Measuring the Size of a Small, Frost World [Charon] (Forwarded)

ESO Education and Public Relations Dept.

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Contacts

Bruno Sicardy
Paris Observatory, France
Email: Bruno.Sicardy @ obspm.fr

Olivier Hainaut
ESO, Chile
Phone: +56 55 43 5336 or +56 2 463 3118
Email: ohainaut @ eso.org

Under Embargo till January 4, 2006, 19:00 CET

ESO 02/06 - Science Release

Measuring the Size of a Small, Frost World

Stellar occultation allows VLT to determine Charon's size and
to put upper limit on its atmosphere

Observing a very rare occultation of a star by Pluto's satellite
Charon from three different sites, including Paranal, home of
the VLT, astronomers were able to determine with great accuracy
the radius and density of the satellite to the farthest planet.
The density, 1.71 that of water, is indicative of an icy body
with about slightly more than half of rocks. The observations
also put strong constraints on the existence of an atmosphere
around Charon.

ESO PR Photo 02a/06
Artist's Impression of the Pluto-Charon system

Since its discovery in 1978, Charon and Pluto have appeared
to form a double planet, rather than a planet-satellite
couple. Actually, Charon is about twice as small as Pluto in
size, and about eight times less massive. However, there have
been considerable discussions concerning the precise radii of
Pluto and Charon, as well as about the presence of a tenuous
atmosphere around Charon.

In August 2004, Australian amateur astronomer Dave Herald
predicted that the 15-magnitude star UCAC2 26257135 should be
occulted by Charon on 11 July 2005. The occultation would be
observable from some parts of South America, including Cerro
Paranal, in the northern Atacama Desert, the location of
ESO's Very Large Telescope (VLT).

Stellar occultations have proved to be powerful tools to both
measure sizes -- at km-level accuracy, i.e. a factor ten better
than what is feasible with other techniques -- and detect very
tenuous atmosphere -- at microbar levels or less. Unfortunately,
in the case of Charon, such occultations are extremely rare,
owing to the very small angular diameter of the satellite on
the sky: 55 milliarcsec, i.e. the size of a one Euro coin
observed from 100 km away!

This explains why only one occultation by Charon was ever
observed before 2005, namely on 7 April 1980 by Alistair
Walker, from the South Africa Astronomical Observatory.

Similarly, only in 1985, 1988 and 2002 could astronomers
observe stellar occultations by Pluto. Quite surprisingly,
the 2002 event showed that Pluto's atmospheric pressure had
increased by a factor of two in four years (ESO PHOT 21/02).

"Several factors, however, have boosted our odds for witnessing
occultations of Charon," said Bruno Sicardy, from Paris
Observatory (France) and lead author of the paper reporting the
results. "First, larger telescopes now give access to fainter
stars, thus multiplying the candidates for occultations.
Secondly, stellar catalogues have become much more precise,
allowing us to do better predictions. And, finally, the Pluto-
Charon system is presently crossing the Milky Way, thereby
increasing the likelihood of an occultation."

ESO PR Photo 02b/06 ESO PR Photo 02c/06
The Pluto-Charon System Charon's Occultation on July 11,
(NACO/VLT) 2005

The July 2005 event was eventually observed from Paranal with
Yepun, the fourth Unit Telescope of the VLT, equipped with
the adaptive optics instrument NACO, as well as with the 0.5m
"Campo Catino Austral Telescope" at San Pedro de Atacama
(Chile), and with the 2.15m "Jorge Sahade" telescope at Cerro
El Leoncito (Argentina).

An accurate timing of the occultation seen at the three sites
provides the most accurate measurement of Charon's size: its
radius is found to be 603.6 km, with an error of the order of
5 km.

This accuracy now allows astronomers to pin Charon's density
down to 1.71 that of water, indicative of an icy body with
about slightly more than half of rocks. Quite remarkably,
Charon's density is now measured with much more precision
than Pluto's.

Thanks to these observations, Sicardy and his collaborators
could determine that if an tenuous atmosphere exists on
Charon, linking it to the freezing -220 degrees centigrade
or so surface, its pressure has to be less than one tenth of
a millionth that at the surface of the Earth, or 0.1 microbar,
assuming that it is constituted entirely of nitrogen.

A similar upper limit is derived for a gas like carbon
monoxide. This is more than a factor one hundred smaller
than Pluto's surface pressure, which is estimated to be in
the range 10-15 microbars.

"Comparing Pluto and Charon, we seem to cross a borderline
between bodies which may have bound atmospheres -- like
Pluto -- and airless bodies like Charon", said Olivier
Hainaut, from ESO and member of the team.

The observations also indicate that methane ice, if present,
should be restricted to very cold regions of the surface.
Similarly, nitrogen ice would be confined at best to high
northern latitudes or permanently shadowed regions of Charon.

As Pluto and its satellite sweep across the Milky Way,
observations of more occultations will be tempted from the
ground, while the NASA's Pluto-Kuiper Belt Mission, to be
launched in January 2006, will be travelling towards the
planet, that it should reach in July 2015.

A report of these results is to be published in the January 5,
2006 issue of Nature ("Charon's size and upper limit on its
atmosphere from a stellar occultation", by B. Sicardy, A.
Bellucci, E. Gendron, F. Lacombe, S. Lacour, J. Lecacheux, E.
Lellouch, S. Renner, S. Pau, F. Roques, T. Widemann, F. Colas,
F. Vachier, N. Ageorges, O. Hainaut, O. Marco, W. Beisker, E.
Hummel, C. Feinstein, H. Levato, A. Maury, E. Frappa, B.
Gaillard, M. Lavayssière, M. Di Sora, F. Mallia, G. Masi,
R. Behrend, F. Carrier, O. Mousis, P. Rousselot, A. Alvarez-
Candal, D. Lazzaro, C. Veiga, A.H. Andrei, M. Assafin, D.N. da
Silva Neto, R. Vieira Martins, C. Jacques, E. Pimentel, D.
Weaver, J.-F Lecampion, F. Doncel, T. Momiyama, and G.
Tancredi).

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