Astronomers Seize Rare Opportunity to Measure Distant Charon (Forwarded)

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Astronomers Seize Rare Opportunity to Measure Distant Charon

Being in the right place at the right time gave a group of Massachusetts
research astronomers a unique opportunity to study Pluto's largest moon
Charon. The resulting measurements, to unprecedented accuracy, of
Charon's size and possible atmosphere provide insight into the way this
distant world may have formed.

On July 10, 2005, astronomers from the Massachusetts Institute of
Technology (MIT) in Cambridge and Williams College in Williamstown
observed the light from a star as it disappeared behind Charon and
reappeared on the other side -- an event known as a stellar occultation.
Occultations provide important information about the size of remote
bodies, as well as the makeup of their atmospheres (if they have them).

According to team member Jim Elliot, a professor in MIT's Department of
Earth, Atmospheric, and Planetary Science and in the Department of
Physics, observations of a stellar occultation like this one have been
made only once before, from South Africa in 1980. "We have been waiting
many years for this opportunity," he said. "Watching the star vanish as
it was blocked by Charon was spectacular."

Although the star disappeared behind Charon for less than a minute, data
from the observations provided considerable information about this tiny
moon. In a paper released in the January 5, 2006, edition of Nature, the
MIT-Williams collaboration determined Charon's radius to be 606 +/- 8
km. For perspective, this radius is roughly twice the width of
Massachusetts with an error of only 5 miles. The size was combined with
mass measurements from Hubble Space Telescope data to establish a
density for Charon of 1.72 g/cm3. This density, roughly 1/3 that of the
Earth, reflects Charon's rocky-icy composition.

What makes this achievement so remarkable is that the observation could
only be made from a narrow, 650-mile wide region in South America. The
MIT-Williams observers were located at four telescopes in Chile and one
telescope in Brazil for the event.

The largest telescope employed by the consortium was the 8-meter Gemini
South Telescope on Cerro Pachón. The observations utilized the
Acquisition Camera, a guider instrument that is typically used for
telescope pointing and target selection, as a high-speed photometer.
Portable camera systems constructed by the MIT-Williams group were
mounted on the other telescopes: the 6.5-meter Clay and 2.5-meter du
Pont at Las Campanas Observatory in La Serena, Chile, the 0.8-meter at
the Observatório Cerro Armazones in Anofagasta, Chile, and the 0.6-meter
at Observatório Pico dos Dias, Itajubá, Brazil. Observations were
successful at all stations excluding Brazil, which was clouded out.

Jay Pasachoff, Professor of Astronomy at Williams College and a
collaborator in the effort, praised the team doing the work. "It's
astounding that our group could be in the right place at the right time
to line up a tiny body three billion miles away," he said. "The
successful observations are quite a reward for all of the people who
helped predict the event, constructed and integrated the equipment, and
traveled to the telescopes."

Observations taken at a high rate, 10 frames per second, from the
6.5-meter Clay telescope (which was built by a collaboration of
institutions including MIT) detected subtle optical effects caused when
the starlight passed the edge of Charon's disk. By analyzing these
effects, known as diffraction fringes, the MIT-Williams team concluded
that any atmosphere on Charon is less than one millionth the density of
Earth's atmosphere. Their analysis provided very strict limits on the
amounts of various gases that could be present. Three years earlier, the
team previously used the technique of stellar occultation to study
Pluto's thin atmosphere, showing that it was subject to slight global
warming.

The results of the observations argue against the theory that Pluto and
Charon were formed by the cooling and condensing of the gas and dust
known as the solar nebula. Instead, astronomers think that Charon was
formed in a collision between two objects early in the formation of the
solar system.

"Our observations show that there is no substantial atmosphere on
Charon, which is consistent with an impact formation scenario," said
Nature lead author Amanda Gulbis. "We also find that Charon contains
roughly 10% less rock by mass than Pluto. This difference suggests that
either, or both, objects involved in a Charon-forming collision had
concentrations of heavier materials in their cores." A collisional
formation like this has a parallel in theories for the formation of the
Earth-Moon system.

Pluto has recently received considerable attention, with NASA's New
Horizons mission to be launched in January 2006, the discovery of two
new moons, and the discovery of several Kuiper belt objects that are
Pluto-sized (or even larger). The success of the MIT-Williams team in
observing the Charon occultation bodes well for their ability to observe
occultations of different stars by these newly discovered objects.

The so-called "10th planet" (2003 UB313), recently discovered by
scientists from Caltech, is a prime candidate for stellar occultation
observations. Although this object is approximately twice as far away
from the Earth as Charon, it is thought to be twice as large. 2003 UB313
thus covers the same angular extent in the sky as Charon, just as the
Moon and the Sun appear to be the same size although the Sun is
physically larger.

"We are eager to use the occultation technique to probe for atmospheres
around large Kuiper belt objects," remarked Jim Elliot, who has been
observing stellar occultations by bodies in the solar system for more
than three decades.

Members of the MIT team were Jim Elliot, Amanda Gulbis, Michael Person,
Elisabeth Adams, and Susan Kern, with support from undergraduate Emily
Kramer. The Williams College team included Jay Pasachoff, Bryce Babcock,
Steven Souza and undergraduate Joseph Gangestad.

The article describing this research is "Charon's Radius and Atmospheric
Constraints from Observations of A Stellar Occultation," by A.A.S.
Gulbis, J.L. Elliot, M.J. Person, E.R. Adams, B.A. Babcock, M. Emilio,
J.W. Gangestad, S.D. Kern, E.A. Kramer, D.J. Osip, J.M. Pasachoff, S.P.
Souza, and T. Tuvikene. It will appear in the January 5, 2006, edition
of Nature.

A team led by French astronomer Bruno Sicardy and a team led by American
astronomer Leslie Young also observed the occultation from telescopes in
South America. American astronomer David Tholen discussed the
significance of the various results in a "News and Views" introductory
article in the same issue of Nature as the MIT-Williams and Sicardy-team
articles.

The MIT-Williams electronic equipment and expeditions are supported by
grants from NASA.

Is there any way to learn about the star by observing an occultation
like this? It's radius, for instance? Or it's corona?

Thanks,
Greg

In message .com,
writes
Is there any way to learn about the star by observing an occultation
like this? It's radius, for instance? Or it's corona?


Occultations by the moon certainly are (or were) used to determine the
size of giant stars, but I'd guess that an ordinary star is effectively
a point source, and its corona is too faint to observe (looking it up, I
found that the corona is about one millionth as bright as the sun, so a
first-magnitude star has a corona of magnitude 16).
Some stars have been shown to be very close doubles by observing
occultations.