Forming Binary Near-Earth Asteroids From Tidal Disruptions

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4.2 Steady-State Results and DiscussionWe first present the nominal case, which includes the best estimate for each of the manyvariable parameters included in the steady-state model. In subsequent sections we examinethe individual effects of each of the main model parameters, and their overall effecton the results. Our nominal case has the following properties:1. Progenitors follow the shape and spin distributions discussed above (section 4.1),matching estimates for SMBAs,2. Tidal evolution actively changes binary properties, with parameters from Chapter2,3. 10% of Main Belt asteroids entering the simulation are binaries, with propertiesfrom Durda et al. (2004) (excluding unit-mass-ratio systems) with 100 Myr of tidalevolution,4. Binary encounters with Earth are handled via a look-up table of 3-body encounters.4.2.1 Nominal caseThe nominal case is a 1 Gyr simulation using 2000 asteroids. Figure 4.5 shows the evolutionof binary number over time for the simulation, showing the quick decline in thenumber of remaining MBA binaries. This rapid decline is due to the comparatively largesemi-major axes of the binary MBAs and their very short lifetimes against disruption. Thesteady-state number of MBA binaries in the population hovers close to 0.2%. The fractionof binaries formed via tidal disruption is 1.2% and has comparable small fluctuations innumbers throughout the simulation.The properties of the binaries show strong effects of tidal evolution. There is a verystrong peak in eccentricity between 0.0–0.1, which shows significant tidal damping of95
the original eccentricity distribution and matches the observed population well (Fig. 4.6).The distribution of semi-major axes is mildly dependent on the formation mechanism,tidal disruption, or collisional remnant in the Main Belt. The tidal disruption remnantshave semi-major axes almost entirely below 10 R pri , where the binaries from the MainBelt have a number of bodies with larger semi-major axis.The low steady-state percentage of both NEA and MBA binaries is due to short lifetimesagainst disruption during planetary encounters. The average lifetime before a NEAbinary is disrupted is ∼ 1.2 Myr, while for an MBA formed binary that time is ∼ 0.3 Myr(Fig. 4.7). While increasing the percentage of MBA binaries will increase the number migratinginto the population (see Sec. 4.2.2), the properties of the binaries will determinetheir lifetime, and the sustainable binary fraction. Hence, for the distribution of binaryproperties used in this nominal case for both the tidal disruption and MBA migrated binaries,the steady-state fraction is dominated by short binary lifetimes against disruption.4.2.2 Influence of MBA binary percentageTests were run varying the binary percentage of MBA progenitors between 10 (the nominalcase), 20, 50, and 80%. The contribution to the NEA binary population from migratedMBA binaries is quite low for the binary properties used, below 1.2% for all four testsvalues run (Fig. 4.8).The binaries that migrate in from the MBA population are disrupted via a close encounterwith Earth quite quickly. The average lifetime for a migrated binary is ∼ 0.3Myr. The properties of the binary MBAs will drastically affect their lifetime, and alsotheir overall contribution to the steady-state population (see Fig. 4.7). The propertiesfor the binary MBAs in this work from Durda et al. (2004) represent the results of themost recent asteroid collision simulations. A different formation mechanism for producingsmaller-separation binary MBAs could theoretically provide a different set of binary96
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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
Page 57 and 58: prolate-like), the oblate-like bodi
Page 59 and 60: Figure 2.9: Plots comparing T-PROS
Page 61 and 62: Figure 2.10: Comparison of S-class
Page 63 and 64: 2.3.5 Triples and hierarchical syst
Page 65 and 66: a/R pri based on a simple conversio
Page 67 and 68: Figure 2.13: The eccentricity dampi
Page 69 and 70: Main Belt. Work by Chauvineau & Far
Page 71 and 72: (1992), with a target range of slig
Page 73 and 74: The observations were made from the
Page 75 and 76: peaked period, with a lower signifi
Page 77 and 78: and suggests that it is possibly a
Page 79 and 80: fit for the period of the lightcurv
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
Page 91 and 92: Typically observations of eclipse o
Page 93 and 94: Chapter 4Steady-State Model of the
Page 95 and 96: values between 0.2-0.6. There is on
Page 97 and 98: asteroids in the Main Belt is suffi
Page 99 and 100: Figure 4.2: Percent of surviving NE
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: Figure 4.4: The total number of bin
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
Page 117 and 118: quickly altered, or involved in bin
Page 119 and 120: tidal disruption. However, the impl
Page 121 and 122: Chapter 5ConclusionsA general concl
Page 123 and 124: small additions to the angular mome
Page 125 and 126: cently this large pool of observers
Page 127 and 128: differences in results when a lower
Page 129 and 130: Table A.1.Number of binaries produc
Page 131 and 132: Thus the reaccumulated bodies in th
Page 133 and 134: of 2.2 h observed is significantly
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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