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Two planets may
shape stellar ring
By Nadia Drake
Two small planets might be sculpting an
enormous dust ring around the star Fomalhaut, 25 light-years away from Earth.
But it’s not likely that either planet is
the notorious Fomalhaut b, which once
held the title of the first directly imaged
exo-world (SN: 2/25/12, p. 12).
New observations have revealed that
the ring is thin, with sharp inner and
outer edges — an unlikely shape, unless
something is keeping the ring particles
neatly in line.
The new work shows a dusty ring with
unexpectedly well-defined edges — the
trademark of what are called shepherding bodies. “We do see a similar scenario
in our solar system, with the Epsilon ring
of Uranus being herded by two small
moons,” says Ray Jayawardhana, a University of Toronto astrophysicist.
Instead of moons, though, shepherd
planets might be responsible for the
Fomalhaut ringscaping — one brushing
up the inner edge, and the other tidying
the outer edge. The orbs are each probably between the size of Mars and a super-Earth, too small to be directly detected.
“These sharp edges of the disk do indicate that there should be some planet in
there that hasn’t necessarily been seen
yet,” says astronomer Markus Janson of
Princeton University. But he advises caution until more observations are made.
Neutrino search comes up empty
Gamma-ray bursts elude blame for high-energy cosmic rays
By Nadia Drake
Instead of clearing up a half-century-old mystery, scientists have tossed a
bit of mud into an already murky pool
of suspects behind the highest-energy
cosmic rays. New findings cast doubt on
gamma-ray bursts as producers of these
enigmatic particles, which carry energy
exceeding 10 billion billion electron volts.
But there’s some wiggle room in the
evidence. If theorists rejigger equations
describing the cosmic objects, gamma-ray bursts could still be in the lineup,
scientists report in the April 19 Nature.
The results come from the IceCube
neutrino telescope, a cubic kilometer
of detectors buried beneath the South
Pole. Over a period of two years, the telescope didn’t detect any of the neutrinos
expected to arrive following 307 gamma-ray bursts. Neutrinos act as proxies indicating that cosmic rays are produced.
“Either gamma-ray bursts cannot be
A detector under the South Pole’s IceCube Laboratory (shown) did not spot
neutrinos after gamma-ray bursts, suggesting that something else is responsible for ultrahigh energy cosmic rays.
the source of all ultrahigh energy cosmic
rays, or there has to be some physics
going on inside the gamma-ray burst
that makes neutrino production different” from what scientists expected,
says Abigail Vieregg, a physicist at the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Ultrahigh energy cosmic rays, which
are actually charged subatomic particles,
arrive at Earth with millions of times the
energy generated inside CERN’s Large
Hadron Collider. Since first detecting
these rays in 1962, scientists have sought
the astrophysical accelerator whence the
rays come — an object capable of super-flinging particles across the cosmos.
“You expect it to be something very
violent,” says astrophysicist Nathan
Whitehorn of the University of Wisconsin–Madison, a study coauthor. Gamma-ray bursts are prime suspects.
For two years, IceCube scientists
watched for neutrinos after gamma-ray
bursts, focusing on the muon neutrino
subtype. It leaves long footprints that
point toward home. “If we measure the
arrival direction on Earth, we can look
back and see where they came from,”
says astrophysicist and study coauthor
Spencer Klein of Lawrence Berkeley
National Laboratory in California.
But no neutrinos arrived.
“I’m kind of inclined to think that it’s
because gamma-ray bursts are not the
source of the ultrahigh energy cosmic
rays in the universe,” Klein says.
S. LIDSTROM/NSF
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