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Ultra-Slow Rotating Outer Main Belt and Trojan Asteroids: Search for Binaries

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Ultra-Slow Rotating Outer Main Belt and Trojan Asteroids: Search for Binaries 45th Lunar and Planetary Science Conference (2014) 1703.pdf ULTRA-SLOW ROTATING OUTER MAIN BELT AND TROJAN ASTEROIDS: SEARCH FOR BINARIES. K.S. Noll1, S.D. Benecchi2, E.L. Ryan3, and W.M. Grundy4, 1NASA Goddard Space Flight Center, Code 693.0, 8800 Greenbelt Rd., Greenbelt, MD 20771, [email protected], 2Planetary Science Institute, 3NASA Postdoctoral Program Fellow 4Lowell Observatory. Introduction: Binaries are already known to exist the binary fraction of this population subset. However, in the Outer Main Belt, Hilda, and Trojan (OMB+) when combined with other published searches of Tro- populations and appear to fall into two main catego- jans [7], regardless of rotation period, it appears that ries. The first are objects like 624 Hektor, 121 Hermi- the binary fraction in the OMB+ population is lower one, 107 Camilla, and 87 Sylvia which contain elon- than among similar sized objects in the Kuiper Belt. gated/bilobed primaries with small satellites, a rapidly Colors and Classification: Spectral classes have rotating primary, and densities greater than 1000 kg/m3 been estimated from for four of the eight targets from [1-4]. On the other hand, 617 Patroclus, 17365, and Sloan colors [8], most are SMASSII X class as ex- 29314 have have similar-sized components, are syn- pected. Using HST B, V, and I colors we will be able chronously locked (or in contact), and have densities to assign approximate classifications to the remaining below 1000 kg/m3[5-6]. These two classes of binaries objects in our sample. may reflect different formation mechanisms (collision References: [1] Marchis, F. et al. (2006) IAUC vs. primordial), differences in the origin of the plane- 8732; Marchis, F. et al. (2012) ACM 2012, #1667. [2] tesimals and/or their subsequent dynamical and colli- Merline, W. et al. (2002) IAUC 7980; Marchis, F. et sional evolution. As a test, we have conducted a search al., (2005) Icarus 178, 450-464; Descamps, P. et al. for other possible binary systems in the OMB+. (2009) Icarus 203, 88-101. [3] Storrs, A. et al. (2001) Observations: Observations were obtained with IAUC 7599. [4] Marchis, F. et al. (2013) DPS 45, the Wide Field Camera 3 on the Hubble Space Tele- 112.07. [5] Merline, W. et al. (2001) IAUC 7741; scope (HST) using four filters, the F336W, F438W, Marchis, F., et al. (2006) Nature 439, 565-567. [6] F555W and F814W (approximating U, B, V, and I Mann, R. K. et al. (2007) AJ 134, 1133-1144. [7] bands). Four exposures were obtained for each filter in Merline, W. et al. (2007) DPS 39, 60.09. [8] Carvano, a standard sub-pixel box dither pattern centered on the J. M. et al. (2010) A&A 510, A43. UVIS2-C1K1C subarray. Binary Search Results: We targeted eight objects Acknowledgements: Observations made with the with periods of 56-540 hours that, if tidally locked Hubble Space Telescope as part of program 12891. binaries with the orbital period equal to the observed Support for program 12891 was provided by NASA lightcurve period (like Patroclus), would have separa- through a grant from the Space Telescope Science In- tions resolvable using HST. Observations were com- stitute, which is operated by the Association of Univer- pleted in June 2013 and none of the objects show clear sities for Research in Astronomy, Inc., under NASA evidence of being binary. (PSF-fitting is required to contract NAS 5-26555. rule out partially resolved systems.) A negative result for eight slow rotators places only weak constraints on Object H Trot a e I D1 D2/D1 ρ (mag) (h) (AU) (°) (°) (km) (kg/m3) 121 Hermione 7.31 5.55 3.45 0.143 7.58 230x120 0.10 1400 107 Camilla 7.08 4.84 3.48 0.079 10.04 240x180 0.04 1880 87 Sylvia 6.94 5.18 3.49 0.080 10.87 380x232 0.06 1300 624 Hektor 7.5 6.92 5.23 0.024 18.19 363x207 0.05 2400 617 Patroclus 8.2 102.8 5.23 0.138 22.03 122 0.92 800-1000 29314 1994 CR18 11.1 15.0 5.25 0.073 15.26 32 0.75 590 17365 1978 VF11 10.3 12.7 5.27 0.078 11.65 92 0.84 780 Table 1. Known Outer Main Belt and Trojan binaries [1-6] fall into two distinct classes that may reflect differ- ing formation mechanisms. 45th Lunar and Planetary Science Conference (2014) 1703.pdf Object H Trot a e I D q (mag) (h) (AU) (°) (°) (km) (arcsec) 1208 Troilus 9.0 56.2 5.243 0.093 33.56 103 0.14 1512 Oulu 9.6 132. 3.944 0.151 6.50 83 0.19 5511 Cloanthus 9.6 336. 5.234 0.118 11.18 77 0.23 1042 Amazone 9.8 540. 3.236 0.093 20.68 74 0.54 5025 1986 TS6 9.8 250. 5.203 0.076 11.02 58 0.16 7119 Hiera 9.8 400. 5.168 0.103 19.30 76 0.29 16974 1998 WR21 9.8 78.9 5.204 0.069 15.02 55 0.07 13331 1998 SU52 11.2 375 5.078 0.110 2.29 30 0.11 Table 2. Targeted Trojan, Hilda and OMB asteroids with Trot > 56 hours are listed here. D is the effective diam- eter for a single object determined from measured albedos (or estimated using an assumed albedo of 5%). The max- imum angular separation of a hypothetical binary, q, is computed assuming a tidally evolved equal mass binary with ρ = 900 kg/m3. Actual separation would depend on orbital phase and orientation of the orbit plane. The lack of de- tection of any companions in the search suggests they are not prevalent among slow rotators. .
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