**Next:** The
``Reflection'' Effect **Up:** Ejectors
in Massive Binary **Previous:** The
Radiopulsar PSRB1259-63 - a

eccentricity?

The radiopulsar PSRB1259-63 in a massive binary has an extremely high
eccentricity
(Johnston *et al.*, 1994[79]).
The probability of finding a pulsar in an orbit with this eccentricity
is at least 0.77.
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 s and without it. The calculation has been done for all pulsars in pairs with normal stars and separately for visible pulsars (the optical depth 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 s
(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

Sat Feb 22 18:38:13 MSK 1997