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)
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
Astrometric Predictions
Williams Light Curve
Future Work
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.
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.
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.
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 PlutoCharon 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.
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.
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.
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.
References
Elliot, J.L., & Young, L.A. 1992, AJ, 103, 991
Gulbis, A.A.S., Elliot, J.L., et al. in preparation
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.
Analysis
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.
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?).