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Colors of Dynamically Associated Asteroid Pairs Icarus 221 (2012) 63–71 Contents lists available at SciVerse ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Colors of dynamically associated asteroid pairs Nicholas A. Moskovitz Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, Washington, DC 20008, USA article info abstract Article history: Recent dynamical studies have identified pairs of asteroids that reside in nearly identical heliocentric Received 5 April 2012 orbits. Possible formation scenarios for these systems include dissociation of binary asteroids, collisional Revised 12 July 2012 disruption of a single parent body, or spin-up and rotational fission of a rubble-pile. Aside from detailed Accepted 12 July 2012 dynamical analyses and measurement of rotational light curves, little work has been done to investigate Available online 25 July 2012 the colors or spectra of these unusual objects. A photometric and spectroscopic survey was conducted to determine the reflectance properties of asteroid pairs. New observations were obtained for a total of 34 Keywords: individual asteroids. Additional photometric measurements were retrieved from the Sloan Digital Sky Asteroids Survey Moving Object Catalog. Colors or spectra for a total of 42 pair components are presented here. Photometry Spectroscopy The main findings of this work are: (1) the components in the observed pair systems have the same colors within the uncertainties of this survey, and (2) the color distribution of asteroid pairs appears indistin- guishable from that of all Main Belt asteroids. These findings support a scenario of pair formation from a common progenitor and suggest that pair formation is likely a compositionally independent process. In agreement with previous studies, this is most consistent with an origin via binary disruption and/or rotational fission. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction the parent would fission into a proto-binary system and eventually disrupt under its own internal dynamics to form an unbound aster- Analyses of osculating orbital elements of Main Belt asteroids oid pair (Jacobson and Scheeres, 2011). The estimated size ratios have revealed over 80 pairs of asteroids that reside in nearly iden- and observed rotational properties of known pair systems are con- tical heliocentric orbits (Vokrouhlicky´ and Nesvorny´ , 2008; Pravec sistent with a formation scenario via rotational fission (Pravec, and Vokrouhlicky´ , 2009; Rozek_ et al., 2011). These objects are dis- 2010). Progressive mass shedding due to YORP spin-up and accre- tinct from binary asteroids as they are not on bound orbits around tion of a dynamically unstable proto-satellite offers a similar path- a common center of mass, and it is unlikely that their proximity is way to pair formation (Walsh et al., 2008). The close spectral due to random fluctuations of asteroid densities in orbital element similarity between components in one pair system (Duddy et al., space. Backwards integration of these pairs’ heliocentric orbits sug- 2012) and the photometric similarity between components in an- gests they may have separated recently into an unbound state, in other (Willman et al., 2010) support these scenarios. some cases much less than a Myr ago (Vokrouhlicky´ and Nesvorny´ , Though a fission origin is consistent with the size ratios and 2009; Vokrouhlicky´ , 2009, 2011; Pravec, 2010; Duddy et al., 2012). rotation properties for a large number of pairs (Pravec, 2010), col- As such these are interesting objects for studying phenomena, such lisions provide another possible formation mechanism that may be as space weathering and radiation pressure forces, that are rele- relevant to a subset of systems. In this scenario a catastrophic col- vant to the ongoing dynamical, physical and chemical evolution lision would produce a distribution of fragments, of which only the of Main Belt asteroids. largest two are observed as an associated pair. Collisions between The components of known pairs are typically a few km in size small bodies can result in compositionally complex outcomes (e.g. and consist of a primary and a secondary (respectively defined as Leinhardt and Stewart, 2009), but it is unclear how collisional for- the larger and smaller components based on measured absolute mation would affect the relative colors and/or spectra of the km- magnitudes). One formation scenario for these systems (Scheeres, scale objects found in pair systems. Aside from any spectroscopic 2007; Pravec, 2010) involves parent asteroids that were spun up or photometric implications, hydrodynamic simulations of impact to a critical frequency by the YORP effect, i.e. a change in angular events make predictions about the resulting orbital properties and momentum due to anisotropic emission of thermal photons size ratios of collisional fragments (Nesvorny´ et al., 2006; Durda (Rubincam, 2000; Bottke et al., 2006). At this critical frequency et al., 2007). Unfortunately, due to incompleteness for sub-km bodies in the Main Belt, it is currently not possible to fully test E-mail address: [email protected] these predictions (Vokrouhlicky´ and Nesvorny´ , 2008). 