Via Scientific Computing:
The January issue of Nature has an article (subscribers only) about a solution to the problem how pulsars get their spin. This was quite a mystery, for a pulsar should have the same spin as the star it used to be, but faster because it’s contracting during the collapse. However, it somehow didn’t sum up – this theory would only be true for very fast spinning pulsars – those with a rotation-time less than a second or so.
Now some fellow from the Department of Energy and the North Carolina State University used Oak Ridge‘s Phoenix (rated 32 in the TOP500), a Cray X1E supercomputer, to perform detailed simulations about what happens during the collapse.
The results were interesting; apparently the spin of the neutron star is not determined by the spin of the star, but from the shock waves which occur when the solid iron-core of the star collapses:
“That shock wave is inherently unstable, a discovery the team made in 2002, and eventually becomes cigar-shaped instead of spherical. The instability creates two rotating flows — one in one direction directly below the shock wave and another, inner flow, that travels in the opposite direction and spins up the core.
“The stuff that’s falling in toward the center, if it hits this shock wave that is not a sphere any more but a cigar-shaped surface, will be deflected,” Mezzacappa said. “When you do this in 3-D, you find that you wind up with not only one flow, but two counter-rotating flows.”
Interesting stuff, although I must admit that I didn’t really understand everything. A pity that no animations or better pictures than the one published at Scientific Computing are available, it could enlight the uninitiated. Kudos to Dorigo, for he presents his results in a way that even the dumbest high-school-grade physics-adept understands it :) (if brain is enabled)
Spin, spin, spin! I get all dizzy.
P.S.: The ORNL website seems to be down at the moment. Wasn’t me!