1. Occultation of C313.2 by Charon J.M. Pasachoff, J.W. Gangestad, S.P. Souza, and B.A. Babcock (Williams College); J.L. Elliot, A.A.S. Gulbis, M.J. Person (MIT); T. Tuvikene (Vrije Universiteit, Brussels); M. Emilio (Universidade Estadual de Ponta Grossa, Brazil) Stellar Occultations A stellar occultation occurs when the light from a star is blocked by an intervening body (such as a planet, moon, ring or asteroid). We can use the light of the occulted star to probe the atmosphere of a body in the outer Solar System, as well as determine the body’s physical size from the duration of the occultation. As has been observed with Pluto (Fig. 1), if a planet has an atmosphere the light from the occulted star will not drop instantly, but slope down to a minimum and then rise up again in a similar manner at the end of the occultation. Refraction is the primary cause for the dip in intensity - extinction becomes significant only at low altitudes (where the atmosphere becomes adiabatic), as is evident in Fig. 1 where the slope becomes steeper about halfway through the event. By measuring the slope of the light curve during immersion and emersion, and comparing them to input quantities of a model, an occultation can provide a wealth of information about a planet’s atmosphere: density, temperature, composition, and more. The MIT-Williams group had been working on occultations of Pluto for several years when attention shifted in 2004 to a predicted occultation in July 2005 by Charon of C313.2, an otherwise insignificant star in the US Naval Observatory catalog. Williams Light Curve Bryce Babcock and Joseph Gangestad from Williams College and Michael Person from MIT successfully observed the occultation of C313.2 at Cerro Armazones. Jay Pasachoff and Steve Souza at Pico dos Dias were unfortunately clouded out. Data at Cerro Armazones were taken at a cadence of 2 Hz with POETS (Portable Occultation, Eclipse and Transit System) and after reduction were made into a light curve (Fig. 3). Although conditions at the telescope were not optimal (resulting in low signal-to-noise), the occultation was unquestionably observed. Figure 3. The MIT-Williams light curve. Creation of the light curve involved the integration of Pluto, Charon and C313.2 together, resulting in only a 25% decrease in total flux during the occultation. Astrometric Predictions Predicting an occultation is an extremely sensitive matter. Due to the great distance of the Pluto-Charon system from the Earth (about 6 billion kilometers), the shadow of Charon which is cast on the Earth during an occultation is extremely narrow. To accurately predict where this shadow will fall, the positions of both C313.2 and Charon had to be known to within milliarcseconds, a level of precision uncommonly high for most astronomical applications. Jim Elliot’s team at MIT has developed a suite of software with Mathematica which employs differential astrometry to track the motion of Pluto-Charon in relation to the target star and predicts where on Earth the occultation will be visible. For C313.2, the predictions pointed to a path over South America (Fig. 2), a part of the world already sprinkled with telescopes both large and small. Three main groups were working on this occultation: a French group led by Bruno Sicardy, a group from the Southwest Research Institute led by Leslie Young, and the MIT-Williams group, led by Jim Elliot at MIT and Jay Pasachoff at Williams College. The Williams team split into two - Jay Pasachoff and Steve Souza traveled to the Pico dos Dias Observatory in Brazil, and Bryce Babcock and Joseph Gangestad observed from the Cerro Armazones Observatory in the Atacama Desert near Antofagasta, Chile. A stellar occultation, the obstruction of the light of a star by an intervening body, is the most accurate method of measuring the size of planets, comets, and other solid bodies in the outer Solar System and elsewhere, and can provide a wealth of other information, such as parameters of the occulting body’s atmosphere. On 11 July 2005 (UT) an occultation by Charon of the star C313.2 was observed by several teams from MIT and Williams College. One team from Williams successfully observed the occultation from the Cerro Armazones observatory in Chile, while the other Williams team in Brazil was clouded out. Data from Cerro Armazones and from other telescopes manned by MIT teams have allowed us to improve the value of Charon’s radius, as well as better constrain values for the moon’s density and rock-ice fraction; upper limits on any Charonian atmosphere have been determined. The methods employed in these analyses can be applied to recently-discovered Kuiper Belt Objects, details of which are sketchy so soon since their discovery. Introduction Pluto and its moon Charon are the only planets (aside from recently discovered Kuiper Belt Objects) not yet visited by spacecraft from Earth, so everything we know about this system comes from Earth-bound telescopes. Pluto-Charon are about 6 billion kilometers from Earth, but separated by only nineteen thousand, so resolving their half-arcsecond separation on Earth requires the largest telescopes available, and even then only the grossest surface features can be resolved. Answering deeper questions, such as whether these bodies have atmospheres, requires an alternative method, namely an occultation. Figure 1. Stellar occultation data and model ( from Elliot & Young 1992). The stellar-occultation light curve observed with the Kuiper Airborne Observatory has been plotted as points and the best-fitting model as a line. The residuals from the model fit are the points in the lower part of the figure. The few points with the largest residuals occur where the light curve has the largest slope, and they represent unmodeled density variations in Pluto’s atmosphere. Figure 2. Track of Charon’s shadow across South America. Ideally, observations are made at several chords (several lines of latitude) to improve the accuracy of astrometric calculations later. Analysis Unlike with Pluto’s light curve (see Fig. 1) one sees no slope associated with this star’s waning light. The light curve shows the light of C313.2 cut off in only one frame - a time of 0.5 seconds - and there is only one intermediate frame during emersion. A similar light curve was observed with one of the Magellan telescopes at Cerro Tololo, where observations were made at a 10 Hz cadence. This does not mean that Charon has no atmosphere, only that it is below our detectable threshold. Analysis of the light curves from all the MIT-Williams teams has determined a radius for Charon significantly improved from previous estimates, as well as better constraints on the moon’s density and rock-ice fraction and on the bulk density of the Pluto-Charon system. These results are consistent with a collisional formation scenario for the Pluto-Charon system during which either of the objects may have been differentiated. Upper limits on any possible Charonian atmosphere have been determined for a variety of likely gases, although these limits are miniscule compared even to Pluto (quote Nature paper in preparation?). Future Work All analyses performed thus far have assumed a spherical Charon, but further analysis of the light curves may reveal possible asymmetry in the shape of Charon. The new values for the moon’s rock-ice fraction and the system’s bulk density will allow a reevaluation of models which describe the creation of this system and other bodies in the outer Solar System. A better-constrained value for Charon’s radius will prompt a revisiting of mutual-occultation data of Pluto- Charon from the 1980s, which will allow us to put new constraints on the physical parameters of Pluto. The recent discovery of many large bodies in the Kuiper Belt, some potentially larger than Pluto, provides an exciting opportunity both to test POETS and to determine atmospheric and physical characteristics of these new planets, details of which are sketchy so soon since their discovery. References Elliot, J.L., & Young, L.A. 1992, AJ, 103, 991 Gulbis, A.A.S., Elliot, J.L., et al. in preparation Acknowledgments We thank N. Vogt and M Murphy of Universidad Católica del Norte for their assistance in our use of Cerro Armazones. Support for this work is provided by NASA Planetary Astronomy.