How do close binary pulsars form?

[Tom Mazanec]

They are only millions of miles apart to radiate gravity waves strong enough to collide in a reasonable length of time, like PSR B1913+16 (exactly when WILL it collide, BTW?). Yet their stars must have been large red giants, if not supergiants, to become pulsars, much larger than their separations. And they must have supernovaed, so why weren't the systems blown apart? BTW, do we have GR tests on other binary pulsars (and when will THEY collide, if we do?)?

[antoniseb]

Binary pulsars most likely did not form from stars that started only millions of miles apart. They probably did spiral closer together during the super giant phases. When the first one exploded, the second one certainly lost some outer material, but a quick calculation shows that only a minor (and inert) amount of the mass of the second one would be lost in the explosion.

When the second star becomes a supergiant, the first neutron star should be able to orbit through the material of the other star like a truck driving through fog. The magnetic field of the first neutron star would have an impact on the supergiant though. Eventually, the second star explodes, and the energy transfered to the first neutron star is minimal compared to the forces holding it together, and keeping it in an orbit.

As far as I know this pair is the only known case where both neutron stars appear to us as pulsars. There are other known cases of pulsars orbiting neutron stars.

This pair looks like they will join in something like 85 million years if I remember correctly.

[trinitree88]

They are only millions of miles apart to radiate gravity waves strong enough to collide in a reasonable length of time, like PSR B1913+16 (exactly when WILL it collide, BTW?). Yet their stars must have been large red giants, if not supergiants, to become pulsars, much larger than their separations. And they must have supernovaed, so why weren't the systems blown apart? BTW, do we have GR tests on other binary pulsars (and when will THEY collide, if we do?)?

Tom. Antoniseb's scenario is certainly valid, but it's not the only one. Pulsars are often kicked to high transverse/radial velocities. Extended gas clouds often form multiple stars with matched magnetic polarizations (Steve Strom, UMass). Supernovae in such groups should eject pulsars in similar directions.(Parity, Pulsars and Supernova Remnants. Vassar 92) Those not exceeding galactic escape velocities (~285 km/sec), will travel to the galactic halo, and orbit within it, passing through the galactic plane. Nothing precludes two of these objects from finding common space, and capturing each other in a gravitational dance...except the slim statistics..but that's not forbidding.pete.

[Tom Mazanec]

I thought two objects cannot capture each other in orbit without a third helping body (like one of Jupiter's moons for Shoemaker-Levy 9, IIRC).

[antoniseb]

I thought two objects cannot capture each other in orbit without a third helping body (like one of Jupiter's moons for Shoemaker-Levy 9, IIRC).

Yes, that is correct, though the ejection of a third body within reason as a way that two neutron stars could be in a close orbit. Trinitree88's scenario is a little far fetched. n.b. it IS possible for two black holes to pass close and capture each other through the extreme loss of energy via gravity waves, but that is not what we are talking about here.