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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:
http://adsabs.harvard.edu/
(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 from the Lab for
High Energy Astrophysics at NASA/GSFC)
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