The stellar occultation by Charon of 11 july 2005

A very rare phenomenon allowed astronomers to derive an accurate value for Charon's size: on 11 July 2005, Pluto's satellite occulted the 15th magnitude star UCAC2 2625713. The phenomenon was observed by several teams in South America, during a campaign organized by a team from Paris Observatory. This observation provides the satellite radius, 603.6 +/- 5 km, as well as its density, 1.71 +/- 0.08 g cm-3.

Predicting this event was a challenge, as Charon's diameter subtends an angle of 55 milli-arcsec (mas), the equivalent of a one Euro coin observed 100 km away! The smallness of this diameter explains the rarity of such events: only one occultation by Charon has been observed 25 years ago in South Africa - on 7 April 1980 - and from only one station, which did not permit to derive its radius.

Measurements made at Bordeaux Observatory and in Brazil for several months prior to the July 2005 event allowed astronomers to obtain an accuracy of about 20 mas on the prediction, showing that the occultation could be observable with several large telescopes in Chile, as well from smaller telescopes in Brazil, Paraguay, Uruguay and Argentina (Figure 1).

Figure 1: Track of Charon's shadow on 11 July 2005. The green symbols indicate sites where observations were planned. The blue circles show the sites where the occultation was detected. However, only the stations at San Pedro de Atacama, Paranal and El Leoncito provided data with sufficient signal to noise ratio to derive Charon's radius.

Luck was with astronomers this time, as Charon's shadow swept South America, including all the large telescopes involved for this event: VLT, Magellan, CTIO, Gemini, SOAR, all 4-m class, or larger, telescopes.

The Paris Observatory team could then gather data from Paranal, obtained with the NACO adaptive optics camera installed on the "Yepun" 8.2-m Very Large Telescope (Figure 2). Data were also obtained from El Leoncito in Argentina, with the 2.15-m "Jorge Sahade" telescope, and from San Pedro de Atacama (Chile), with the Italian 50-cm amateur "Campo Catino Austral Telescope".

Figure 2: Images of the Pluton-Charon couple before occultation. Is shown here a composite of 13x13 arcsec images taken with the adaptive optics camera NACO on one of the VLT's of the European Southern Observatory (Paranal site). The images were taken in K band (2.2 microns) from about three hours to about one hour before the occultation. All images were centered on Pluto and Charon, the double object visible near the top, with a separation of about 0.9 arcsec. Because of Pluto's motion, the occulted star, at the right of Pluto, appears as a dotted trail, as well as another star in the lower left corner.
Note the great quality of the image of the giant telescope, with the low atmospheric turbulence and the adaptive optics, which allows to clearly separate Pluton and Charon, although they are less than one arscec distant.

The accurate timing of this event from these three sites allowed the team to reconstruct the "occultation chords" with accuracies of a fraction of a second, equivalent to a few km at Charon (Figure 3). It was then possible to derive Charon's radius, R= 603.6 km with a formal error of +/- 1.4 km. This error bar must be increased to about +/- 5 km, to account for possible topographic features on Charon's surface, a for a possible oblateness of the satellite.

Figure 3: Reconstruction of the occultation chords from the occultations observed at the three sites of San Pedro de Atacama, Paranal (Chile) and El Leoncito (Argentina). A fit to the chord extremities provides Charon's radius, 603.6 km. The arrow indicates the satellite rotation.

This size provides Charon's density, 1.71 +/- 0.08 g cm-3, indicating a ice/rock mixture, with a rock fraction of typically 55-60%. Up to now, Charon's size was estimated to lie somewhere between 600 and 650 km, and its density, between 1.4 and 1.8 g/cm-3.

Charon's radius determination could have indirect consequences on Pluto's size (and then also density) and on Pluto's atmospheric structure. One can now re-analyze the "mutual events" (eclipses et occultations) of Pluto and Charon observed during the 1980's by fixing Charon's radius to its value now determined by the occultation, and the satellite's semi-major axis to its value derived from Hubble Space Telescope observations. There is then only one free parameter, Pluto's radius, which can be derived with much better accuracy. This radius can in turn improve the atmospheric models for the planet, by discriminating clear atmospheres with a strong inversion layer near the surface, from hazy atmospheres, with a possible convective layer.

This occultation finally provided an upper limit for a hypothetical Charon's atmosphere. For instance, we can give an upper limit of about 110 nanobar in pressure at the surface for an isothermal nitrogen atmosphere. This upper limit can be decreased to 15 nanobar for a methane atmosphere, a value one thousand times smaller than for Pluto's atmosphere (Figure 4).

Figure 4: Composite of the occultation light curves obtained at Paranal and El Leoncito. The time axis has be translated into distance to Charon's center. The light grey model show the effect that would be caused by an isothermal nitrogen atmosphere with surface pressure of 110 nanobars. The darker grey model shows the effect that a 15 nanobars methane atmosphere would cause on the light curve. These two models are examples of upper limits for possible atmospheres around Charon.

Pluto just entered in the sky region covered by the Milky Way, a promise for several more occultations till 2015. Meanwhile, in January 2006, the NASA "Pluto-Kuiper Belt Mission" should be launched for an expected arrival at Pluto in July 2015.

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. Ageorge, O. Hainaut, O. Marco, W. Beisker, E. Hummel, C. Feinstein,H. Levato, A. Maury, E. Frappa, B. Gaillard, M. Lavayssi╦re, 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, G. Tancredi:
"Charon's size and upper limit on the atmosphere from a stellar occultation" Nature, 5 January 2006

Bruno Sicardy (Observatoire de Paris, LESIA)
Thomas Widemann (Observatoire de Paris, LESIA)