[meteorite-list] Cosmic Diamonds May Be Hidden in 'Carbon Onions'

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 18 Dec 2008 10:47:44 -0800 (PST)
Message-ID: <200812181847.KAA10209_at_zagami.jpl.nasa.gov>

http://www.newscientist.com/article/dn16259-cosmic-diamonds-may-be-hidden-in-carbon-o
Cosmic diamonds may be hidden in 'carbon onions'
by Rachel Courtland
New Scientist
12 December 2008

An exotic carbon structure could explain why astronomers have seen very
little diamond dust in the universe.

Since the 1980s, researchers have puzzled over the origin of
"nanodiamonds", tiny diamond deposits preserved in meteorites, such as
the Allende meteorite that landed in Mexico in 1969.

These tiny diamonds make up roughly 3% of the carbon in the rocks. That
suggests nanodiamonds should abound in clouds of interstellar gas and dust,
possibly forged in the fiery blasts of previous supernovae.

But so far, signs of diamonds have only been found in the dusty discs
around three young stars. Strangely, the diamonds are found close to the
stars, as opposed to being distributed more evenly in the space around
them. That hints that they were not left over from ancient stellar
explosions but may have formed near the stars, at comparatively low
pressures.

Now researchers led by Miwa Goto <http://www.mpia-hd.mpg.de/~mgoto/> of
the Max Planck Institute for Astronomy in Heidelberg, Germany, say they
have come up with an explanation.

Nanodiamonds may be hidden from view in many places because they form
within "carbon onions", exotic structures made up of concentric layers
of graphite that can form in dusty material that is blasted with
high-energy particles, the researchers say.
      
Right conditions

Carbon onions have been used to forge nanodiamonds on Earth. At high
temperatures, researchers bombard the layered graphite with electrons.
This knocks atoms out of the onions' outer shells, forcing the remaining
atoms in the shells to rearrange and close ranks. The shells therefore
squeeze inwards, eventually creating enough pressure at the centre of
the onion to form diamond.

But conditions must be just right to produce diamonds in carbon onions
around stars. A star must eject enough charged particles to turn onions
into pressure cells.

Then, in order for astronomers to see the diamonds, one of two
conditions must be met. There must either be a nearby source of X-rays
that could slough off the outer layer of the onion to reveal the
underlying gems, or the onions themselves must be warm enough - more
than 300 Degrees C - for the entire carbon onion to be turned into a
nanodiamond.

The three stars that boast signs of diamonds are all massive young stars
called Herbig Ae/Be stars. Notably, they all seem to have the required
conditions to create carbon onion diamonds, Goto says.

Two of the stars have binary companion stars that emit large flares that
could blast charged particles towards the onions. The other has a nearby
X-ray source that has yet to be identified. All have dusty discs that
are warm enough to sustain diamond growth.

Diamonds in the rough

"It's a new idea that should definitely be taken seriously," says Louis
Allamandola of NASA's Ames Research Center in Moffett Field, California,
who has studied cosmic diamonds.

But he adds that it's not clear how well the diamonds would survive
continued irradiation by stellar flares. "If they can't cool down fast
enough, they just basically erode, atoms just boil off," Allamandola
told New Scientist.

Light from carbon onions may resemble that from other forms of carbon,
making it difficult to confirm whether such structures orbit the star.

But carbon onions have been found in meteorites, including Allende. If
future studies reveal nanodiamonds within those carbon onions, it would
bolster the case that the universe is sprinkled with diamonds in the rough.

Journal reference: Astrophysical Journal
<http://www.journals.uchicago.edu/apJ/> (forthcoming)
Received on Thu 18 Dec 2008 01:47:44 PM PST