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EMBARGOED UNTIL 11:00 A.M. EST, May 27, 2003

Hot Gas Around Cold Dust Cloud Surprises Astronomers

New features may make southern sky's "Coalsack" ideal for further
study

Stargazers call a prominent dark black region in the Southern 
Hemisphere's night sky the Coalsack. Even for naked-eye observers,
the cloud of cold gas that makes up the Coalsack is hard to miss: it
covers a part of the misty luminescence of the Milky Way, blocking
out the band of distant stars in the disk of our galaxy with deep
black shades that earned the Coalsack its name.

However, a newly discovered aspect of the Coalsack may soon have 
astronomers thinking of it more like a treasure chest. At an American 
Astronomical Society Meeting in Nashville this week, astronomers will 
reveal evidence that the Coalsack has hot gases on its perimeter, a 
finding that means the Coalsack will likely provide many outstanding 
opportunities to learn more about interactions between regions of hot 
and cold gas, processes that are essential to star formation and 
distribution of the elements that make up life forms and the planets. 
Findings from the Coalsack may also help scientists refine their
models of energy production in the galaxy.

"Every once in a while nature gives us a break and sets things up so 
that we can study the key processes fairly easily," says B-G
Andersson, associate research scientist in the Krieger School of Arts
and Sciences at Johns Hopkins and lead author of the AAS
presentation. "For astronomers, this is a bit like finding a living
dodo -- instead of trying to make inferences about how the dodo
walks, which is what we normally have to do, we can get direct,
detailed observations of it walking."

The Coalsack is relatively nearby in cosmological terms, about 650
light years away in the same spiral arm of the Milky Way Galaxy as
Earth. Because it is close by, structures in the Coalsack can be
studied in great detail. It's also nicely backlit from Earth's
point-of-view by bright stars in the next arm of the Milky Way
Galaxy, allowing scientists to use spectroscopy to acquire a fairly
comprehensive sense of the chemical ingredients of the cloud.

The hot gas on the perimeter may indicate that the Coalsack is
contained within a region of active massive star formation and
supernovae known as the Upper Centaurus-Lupus super-bubble. This
region has produced large hot stars that burn out quickly and die
explosively, sometimes heating interstellar gas to high temperatures.
Naked-eye observers can still see the darkness of the cold gas in the
Coalsack because most of the surrounding hot gas is too warm to emit
light in the visible portion of the electromagnetic spectrum.

Andersson and coauthors David Knauth and Robin Shelton of Johns
Hopkins, S.L. Snowden of Goddard Space Flight Center and Peter
Wannier of the Jet Propulsion Laboratory built the case for their
finding with data taken by the orbiting Far Ultraviolet Spectroscopic
Explorer (FUSE) and Roentgen Satellite (ROSAT) observatories. They
had been using FUSE to study how cold gas clouds dissipate at their
edges when their observations of the Coalsack came back with signs of
oxygen VI , oxygen atoms with five of their eight surrounding
electrons stripped away. 

Astronomers know that considerable energy is required to knock that
many electrons off an oxygen atom, and as a result have long
interpreted the presence of oxygen VI as a sign that very hot gases
are entering a cooling process. How this cooling process occurs --
through turbulent mixing of the gas with colder gas clouds or through
conduction of electrons -- is a topic of debate among astronomers
that further study of the Coalsack may help resolve.

Tipped off by the FUSE readings, Andersson and his colleagues
analyzed X-ray data from ROSAT, which surveyed X-ray emissions from
the entire sky, and found that the perimeter of the Coalsack,
particularly its southeast edge, lights up in X-rays -- another sign
of the hot gas cloud. Through further analysis, they were able to
show that the two readings appeared to be coming from the same region
around the Coalsack. 

"If our model of the Coalsack is right, then you can use it to test 
various theories of oxygen VI generation, and this may help us better 
understand what are the mechanisms behind previously detected oxygen
VI production in many more distant parts of the galaxy," Andersson
comments. "This could tell us something about energy production in
the galaxy, and that could in turn tell us more about star
formation." 

Andersson noted that astronomers have been studying the Coalsack
since the early 19th century, and plenty of good data on many
features of the cloud are already available.

"This is starting to look like a really good laboratory for
conducting these kinds of experiments," he said. "There are certainly
other cases where oxygen VI has likely been associated with a cloud,
but never before have we had such a nearby cloud with a reliable
distance determination or as many background stars behind the cloud
to allow us to look at absorption readings."

This research was funded by NASA.

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