Physicist: Stars can be strange (Forwarded)

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Dec. 14, 2006

Physicist: Stars can be strange

Exploring the Stange Matter Hypothesis

By Alison Drain

According to the "Strange Matter Hypothesis," which gained popularity in
the paranormal 1980's, nuclear matter, too, can be strange. The hypothesis
suggests that small conglomerations of quarks, the infinitesimally tiny
particles that attract by a strong nuclear force to form neutrons and
protons in atoms, are the true ground state of matter. The theory has
captivated particle physicists worldwide, including one of Washington
University's own.

Mark Alford, Ph.D., Washington University in St. Louis assistant professor
of physics in Arts & Sciences, and collaborators from MIT and the Lawrence
Berkeley National Laboratory and Los Alamos National Laboratory, have used
mathematical modeling to discover some properties of theoretical "strange
stars," composed entirely of quark matter. Alford and his colleagues have
found that under the right conditions the surface of a strange star could
fragment into blobs of quark material called "strangelets," forming a
rigid halo that contradicts traditional strange star models. This means
that collapsed stars' nuclear leftovers, like the famously resplendent
Crab Nebula, could be stranger than physicists think.

Alford and his colleagues recently published their findings in Physical
Review D 73, 114016 (2006). The standard account of the dramatic death of
a heavy star is that, after exploding in a supernova that rivals a whole
galaxy in brightness, what is left is a "neutron star," a profoundly dense
remnant, made mostly of neutrons, with a mass one and a half times that of
our sun, crammed into an area with the radius of Saint Louis.

A strange star is an alternate ending of this story. If the Strange Matter
Hypothesis is correct, then what is left behind is not a neutron star but
an even denser strange star, made of quark matter rather than neutrons.
And until recently, physicists thought that the two presented very
different faces to the world.

A neutron star has a complicated multilayered surface. According to a
description by M. Coleman Miller, Ph.D., of the University of Maryland,
the deeper portions of the crust have voids that can be likened to Swiss
cheese, overlaid by regions with sheets like lasagna, rods like spaghetti,
and finally blobs like sprinklings of meatballs on the outside.

A strange star, on the other hand, was generally assumed to have a much
simpler surface, consisting of a sharp interface between strange matter
and the vacuum of surrounding space.

"A sharp interface between quark matter and the vacuum would have very
different properties from the surface of a neutron star," noted Alford.
But couldn't strange stars also have complicated surfaces? And if they
did, could we even tell neutron stars and strange stars apart?

Kaleidoscopic aura of matter

Earlier this year, Alford's colleagues concocted a radical proposal. If
blobs of quark matter (strangelets) have the right properties, maybe the
strange star crust is something more like a kaleidoscopic aura of matter
than a melon rind. "The idea was that the surface of a quark star might be
as complicated as that of a neutron star, with a sort of crystalline halo
or crust of strangelets," Alford explained. "If strangelets exist in
reality, they will have a preferred size. If small strangelets are
preferable, then big ones will split up into smaller ones. Conversely, if
big strangelets are more stable, then small ones could fuse with other
small ones -- if they happened to bump in to each other -- to make big
ones."

If strangelets prefer to be big, then the strange star's surface will be
the conventional simple sharp interface, with particles fused into the
main body of the star. But if strangelets prefer to be small, then the
surface will evaporate small strangelets to form a crystalline aura of
strangelets floating in a sea of electrons.

His colleagues found that if surface tension along the interface and
electrical forces within the charge distribution were neglected, then
strangelets prefer to be small, and the strange star's surface indeed
fragments into strangelets.

To follow, Alford joined the researchers in a more definitive
investigation, addressing key parameters like surface tension and
electrical forces that were neglected in the original study. Their results
show that as long as the surface tension is below a low critical value,
the large strangelets are indeed unstable to fragmentation and strange
stars naturally come with complex strangelet crusts, analogous to those of
neutron stars.

Their results will fuel the ongoing debate among astrophysicists about the
nature and existence of strange stars. "A strange star believer would say:
See, they showed that if the quark matter surface tension was low, then a
strange star would have this strangelet crust, so perhaps some of the
objects we think are neutron stars could actually be strange stars,"
Alford explained. "A strange star skeptic would say: Oh well, but the
surface tension would have to be absurdly low for that to happen. These
results basically show that for any reasonable value of the surface
tension there is no crust, and strange stars are completely different."

Both conclusions are valid.

The strange star theory has its staunch defenders, but most physicists
think it's merely an interesting, though improbable idea. But Alford and
his colleagues are keeping its possibility afloat.

"There is still enough doubt about our understanding of these things," he
said, "to leave room for speculation that there may be strange stars out
there."