrap_inline894577 percent.

next up previous contents index
Next: The ``Reflection'' Effect Up: Ejectors in Massive Binary Previous: The Radiopulsar PSRB1259-63 - a

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