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.07.011 64 N.A. Moskovitz / Icarus 221 (2012) 63–71 A final formation mechanism involves the dynamical dissocia- Table 1 tion of bound binary systems. Perhaps the best evidence for ongo- Asteroid pairs observed in this study. ing pair formation via binary disruption comes from the system of Object UT date D (AU) R (AU) a (°) asteroids associated with 3749 Balam. Adaptive optics and light (2110) Moore-Sitterly 2010-02-28 1.661 2.513 14.2 curve observations (Merline et al., 2002; Marchis et al., 2008a,b; (15107) Toepperwein 2010-02-28 1.766 2.587 14.9 Polishook et al., 2011) have shown that Balam has two bound sat- (17288) 2000 NZ10 2010-03-08 2.071 2.488 22.9 ellites, making it a rare triple system. One of its two satellites is on (17288) 2000 NZ10 2010-08-31 1.992 2.144 27.9 (21930) 1999 VP61 2010-02-27 3.670 4.636 3.1 a highly eccentric orbit (e 0.9), while the other orbits at a dis- (22647) Levi-Strauss 2010-03-07 3.775 4.761 1.5 tance of only 20 km (Balam itself is about 5–10 km in size). In addi- (32957) 1996 HX20 2010-08-31 2.044 2.330 25.6 tion, this triple system has a dynamically associated pair, Asteroid (38395) 1999 RR193 2010-08-29 3.443 3.722 15.6 (312497) 2009 BR60. Backwards numerical integrations show that (51609) 2001 HZ32 2011-03-05 1.669 2.654 3.2 the orbits of Balam and 312497 converge within the past 0.5 Myr (54041) 2000 GQ113 2010-02-28 1.774 2.611 14.1 (69142) 2003 FL115 2010-08-30 1.903 2.041 29.4 (Vokrouhlicky´ , 2009). Numerical models suggest that a cascade (70511) 1999 TL103 2010-08-29 1.404 2.373 8.9 of fragments, like that seen in the Balam system, can result from (84203) 2002 RD133 2010-08-29 1.069 1.850 26.3 repeated rotational fission events (Scheeres and Sánchez, 2011). (92652) 2000 QX36 2010-03-08 2.000 2.465 22.8 It seems in this case that YORP fission and binary dissociation (99052) 2001 ET15 2010-03-08 1.666 2.548 12.7 (117306) 2004 VF21 2010-03-08 1.952 2.509 21.4 may be closely related processes. (127502) 2002 TP59 2012-02-14 0.928 1.888 10.1 However, it is unclear how the reflectance properties of asteroid (139537) 2001 QE25 2010-08-31 1.751 2.527 17.7 pairs would reflect an origin due to the dissociation of binaries. (141513) 2002 EZ93 2010-08-30 3.656 3.732 15.7 Component-resolved spectra have been obtained for a very small (143662) 2003 SP84 2010-02-26 1.081 1.950 18.8 number of bound binary systems (e.g. Polishook et al., 2009; Mar- (143662) 2003 SP84 2010-02-27 1.085 1.951 19.0 (189994) 2004 GH33 2010-03-07 2.059 2.390 24.3 chis et al., 2011; DeMeo, 2011). This is in part due to the need for (194083) 2001 SP159 2010-03-06 1.322 2.280 8.4 either adaptive optics or space based observations to resolve the (195479) 2002 GX130 2010-03-08 1.550 2.516 6.7 individual components. Available spectra suggest that the compo- (195479) 2002 GX130 2010-02-28 1.587 2.512 10.2 nents of binaries are compositionally similar, however data is (203489) 2002 AL80 2010-03-07 1.905 2.608 18.1 scarce and more information is required before generalized state- (216177) 2006 TE23 2010-08-22 1.473 2.393 12.9 (220143) 2002 TO134 2011-03-04 1.747 2.624 12.4 ments can be made. (229991) 2000 AH207 2010-03-01 1.133 2.088 9.8 In light of these various formation mechanisms, there remain (237517) 2000 SP31 2010-03-06 1.798 2.709 10.3 several open issues regarding the formation and evolution of aster- (279865) 2001 HU24 2010-03-06 1.755 2.529 16.9 oid pairs. As indicated above, the relationship between pairs and (284765) 2008 WK70 2010-03-06 1.739 2.547 15.7 (291188) 2006 AL54 2010-03-01 1.396 2.256 16.0 bound multi-component systems is unclear. In addition, little is (291788) 2006 KM53 2010-03-07 1.628 2.581 7.6 known about the compositions/taxonomic types of pairs. Different (303284) 2004 RJ294 2010-03-08 1.334 2.086 22.4 formation mechanisms could affect the relative reflectance proper- 2008 TS51 2010-03-07 1.797 2.699 10.8 ties of pair components in different ways.
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