A team of scientists led by Dr Franck Marchis from the Carl Sagan Center of the SETI Institute teamed up with amateur astronomers to discover that the main-belt asteroid Sylvia has a complex interior thanks to the presence of two its moons – Romulus and Remus.
This is an artist’s impression of the triple asteroid system Sylvia. The two moons, Romulus and Remus, are shown to be strongly elongated, and composed of two lobes. Image credit: Danielle Futselaar / SETI Institute.
Discovered in 1866 by the English astronomer Sir Norman Robert Pogson, Sylvia is one of the largest main-belt asteroids.
With a diameter of 286 km and two small moons, Romulus and Remus, Sylvia orbits around the Sun between Mars and Jupiter. The Romulus was discovered in 2001, and the 7-km-wide Remus was discovered in 2004.
Dr Marchis’ team analyzed 66 adaptive optics observations of this triple system from 8-10m class telescopes including those at the W. M. Keck Observatory, the European Southern Observatory, and Gemini North.
“Because Sylvia is a large, bright asteroid located in the main belt, it is a great target for the first generation of adaptive optics systems available on these large telescopes. We have combined data from our team with archival data to get a good understanding of the orbits of these moons,” said Dr Marchis, who reported the results at the 45th annual Division of Planetary Sciences meeting in Denver, Colorado.
The astronomers also developed a dynamical model of the system, allowing them to predict the position of the moons around the asteroid at any time.
The drop test of this work was the prediction of the relative positions of the moons during an occultation on Jan. 6, 2013. Observers equipped with small telescopes located on a narrow path across the south of France, Italy and Greece could see Sylvia passing in front of a bright 11-mag star. Observers at different locations see different parts of the asteroid, or its moons, passing in front of the star. Such occultations allow exquisitely precise measurements of the relative positions and sizes of the occulting objects.
The team successfully motivated more than 50 observers to watch the event. Twelve of them detected the occultation by the primary of the system which lasted between 4 and 10 seconds depending on the observer’s position on Earth.
The differentiated interior of Sylvia is shown through a cutaway diagram. The asteroid may have a dense, regularly-shaped core, surrounding by a fluffy or fractured material. Image credit: Danielle Futselaar / SETI Institute.
“Additionally, four observers detected a two-second eclipse of the star caused by Romulus, the outermost moon, at a relative position close to our prediction. This result confirmed the accuracy of our model and provided a rare opportunity to directly measure the size and shape of the moon,” said co-author Dr Jérôme Berthier from the Observatoire de Paris.
The chords of this occultation observations revealed that Romulus is 24 km in diameter with an extremely elongated shape, possibly made of two lobes joined together like a dumbbell. This is not surprising if the moon formed from the accretion of fragments created by the disruption of a proto-Sylvia by an impact, several billion years ago.
The team derived the shape of the primary asteroid Sylvia by combining data from the occultation of the asteroid with other sources of information.
These included archived recordings of the variation of light caused by the spin of the asteroid, and direct imaging by adaptive optics systems. Because the moons’ orbits do not seem to be affected by the irregular shape of the asteroid, the team concluded that the large asteroid is most likely differentiated. The asteroid likely has a spherical core of dense material, surrounded by a fluffy or fractured outer surface layer.
“Combined observations from small and large telescopes provide a unique opportunity to understand the nature of this complex and enigmatic triple asteroid system,” Dr Marchis concluded.
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Bibliographic information: Franck Marchis et al. New insights on the main-belt triple asteroid (87) Sylvia. AAS/Division for Planetary Sciences Meeting Abstracts, 45, paper #112.07
