Bi-lobated Shape of 121 Hermione
F. Marchis, C. Laver (University of California at Berkeley), J. Berthier,
P. Descamps, D. Hestroffer (Institut de Mécanique céleste et de Calcul des Ephémérides)
and I. de Pater (UC-Berkeley)
Abstract
Using the Keck-10m AO system and its NIR camera NIRC-2, we observed 121 Hermione on Decembee 6.521 and December 7.528 UT in K band (2.2 microns). The images indicate that the main-belt asteroid has a bi-lopated shape. Two models are proposed, (i) a peanut shape of 230 km (0.126") long and two lobes of R=60 and 50 km separated by 120 km, and perhaps linked by a bridge of matter og 80 km wide, (ii) a snowman shape with two connected components of R=90 and 60 km separated by a center-to-center distance of 115 km. The Angular resolutions provided on these images cannot discern between these models but our images clearly rule out a simple ellipsoid shape.
Comparison with physical ephemeris indicates the pole solution (LONG=342.0,LAT=+30.0 deg. ecliptic B1950 ) of de Angelis et al. Planet. Sp. Sci., 43, 1995 and confirms a rotation rate of 5.551h as measured recently by R. Behrend (Genova Observatory) on the base of new photometric observations made by R. Roy (Blauvac, France) and Ph. Baudoin (Le Havre, France). Finally, note that Hermione is also known to have a moonlet companion orbiting at 790 km around the primary (cf. IAU 7980)". An occultation involving this extremely interesting target will occur on Feb. 16, 2004 UT and will be visible mainly from Europe.
This web-page describes the AO data and give brief information for the Occultation.
This result was published in the IAU circular #8264.
Observations were performed with the Keck-10m telescope, its AO system and the NIRC-2 infrared camera. Data were recorded in K band with the 9.96mas/pixel camera under excellent seeing conditions (estimated to 0.6" in K band). Ephemeris indicate that at the date of observations, the visible magnitude of the asteroid 121 Hermione was 12.7, its distance to Earth 2.567079 AU, its elongation 218 degrees. Visible wavefront analysis were performed using the asteroid itself. A PSF star (a star with the same magnitude and located nearby the target) was also taken each night.
On each night, our data show that the asteroid presents a bi-lobated shape. More accurate analyses were performed applying MISTRAL (Mugnier et al. 2003) deconvolution process. The sharpness of the data was dramatically increased confirming a "peanut" or "snowman" shape. The angular resolution on the data is 0.054 arcsec, corresponding to a spatial resolution of 90 km.
Images
First observation performed on December 6, 2003. North is up, East is left. Angular resolution is 54 mas (~90 km)
Second observation performed on December 7, 2003.
121 Hermione's size was estimated to be 209 km as derived from the IRAS satellite observations (Tedesco et al., 2002). Our resolved data can also be used to derive the size and the shape of the main-belt asteroid. From the analysis of these images, we can compare the computed physical ephemerides to the observations, and find which published spin-pole orientation better matches the observed orientation and apparent shape of Hermione. This enables us to determine the direction of rotation of Hermione and make some estimates of the spin-axis orientation, without the usual ambiguity that cannot be removed from some photometric lightcurves analysis methods alone. Among the different solutions (See http://www.astro.amu.edu.pl/Science/Asteroids/) for the spin direction of Hermione given to about +/-10 degrees accuracy, the pole coordinates (in ecliptic B1950) 342degrees,+30degree (de Angelis et al, 1995) provide the best overall matching between the observed and the computed values of the b/a ratio and position angle of the apparent ellipse. Note that our previous observations in January 2003 (Marchis et al., 2003, submitted to Icarus) with the VLT when Hermione was located at 2.81 AU from Earth (spatial resolution = 120 km) also indicate this pole solution with a computed ellipsoidal model a:b:c = 128.2: 107.1: 77 km.
The bi-lobated shape of Hermione can be simulated through several models due to the lack of angular resolution. It can be regarded as a "peanut" shaped asteroid, 230 km (0.126") long with two lobes of radius 60 and 50 km separated by 120 km and, perhaps, linked by a bridge of matter of 80 km wide, or as two connected components of radius 90 and 60 km separated by a center-to-center distance of 115 km.
The following figures display Hermione at the date of the observations with the three models. The single ellipsoid one can be ruled out from these Keck observations.
