Forming Binary Near-Earth Asteroids From Tidal Disruptions

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What is the overall steady-state binary fraction for NEAs caused by tidal disruption?1–2%. The steady-state simulations demonstrated that the average lifetime of a tidaldisruption-formed binary was too short (1.2 Myr) to have a higher binary fraction. <strong>Tidal</strong>evolution had a significant impact on the simulations, damping eccentricities rapidly,making most of the population of binaries have values below 0.1. Planetary encountersremoved wide binaries quite rapidly, resulting in a distribution that closely resemblesthat of the observed population, with separations near 5 R pri and below. Overall, tidaldisruption can only account for a 1–2% binary fraction among NEAs.5.1 Future WorkThe questions raised by this work, to be answered by future work include,1. What are the properties of binaries formed by the YORP thermal spin-up mechanism?2. Can the addition of YORP as a formation mechanism explain all of the observedNEA binaries?3. How closely do sub-kilometer MBAs resemble NEAs, in both spin and shape, butalso binary fraction and properties?Many of the simulation techniques used in this work will be directly applicable tothese new questions. N-body simulations are an ideal way to simulate a YORP spinup.The steady-state model that was constructed to model the binary fraction causedby tidal disruption can be easily modified to include YORP or another binary formationmechanism. First steps towards a YORP spin-up model have been taken utilizing a rubblepile model of a strengthless asteroid being slowly spun-up. This is implemented via very109
small additions to the angular momentum of the body and spaced out to allow the bodyto reshape or settle after every boost.Preliminary results from YORP simulations for an initial tri-axial ellipsoid that isnearly spherical with a 4 h rotation, show that the first mass is lost after serious shapedeformations, which occur beyond the limit for cohesionless proloids with a friction angleof 40 ◦ (see Fig. 5.1, Holsapple 2001). This limit has been shown to govern thebreakup of numerically modeled rubble piles, whereas the traditional Jacobi sequence ofellipsoids governs the reaccumulation of a rotationally-disrupted body (Richardson et al.2005; Holsapple 2001). This simulation was run until 20% of the initial mass was lostfrom the primary body. At the end of the simulation all mass bound to the central bodywas in single particles, none had accumulated into larger companions.Though this simulation did not provide a stable large companion from a gradual spinup,it does establish the framework for a more realistic set of simulations. Unlike tidaldisruptions, which are by nature substantially more impulsive, the internal makeup ofeach body will have a large impact on the evolution of its shape and spin. As well,due to a significantly smaller parameter space to explore (initial shape and spin, spin-uprate, resolution, and particle size distribution), more exotic and computationally timeconsumingbody structures can be explored. Experiments with particle size distributionsare needed, as are tests of rubble piles consisting of particle aggregates. Introducing anumerical “strength” to the model may permit super-critical spin, allowing for a moresubstantial disruption and a larger release of mass per breakup. These advances may benecessary to quantify how efficient the YORP effect is as a binary formation mechanism.Lastly, a new observing campaign utilizing larger telescopes will be needed to moreclosely examine sub-kilometer MBAs. Currently the bulk of NEA lightcurves are measuredby amateurs who are directed and organized by scientists. The relative proximityof NEAs allows for their study with modest, commercially-available telescopes.Re-110
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AbstractTitle of Dissertation:Formi
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Forming Binary Near-Earth Asteroids
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PrefaceMuch of the work in this dis
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AcknowledgementsI would not have ma
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2.3 Results and Discussion . . . .
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List of Tables1.1 Binary NEA proper
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4.8 Percentage of migrated binaries
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of the system mass M by way of Kepl
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the largest lightcurve amplitudes o
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1.1.3 Binary Main-Belt asteroidsRec
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Lightcurve discoveriesA lightcurve
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Figure 1.1: (Top) The primary rotat
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1.2.1 CaptureIn this scenario two a
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ever, understanding the mechanism i
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off the equator or off one end of a
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3 Denotes a secure result with no a
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nostic.Recent results have demonstr
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another intermediate source of NEAs
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1.5.2 Rubble pilesThe evidence for
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Chapter 2Formation of Binary Astero
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∼ 60%, making a bulk density of
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shedding mass and distorting prior
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Figure 2.3: Snapshots of a tidal di
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Figure 2.4: Normalized probability
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Figure 2.5: Normalized probability
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Figure 2.6: (a) Satellite eccentric
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various complications of measuring
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etween ω pri and L bin ), with clo
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prolate-like), the oblate-like bodi
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Figure 2.9: Plots comparing T-PROS
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Figure 2.10: Comparison of S-class
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2.3.5 Triples and hierarchical syst
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a/R pri based on a simple conversio
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Figure 2.13: The eccentricity dampi
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Main Belt. Work by Chauvineau & Far
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Page 81 and 82: Table 3.2.Orbital, physical and lig
Page 83 and 84: Figure 3.2: Asteroid lightcurves.70
Page 85 and 86: Figure 3.4: Asteroid lightcurves.72
Page 87 and 88: 3.4.4 Spin and shape properties amo
Page 89 and 90: Figure 3.7: The lightcurve amplitud
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Page 93 and 94: Chapter 4Steady-State Model of the
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Page 101 and 102: 4.1.3 Binary evolutionBasic stabili
Page 103 and 104: Table 4.1.Lifetimes for binary NEAs
Page 105 and 106: which means it should have a critic
Page 107 and 108: Figure 4.4: The total number of bin
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Page 111 and 112: Figure 4.6: Properties of the binar
Page 113 and 114: Figure 4.8: Binary percentage for m
Page 115 and 116: Figure 4.9: Effects of tidal evolut
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Page 119 and 120: tidal disruption. However, the impl
Page 121: Chapter 5ConclusionsA general concl
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Page 129 and 130: Table A.1.Number of binaries produc
Page 131 and 132: Thus the reaccumulated bodies in th
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Page 135 and 136: Benner, L. A. M., Nolan, M. C., Ost
Page 137 and 138: V7.0:LCREF TAB, 35, 4Harris, A. W.,
Page 139 and 140: Storrs, A. D., Close, L. M., & Mena
Page 141 and 142: 122Pravec, P., Kušnirá, P., Hicks
Page 143 and 144: Shepard, M. K., Schlieder, J., Nola
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Forming Binary Near-Earth Asteroids From Tidal Disruptions

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