THE 2005 OBSERVING CAMPAIGN OF THE ASTEROID 90 ANTIOPE

Updated Sep. 9, 2005
Introduction CCD Photometry Predictions Stellar Occultation Conclusion References Contacts

Introduction

OA Antiope image
The purpose of this work is to collect photometric lightcurves as much as possible of the double asteroid 90 Antiope. The large and increasing group of CCD equipped amateur astronomers can provide a valuable service to the study of this asteroid still considered as puzzling.

OA Antiope image
Large telescopes with deformable optics are allowing astronomers to study distant asteroids with unprecedented clarity -- leading to the discovery of new shapes and configurations and presenting scientists with new puzzles to solve. An international team of astronomers led by Dr. William Merline of the Boulder office of Southwest Research Institute (SwRI) released the first-ever images of a large, double asteroid. Each asteroid in the pair is the size of a large city (about 85 kilometers across), separated by about 160 kilometers, mutually orbiting the vacant point of interplanetary space that lies midway between them. The discovery was made using the W.M. Keck Observatory atop Mauna Kea, the tallest mountain in Hawaii. The asteroid pair was once assumed to be a single body, called Antiope, orbiting the sun in the outer parts of the asteroid belt between the orbits of Mars and Jupiter. This adaptive optics image of asteroid 90 Antiope was obtained in May 2005 at the Keck Observatory. The sizes of the two separate components of Antiope are not as large, relative to the 170Km distance between them as it appears. In a conventional telescopic photograph, the two objects would appear to be one big, blurry blob. The apparent separation is only 0.124 arcsec. The left image reveals the apparent aspect of the system at the same time. The system is nearly edge-on which provides rare opportunities to observe mutual events such as eclipses and occultations between components.

Size could be derived in principle by direct imaging. In practice, however, this is not a realistic option, due to the very small sizes of asteroids, requiring exceedingly large optical telescopes. In the case of Antiope this means that a big telescope (larger than 10 m) would be needed, working close to the theoretical diffraction limit and thus equipped with adaptive optics. The goal, as it will be better shown later, is to obtain essential information (mainly sizes and shapes) for Antiope.

CCD Photometry of Mutual events

Lightcurve of 90 Antiope
Photometric lightcurves are an abundant source of information on asteroids. Lightcurves provide information about the physical properties of asteroids (shape, rotational and scattering properties), knowledge of which is essential in understanding the history of the Solar system.

The first photometric observations of 90 Antiope, covering full rotational cycle, were performed on four nights in December 1996 (Hansen et al. 1997). The composite lightcurve with two minima and an amplitude of 0.70 mag yielded a period of 16.509 hours. This lightcurve was not quasi-sinusoidal as shown by most asteroids but similar to a typical lightcurve of an eclipsing binary star. In a binary system of two equal components a decrease in brightness, during central occultation, is 0.75 mag at each minimum. In December 1996 the observed amplitude of the lightcurve of Antiope was 0.70 mag, which suggests that the Earth was nearly exactly in the orbital plane of the Antiope system.

At the end of May 2005, we performed the first photometric observation of 90 Antiope whose the corresponding CCD lightcurve is displayed above. Two deep magnitude drops are clearly visible of about 0.7, evidences of eclipsing/occulting events between the components which proves that the system was nearly edge-on at this time. This observation was done at the South African Astronomical Observatory at Sutherland on the 1-m telescope with the R filter and an exposure time of 60 seconds.

Obtaining lightcurves over several months allows the determination of the shapes and sizes of each component. Accurate timing of observations is an absolute necessity. One way of doing this is to save images in FITS format and have the time, from the PC clock, automatically inserted in to the FITS header. However this tended to loose about 10 secs or more per image so, during an imaging session, became somewhat inaccurate. There are a number of other ways of obtaining accurate time signals in order to set the PC clock, radio controlled clocks, the internet and a Global Positioning System receiver. Note that the flat fields must be taken with the camera in exactly the same position as when imaging the asteroid. Suggested instrumentation: Short focal length telescope with an 8+ inch aperture. Good, commercial CCD. FOV should be large enough, at least 7 arcminutes, to include a sufficient number of comparison stars of similar brightness and, preferably, color. However, this is not always possible. Anything less than 5-6 arcminutes will probably make getting those stars very difficult. A field of at least 10 arcminutes is preferred and the larger the better - up to a point! Too large a field can result in poor image quality or serious vignetting that may be difficult to flat out. 90 Antiope is presently a 13th magnitude object. It may be observed with at least a 20cm diameter telescope. The mean duration of a typical event is on the order of 2.5h. You should get as many data points as possible, but without overkill. A 2 minutes exposure time should be enough to assure a good signal to noise ratio. As the amplitude of the magnitude drop is deep, e.g. 0.5-0.8 mag, you can afford a slightly noisier signal with a SNR of 50. A SNR of 100 translates to about 0.01m precision. You should pause no longer than 3 minutes, i.e., pause this amount of time from the end of one exposure to the start of the next. The more data points, the better any "noise" filters out.

