[meteorite-list] Circumstellar Space: Where Chemistry Happens For The Very First Time

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 31 Jul 2007 17:22:30 -0700 (PDT)
Message-ID: <200708010022.RAA27527_at_zagami.jpl.nasa.gov>

http://news-info.wustl.edu/tips/page/normal/9672.html

Circumstellar space: Where chemistry happens for the very first time

Cool place, hot bodies

By Tony Fitzpatrick
Washington University in St. Louis

July 31, 2007 -- Picture a cool place, teeming with a multitude of hot
bodies twirling about in rapidly changing formations of singles and
couples, partners and groups, constantly dissolving and reforming.

If you were thinking of the dance floor in a modern nightclub, think again.

It's a description of the shells around dying stars, the place where
newly formed elements make compounds and life takes off, said Katharina
Lodders, Ph.D., research associate professor of earth and planetary
sciences in Arts & Sciences at Washington University in St. Louis.

Chemistry for the very first time

"The circumstellar environment is where chemistry happens for the very
first time," said Lodders. "It's the first place a newly synthesized
element can do chemistry. It's a supermarket of things from dust to gas
and dust grains to molecules and atoms. The circumstellar shells enable
a chemistry that produced grains older than our sun itself. It's
generated some popular interest, and this year marks the 20th
anniversary of the presolar grain discoveries."

After the discovery of presolar diamonds in a meteorite in 1987 - the
first stardust found in a meteorite - researchers at Washington
University in St. Louis have been prominent in finding and analyzing
pre-solar grains made of silicon carbide, diamonds, corundum, spinel,
and silicates. The latest discovery - a silicate grain that formed
around a foreign star and became incorporated into a comet in our solar
system - was captured and returned by the STARDUST space mission in 2006.

Lodders said that nucleosynthesis - the creation of atoms - takes place
in a star's interior, made of a plasma far too hot for any molecular
chemistry to take place. The event that enables chemistry is the death
of a star, when elements are spewed out of the core, creating a shell
around the star. As this circumstellar shell cools, the elements react
to form gas molecules and solid compounds.

A star comes of age

Our sun and other dwarf stars of less than about ten solar masses burn
hydrogen into helium in their cores. As they come of age, they become
Red Giant stars and burn the helium to carbon and oxygen. But many heavy
elements such as strontium and barium, even heavier than iron, are also
produced, albeit in much smaller quantities than carbon. At the same
time, the star begins to eject its outer layers into the interstellar
medium by stellar winds, building up a circumstellar shell. So
eventually, most of a star's mass, including the newly produced
elements, is ejected into the interstellar medium through the
circumstellar shell. Most interstellar grains come from such stars.

Heavyweight stars go out more spectacularly, in violent supernovae such
as SN2006gy, first observed late last year, which has turned out to be
the most massive supernova ever witnessed. But no matter what, all stars
like the sun and heavier ones like SN2006gy empty their elements into
their circumstellar environments, where gaseous compounds and grains can
form. From there, the gas and grains enter the interstellar medium and
provide the material for new stars and solar systems to be born.

Lodders presented a paper on circumstellar chemistry and presolar grains
at the 233rd American Chemical Society National Meeting, held March
25-29 in Chicago, where a special symposium was held to track the
evolution of the elements across space and time. A book of proceedings
is being prepared for publication.

Lodders said that just one percent of all known presolar grains come
from supernovas. She said that several million stars have been
catalogued and several thousand individual presolar grains have now been
analyzed.

"Back in the 1960s, astronomers didn't know that presolar grains existed
in meteorites," Lodders said. "They were discovered when researchers
were looking at meteorite samples and studying noble gases. They asked
what is the mineral carrier of the noble gases."

By separating minerals from samples of meteorites, they eventually found
the carriers of the noble gases - presolar diamonds, graphite and
silicon carbide - and thus started the study of presolar grains 20 years
ago.

"So the genuine, micron-size star dust survived despite the potential
chemical and physical processing in the interstellar medium, during
solar system formation, and in the meteorite's parent asteroid," she
said. "Since the star dust preserved in meteorites must have been
already present before the solar system and the meteorites formed,
researchers call this star dust presolar grains."

"Laboratory astronomy of stardust has revealed much about stellar
element and isotope production, and about gas and dust formation
conditions in giant stars and supernovae."
Received on Tue 31 Jul 2007 08:22:30 PM PDT