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The Question

(Submitted February 13, 1997)

I am doing research on the interior structure of neutron stars. I was wondering what evidence there is for the superfluid theories, and where I can find more information about the properties that would be associated with such theories.

The Answer

The best evidence for superfluid cores in neutron stars is "glitches" observed in the evolution of the neutron star spin period. Normally, the neutron star spin period increases as time goes on. However, for some neutron stars, e.g. the Vela pulsar and the Crab pulsar, the spin period will suddenly decrease. These glitches can be understood as resulting from the superfluid core.

There are vortices in the superfluid core which move outward in response to the normal spin down. However, when they reach the boundary between the core and the crust, the vortices get pinned to the crust. This pinning keeps the crust and the core rotating together. However, a frictional drag is set up between the core and the crust, and when the stress becomes great enough, the vortices "unpin" from crust. This results in a momentary decrease in the spin period, which is observed as a "glitch".

This is at least one mechanism. There may be other explanations of exactly what happens, but they all depend on the presence of a superfluid core. For more info, you might try looking at sections 10.9-10.11 of "Black Holes, Neutrons Stars, and White Dwarfs" by Stuart Shapiro and Saul Teukolsky (1983). (This book is written at the advanced undergraduate and 1st year graduate level). In the professional astronomy literature, you might also look at Alpar, M.A. et al, "Giant Glitches and Pinned Vorticity in the Vela and Other Pulsars" in the Astrophysical Journal Letters, Vol 249, p. L29 (1981). Finally, if you're interested in more recent work, I'd suggest you use the NASA Astrophysics Data System:

(Despite its name, it actually is a search engine for the professional astronomical literature.)

I've made an assumption here that you're doing research at the college level or higher. If you're in high school, my suggestions on where to look for further info may not be as helpful, but you might try them anyway. More reasonable possibilities for the high school level would be back issues of Scientific American or Sky and Telescope.

I hope this helps.

Jim Lochner
for Imagine the Universe!
(with help from Dr. Alice Harding)

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