The asteroid moves. On the other hand a complete coverage of an event will require several consecutive nights. If working differential photometry, this means you need to determine a common comparison star average magnitude that can be applied to all measurements. If you're not reducing to standard magnitudes, there are still ways to get the data from night to night to merge. Consider the data from each night (or common field if you are working a fast moving target) to be a "session." In each session, you use the same set of comparison stars to determine an average value for each observation that is used to determine the differential value. Taking the average of the averages gives you an arbitrary base line value, or "zero point". Once you have two or more sessions, you can try to merge the data by keeping one zero point constant and adjusting the zero point of the other sessions until the data from all sessions agrees. This is usually determined by matching the peaks of each session to one another. At this point, you have a close approximation of a common zero point for all observations. Each session will have its own offset to its zero point that brings it into line with the others.

Events prediction

From high resolution direct imaging on big telescopes since 2003, we have derived a full orbit solution of the motion of one component with respect to another. Such a dynamical solution allows us to predict mutual events inside the system. It appears that 2005 should give raise to a several months period of mutual eclipses able to be photometricaly observed from Earth-based facilities. The deepest events in magnitude drop will occur in may and november 2005. Moreover observing conditions of Antiope are particularly favorable in the Southern hemisphere where Antiope will be observable all the night.

Such photometric observations will bring us important and direct information as regard sizes and shapes of the components. Improvement of the orbit solution is expected as well.

List of events to be observed from may to december 2005 (new version). The timing accuracy of the events is of the order of a few minutes.

The 2 stellar occultations by 90 Antiope in September 2005

Another possible direct technique should be quoted, namely the observation of stellar occultations by asteroids. In particular, the duration of the decrease in received stellar flux due to occultation by the asteroid disk can be measured by observers located in the narrow region of the Earth in which the occultation actually occurs. The same occultation can be monitored by different observers, each measuring a different duration of the phenomenon due to the irregular shape of the occulting body. It produces different occultation durations as seen from different directions. In this way, it is possible to obtain a determination of the length of a number of single chords along the apparent disk of the occulting asteroid, leading to a rough shape determination. It is a viable tool for asteroid size determination. As a matter of fact, the computation of the region of observability of an occultation requires a very precise knowledge of both the orbital parameters of asteroid and of the astrometric position of the occulted star.

Occultation of UCAC 21638930 - September 15, 2005

Geometric configuration of the system

Visibility path of the occultation

Antiope at the time of the occutation Occ. path, Sept. 15, 2005 Occ. path, Sept. 15, 2005
Occ. path, Sept. 15, 2005 Occ. path, Sept. 15, 2005

The left figure shows the apparent positions of the components of the Antiope'system at the time of the events, Sept. 15, 2005 at 18h 30m UTC. The right figure shows the path of totality (i.e. the region on the Earth's surface where the occultation can be seen).

The predictions presented here are based on our last model of Antiope's system (updated end of august 2005). The ephemeris of the system are computed :

The shadow will "touch down" on the Earth at 18h 20m UTC. Few minutes later, the occultation becomes visible at sunset at a latitude of 34N, west Maroco. The asteroid shadow then moves eastward toward Algeria, Tunisia, Sicilia, Greece and Turkey. It will "lift off" into space 12 minutes later at 18h 33m UTC. On the map, the red lines represent the places of visibility of the occultation taking into account the diameter of one component (85km). The cyan dotted line represents the places of visibility of the occultation by the other component (diameter = 85km). The black dotted lines represent the north and south limit of visibility of the occultation taken into account an uncertainty of 0.15 arcsec in the ephemeris of the system.

Occultation of UCAC 21873650 - September 26, 2005

Geometric configuration of the system

Visibility path of the occultation

Antiope at the time of the occutation Occ. path, Sept. 26, 2005 Occ. path, Sept. 26, 2005
Occ. path, Sept. 26, 2005 Occ. path, Sept. 26, 2005

The left figure shows the apparent positions of the components of the Antiope'system at the time of the events, Sept. 26, 2005 at 20h 20m UTC. The right figure shows the path of totality (i.e. the region on the Earth's surface where the occultation can be seen).