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Single ellipsoid |
Snowman shape |
Peanut shape without bridge of matter |
It is obvious that there is a slight shift between the two dates of observations and the models. This discrepancy may be due to the uncertainty on the pole orientation, or more likely on a shorter spinning period. In Marchis et al. (2003), we adopted a rotation period of 9.24 hr based on the literature. This value is not very accurate. In fact, recent observations performed by R. Roy (Blauvac, France) and Ph. Baudoin (Le Havre, France) and analyzed by R. Behrend (Genova Observatory) confirms a rotation rate of 5.551h (see http://obswww.unige.ch/~behrend/page1cou.html for details)
The moonlet companion was discovered in September 2002 using the Keck AO system by Merline et al. (IAU, 7980, 2002). It is visible on this image taken 2 hours later with a larger integration time. The Primary does not display a bi-lobated shape because of its rotation.
Individual Frames
The bi-lobated shape of Hermione is visible on each individual frames.
On December 6, we observed the asteroid with an individual integration time of 10s (1s x 1 coadd). The elongated shape is visible on each frame.
On December 7, 2003 the individual integration time on each frame is 60s (10s x 6 coadds).
Other Main-belt asteroids
Additional asteroids were observed on the same nights with the same instrument. Most of them were resolved and we could clearly identify their shapes which are not bi-lobated like Hermione. Here a few examples applying a classical deconvolution method based on the Lucy-Richardson algorithm. Note the presence of Gibbs artifact on the edge of some of them (Europa for instance).
94 Aurora (mv=12.6)
(left basic processed data, Lucy-Richardson deconvolved data)
130 Elektra (mv=11.2)
52 Europa (mv=11.1)
45 Eugenia (mv=12.3)
2 frames taken 1 hour away.
Animations
Using the Movis software, which generates an artificial image of an asteroid based on the model of its shape, its pole solution, its size and its position in the solar system, P. Descamps created this movie showing the rotation of Hermione assuming a snowman shape between the two dates of observations (based on the previously measured 9.5h rotation rate). Additional movies will be available soon.
click on the image to start the mpeg movie
In Conclusion
121 Hermione appeared clearly bi-lobated on these AO data. Improvement in the quality of the data delivered by the Keck AO system was decisive for the detection of Hermione shape. Additional observations must be taken in the following months (closest opposition is on January 4, 2004) to better constrain the shape and the pole solution.
The structure of the primary and the presence of a moonlet companion orbiting at 790 km around the primary must be linked. One can imagine that the fragment is a secondary effect coming from the slow collision of two separated bodies which were orbiting around their center of mass. The fragment was captured into the gravitational field of the bi-lobated system formed. Accurate analysis will be performed to better constrain the origin of this interesting three-body system.
On February 16, 2004 at 22h39min UT, Hermione will occult a 9.2 magnitude star (TYCHO-2 1905 00864). The figures below display the path of the occultation on Earth. The occultation will be mainly visible in Europe. The bolt red line is the visibility path considering the size of the asteroide (~230 km). Additional information can be found on Asteroidal Occultations 2004 of J/ Manek (http://mpocc.astro.cz/2004/).
J. Lecacheux analyzed our entire set of data considering new observations taken with VLT/NACO in January 2004. He derived the path of occulation of the moonlet (published in IAU circular #8285). Details can be found on the following web page: http://www.euraster.net/maps/hermione/satellite/.
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click on the images to magnify |
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References
de Angelis, G., Asteroid spin, pole and shape determinations, Planet. Space Sci 43, 649-682, 1995.
Marchis, F., D. Hestroffer, P. Descamps, J. Berthier, I. de Pater, On the Mass and density of asteroid 121 Hermione from the analysis of its companion orbit, submitted to Icarus, 2003.
Tedesco, E.F., P.V. Noah and S.D. Price, The Supplemental IRAS Minor Planet Survey, AJ, 123, 1056-1085, 2002.
Contact
Franck Marchis (fmarchis@astron.berkeley.edu)
Office number: +1 510 642 3958
(I will be in my office starting on the 11th of January 2004)
Acknowledgments
Data presented herein were obtained at the W.M. Keck Observatory,which is operated
as a scientific partnership among the California
Institute of Technology, the University of California and the National Aeronautics
and Space Administration. The Observatory was made
possible by the generous financial support of the W.M. Keck Foundation. This
work has been supported by the National Science
Foundation Science and Technology Center for Adaptive Optics, managed by the
University of California at Santa Cruz under cooperative
agreement No. AST - 9876783. The authors wish to extend special thanks to those
of Hawaiian ancestry on whose sacred
mountain we are privileged to be guests. Without their generous hospitality,
none of the observations of these asteroids would
have been possible.
Last Modified: January 10th 2004