[meteorite-list] Star stuff

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Thu, 14 May 2009 18:38:49 -0500
Message-ID: <sqap05l5g70v59efslb1h7v978d7o91o59_at_4ax.com>

http://www.sciencedaily.com/releases/2009/05/090513234218.htm

Spitzer Catches Star Cooking Up Comet Crystals

ScienceDaily (May 14, 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 .
Received on Thu 14 May 2009 07:38:49 PM PDT