The predictions presented here are based on our last model of Antiope's system (updated end of august 2005). The ephemeris of the system are computed :

The visibililty of the occultation begins off the coast in the Canada. The shadow will "touch down" on the Earth at 20h 17m UTC. Few minutes later, the occultation becomes visible at sunset at a latitude of 40N, north of Spain. The asteroid shadow then moves northward toward North France and South England, where it will "lift off" into space only 8 minutes later at 20h 25m UTC. On the map, the red lines represent the places of visibility of the occultation taking into account the diameter of one component (85km). The cyan dotted line represents the places of visibility of the occultation by the other component (diameter = 85km). The black dotted lines represent the north and south limit of visibility of the occultation taken into account an uncertainty of 0.15 arcsec in the ephemeris of the system.

Conclusion

It is not required that you have software to reduce the images, though much of the fun in this work is derived from measuring and producing results on your own. You can work with another observer who does have the necessary software to measure the images you take. Sometimes this is actually preferable in a collaboration with others since all data will be measured and reduced using the same software and procedures.

IMCCE will collect photometric observations in order to set up a data base dedicated to Antiope. They will be used to improve the orbital model and to constrain the physical characteristics of the system. All contributors will be associated for the public release of the observations and final results. Contact us for details.

References

Descamps P., Marchis F., Michalowski T., Berthier J., Hestroffer D., Vachier F., Colas F., Birlan M., Vieira Martins R., 2005 Insights on double asteroid 90 Antiope combining AO and lightcurves observations IAU Symposium no. 229, Asteroids, Comets and Meteors, Aug. 7-12, 2005
Kaasalainen, M., Torppa, J., & Piironen, J., 2002 Binary structures among large asteroids Astronomy and Astrophysics, vol.383, p.L19-L22 (2002)
Marchis,F., Descamps, P., D. Hestroffer, Berthier, J., I. de Pater, 2004 Fine Analysis of 121 Hermione, 45 Eugenia, and 90 Antiope Binary Asteroid Systems With AO Observations. 2004, 36th DPS Meeting, 8-12 November 2004.
Merline, W. J., Close, L. M., Shelton, J. C., Dumas, C., Menard, F., Chapman, C. R., Slater, D. C., 2000. IAU Circ., 7503, 3 (2000). Edited by Green, D. W. E. Satellites of Minor Planets
Merline, W. J., Close, L. M., Dumas, C., Shelton, J. C., Menard, F., Chapman, C. R., Slater, D. C. 2000 Discovery of Companions to Asteroids 762 Pulcova and 90 Antiope by Direct Imaging American Astronomical Society, DPS meeting #32, #13.06
Michalowski, T., Colas, F., Kwiatkowski, T., Kryszczy?ska, A., Velichko, F. P., Fauvaud, S. 2002 Eclipsing events in the binary system of the asteroid 90 Antiope. Astronomy and Astrophysics, vol.396, 293-299
Michalowski, T., Kwiatkowski, T., Kryszczynska, A., Colas, F., & Michalowski, J. 2001 IAU Circ., 7757. (90) Antiope
Michalowski, T., Colas, F., Kwiatkowski, T., Kryszczynska, A., Hirsch, R., Michalowski, J. 2001 CCD photometry of the binary asteroid 90 Antiope.Astronomy and Astrophysics, vol.378, p.L14-L16.
Michalowski, T., Bartszak, P., Velichko, F.P., Kryszczynska, A., Kwiatkowski, T., Breiter, S., Colas, F., Fauvaud, S., Marciniak, A., Michalowski, J., Hirsch, R., Behrend, R., Bernasconi, L., Rinner, C., and Charbonnet, S., 2004 Eclipsing binary asteroid 90 Antiope. Astronomy and Astrophysics, vol.423,1159-1168.
Weidenschilling, S. J., Marzari, F., Davis, D. R., & Neese, C. 2001 Origin of the Double Asteroid 90 Antiope: A Continuing Puzzle. 32nd Annual Lunar and Planetary Science Conference, March 12 -16, 2001, Houston, Texas, abstract no.1890.

Contacts:

P. Descamps, IMCCE, Paris Observatory, France
J. Berthier, IMCCE, Paris Observatory, France
F. Marchis, Univ. of Berkeley, California, USA
T. Michalowski, Astronomical Observatory, Poznan, Poland
F. Colas, IMCCE, Paris Observatory, France

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