[meteorite-list] Spitzer Catches Star Cooking Up Comet Crystals

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 13 May 2009 17:42:47 -0700 (PDT)
Message-ID: <200905140042.RAA01428_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2009-083

Spitzer Catches Star Cooking Up Comet Crystals
Jet Propulsion Laboatory
May 13, 2009

Scientists have long wondered how tiny silicate crystals, which need
sizzling high temperatures to form, have found their way into frozen
comets, born in the deep freeze of the solar system's outer edges. The
crystals would have begun as non-crystallized silicate particles, part
of the mix of gas and dust from which the solar system developed.

A team of astronomers believes they have found a new explanation for
both where and how these crystals may have been created, by using NASA's
Spitzer Space Telescope to observe the growing pains of a young,
sun-like star. Their study results, which appear in the May 14 issue of
Nature, provide new insight into the formation of planets and comets.

The researchers from Germany, Hungary and the Netherlands found that
silicate appears to have been transformed into crystalline form by an
outburst from a star. They detected the infrared signature of silicate
crystals on the disk of dust and gas surrounding the star EX Lupi during
one of its frequent flare-ups, or outbursts, seen by Spitzer in April
2008. These crystals were not present in Spitzer's previous observations
of the star's disk during one of its quiet periods.

"We believe that we have observed, for the first time, ongoing crystal
formation," said one of the paper's authors, Attila Juhasz of the
Max-Planck Institute for Astronomy in Heidelberg, Germany. "We think
that the crystals were formed by thermal annealing of small particles on
the surface layer of the star's inner disk by heat from the outburst.
This is a completely new scenario about how this material could be
created."

Annealing is a process in which a material is heated to a certain
temperature at which some of its bonds break and then re-form, changing
the material's physical properties. It is one way that silicate dust can
be transformed into crystalline form.

Scientists previously had considered two different possible scenarios in
which annealing could create the silicate crystals found in comets and
young stars' disks. In one scenario, long exposure to heat from an
infant star might anneal some of the silicate dust inside the disk's
center. In the other, shock waves induced by a large body within the
disk might heat dust grains suddenly to the right temperature to
crystallize them, after which the crystals would cool nearly as quickly.

What Juhasz and his colleagues found at EX Lupi didn't fit either of the
earlier theories. "We concluded that this is a third way in which
silicate crystals may be formed with annealing, one not considered
before," said the paper's lead author, Peter Abraham of the Hungarian
Academy of Sciences' Konkoly Observatory, Budapest, Hungary.

EX Lupi is a young star, possibly similar to our sun four or five
billion years ago. Every few years, it experiences outbursts, or
eruptions, that astronomers think are the result of the star gathering
up mass that has accumulated in its surrounding disk. These flare-ups
vary in intensity, with really big eruptions occurring every 50 years or
so.

The researchers observed EX Lupi with Spitzer's infrared spectrograph in
April 2008. Although the star was beginning to fade from the peak of a
major outburst detected in January, it was still 30 times brighter than
when it was quiet. When they compared this new view of the erupting star
with Spitzer measurements made in 2005 before the eruption began, they
found significant changes.

In 2005, the silicate on the surface of the star's disk appeared to be
in the form of amorphous grains of dust. In 2008, the spectrum showed
the presence of crystalline silicate on top of amorphous dust. The
crystals appear to be forsterite, a material often found in comets and
in protoplanetary disks. The crystals also appear hot, evidence that
they were created in a high-temperature process, but not by shock
heating. If that were the case, they would already be cool.

"At outburst, EX Lupi became about 100 times more luminous," said
Juhasz. "Crystals formed in the surface layer of the disk but just at
the distance from the star where the temperature was high enough to
anneal the silicate--about 1,000 Kelvin (1,340 degrees Fahrenheit)--but
still lower than 1,500 Kelvin (2,240 degrees Fahrenheit). Above that,
the dust grains will evaporate." The radius of this crystal formation
zone, the researchers note, is comparable to that of the
terrestrial-planet region in the solar system.

"These observations show, for the first time, the actual production of
crystalline silicates like those found in comets and meteorites in our
own solar system," said Spitzer Project Scientist Michael Werner of
NASA's Jet Propulsion Laboratory, Pasadena, Calif. "So what we see in
comets today may have been produced by repeated bursts of energy when
the sun was young."

JPL manages the Spitzer Space Telescope mission for NASA's Science
Mission Directorate, Washington. Science operations are conducted at the
Spitzer Science Center at the California Institute of Technology in
Pasadena. Caltech manages JPL for NASA.

More information about Spitzer is at
http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer .

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
Whitney.clavin at jpl.nasa.gov
Received on Wed 13 May 2009 08:42:47 PM PDT