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Binary asteroid population. 2. Anisotropic distribution of orbit poles of small,inner main-belt binaries
Authors:P Pravec  P Scheirich  D Vokrouhlický  AW Harris  P Kušnirák  K Hornoch  DP Pray  D Higgins  A Galád  J Világi  Š Gajdoš  L Kornoš  J Oey  M Husárik  WR Cooney  J Gross  D Terrell  R Durkee  F Marchis
Institution:1. Astronomical Institute, Academy of Sciences of the Czech Republic, Fri?ova 1, CZ-25165 Ond?ejov, Czech Republic;2. Institute of Astronomy, Charles University, Prague, V Hole?ovi?kách 2, CZ-18000 Prague 8, Czech Republic;3. 4603 Orange Knoll Avenue, La Cañada, CA 91011, USA;4. Carbuncle Hill Observatory, W. Brookfield, MA, 01585, USA;5. Hunters Hill Observatory, Ngunnawal, Canberra, Australia;6. Modra Observatory, Department of Astronomy, Physics of the Earth, and Meteorology, FMFI UK, Bratislava SK-84248, Slovakia;7. Leura Observatory, Leura, NSW, Australia;8. Astronomical Institute of the Slovak Academy of Sciences, SK-05960 Tatranská Lomnica, Slovakia;9. Sonoita Research Observatory, 77 Paint Trail, Sonoita, AZ 85637, USA;10. Department of Space Studies, Southwest Research Institute, Boulder, CO 80302, USA;11. Shed of Science Observatory, 5213 Washburn Ave. S, Minneapolis, MN 55410, USA;12. Physics and Astronomy Department, Appalachian State University, Boone, NC 28608, USA;13. Physics and Astronomy Department, University of North Carolina, Chapel Hill, NC 27514, USA;14. Institute of Astronomy of Kharkiv National University, Sumska Str. 35, Kharkiv 61022, Ukraine;15. Crimean Astrophysical Observatory, Simeiz Department, Simeiz 98680, Crimea, Ukraine;p. Center for Solar System Studies, 8300 Utica Ave., Suite 105, Rancho Cucamonga, CA 91730, USA;q. Badlands Observatory, 12 Ash Street, P.O. Box 37, Quinn, SD 57775, USA;r. Department of Space Studies, University of North Dakota, Grand Forks, ND 58202, USA;s. Instituto de Astronomia, Universidad Catolica del Norte, Avenida Angamos 0610, Antofagasta, Chile;t. Isaac Newton Group of Telescopes, La Palma, Apartado de Correos 321, E-38700 Santa Cruz de la Palma, Canary Islands, Spain;u. University of Warwick, Department of Physics, Coventry CV4 7AL, UK;v. University of Bucharest, Faculty of Physics, Str. Fizicienilor nr. 1, CP Mg – 11, Bucharest-Magurele 76900, Romania;w. Argelander Institute for Astronomy, Bonn University, Auf dem Huegel 71 D-53121, Germany;x. Palmer Divide Observatory, 17995 Bakers Farm Rd., Colorado Springs, CO 80908, USA;y. Campo Catino Observatory, I-03016 Guarcino, Italy;z. Via Capote Observatory, Thousand Oaks, CA 91320, USA;11. Obs. Ast. de Linhaceira, Escola Superior de Tecnologia de Tomar, Instituto Politecnico de Tomar, 2300-313 Tomar, Portugal;12. Elginfield Observatory, Department of Physics & Astronomy, University of Western Ontario, London, Ontario, Canada N6A 3K7;13. Andrushivka Astronomical Observatory, Observatorna str. 3-7, Galchyn, District of Andrushivka, 13432, Zhytomyr region, Ukraine;14. Keldysh Institute of Applied Mathematics, Russian Academy of Sciences, Miusskaya sq. 4, Moscow 125047, Russia;15. Carl Sagan Center at the SETI Institute, 189 Bernado Ave., Mountain View, CA 94043, USA;16. Department of Astronomy, University of California at Berkeley, Berkeley, CA 94720, USA
Abstract:Our photometric observations of 18 main-belt binary systems in more than one apparition revealed a strikingly high number of 15 having positively re-observed mutual events in the return apparitions. Our simulations of the survey showed that it cannot be due to an observational selection effect and that the data strongly suggest that poles of mutual orbits between components of binary asteroids in the primary size range 3–8 km are not distributed randomly: The null hypothesis of an isotropic distribution of the orbit poles is rejected at a confidence level greater than 99.99%. Binary orbit poles concentrate at high ecliptic latitudes, within 30° of the poles of the ecliptic. We propose that the binary orbit poles oriented preferentially up/down-right are due to either of the two processes: (i) the YORP tilt of spin axes of their parent bodies toward the asymptotic states near obliquities 0° and 180° (pre-formation mechanism) or (ii) the YORP tilt of spin axes of the primary components of already formed binary systems toward the asymptotic states near obliquities 0° and 180° (post-formation mechanism). The alternative process of elimination of binaries with poles closer to the ecliptic by dynamical instability, such as the Kozai effect due to gravitational perturbations from the Sun, does not explain the observed orbit pole concentration. This is because for close binary asteroid systems, the gravitational effects of primary’s irregular shape dominate the solar-tide effect.
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