Forming Binary Near-Earth Asteroids From Tidal Disruptions

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Figure 2.7: (a) Normalized probability of binaries formed as a function of secondaryto-primarysize ratio. (b) Primary (shaded) and secondary (cross-hatch) obliquity. (c)Primary (shaded) and secondary (cross-hatch) spin period.the progenitor is ∼ 2.0 g cm −3 with a porosity of 35%, neither of which are extreme forobserved NEAs. However, any tensile forces or mechanical strength which could pushcritical spin rate faster, even very briefly, may contribute to the very fast spinning primary.Further work is needed to show whether these caveats are responsible for the differencebetween the observed and simulated spin rate distributions.The obliquity of the primary in simulations is quite low (where obliquity is the angle41
etween ω pri and L bin ), with close to 90% of the binaries having an obliquity less than20 ◦ . The obliquity of the secondary has no such relation, being nearly flat between 0 ◦ and90 ◦ , and falling off from 90 ◦ to 180 ◦ (Fig. 2.7b). With many secondaries formed fromaccreting material in orbit around the primary, retaining an aligned spin axis appears tobe unlikely for a secondary.Shape of the primaryThe shape of the primary was measured along the body’s three principal axes, a, b, andc for the longest, intermediate and shortest axis length. <strong>Near</strong>ly all primaries are in aprincipal axis rotation state, rotating around the shortest axis. Thus the shape irregularitieswhich are associated with a rotational lightcurve are a result of a difference between thelong axis, a and the intermediate axis, b. If the intermediate axis, b, equals a or c, thebody is either oblate (b = a) or prolate (b = c). However, few bodies approach suchperfect classification, so for the sake of simplicity we define a shape indexI = (a − b)/(a − c)where I = 1 means prolate, and I = 0 oblate.The progenitor bodies were created to be simple prolate ellipsoids (b = c), with differentvalues of a/c for varying amounts of elongation. The resulting primaries, as shown inFigure 2.8a, have a distribution of a/c concentrated between 1.0 and 3.0. The progenitorelongations used in the simulation were only varied between 1.0 and 2.0, so elongationwas significantly enhanced during the binary-forming encounters.The distribution of a/b is a more valuable comparison to the shape derived via lightcurvestudies for bodies rotating around their shortest principal axis. This distribution has thesame peak as a/c, at 1.95, but a larger concentration of objects near 1.0 (Fig. 2.8b). Whenthe distribution is separated into bodies with I < 0.5 and > 0.5 (more oblate-like or42
Page 1 and 2:
AbstractTitle of Dissertation:Formi
Page 3 and 4: Forming Binary Near-Earth Asteroids
Page 5 and 6: PrefaceMuch of the work in this dis
Page 7 and 8: AcknowledgementsI would not have ma
Page 9 and 10: 2.3 Results and Discussion . . . .
Page 11 and 12: List of Tables1.1 Binary NEA proper
Page 13 and 14: 4.8 Percentage of migrated binaries
Page 15 and 16: of the system mass M by way of Kepl
Page 17 and 18: the largest lightcurve amplitudes o
Page 19 and 20: 1.1.3 Binary Main-Belt asteroidsRec
Page 21 and 22: Lightcurve discoveriesA lightcurve
Page 23 and 24: Figure 1.1: (Top) The primary rotat
Page 25 and 26: 1.2.1 CaptureIn this scenario two a
Page 27 and 28: ever, understanding the mechanism i
Page 29 and 30: off the equator or off one end of a
Page 31 and 32: 3 Denotes a secure result with no a
Page 33 and 34: nostic.Recent results have demonstr
Page 35 and 36: another intermediate source of NEAs
Page 37 and 38: 1.5.2 Rubble pilesThe evidence for
Page 39 and 40: Chapter 2Formation of Binary Astero
Page 41 and 42: ∼ 60%, making a bulk density of
Page 43 and 44: shedding mass and distorting prior
Page 45 and 46: Figure 2.3: Snapshots of a tidal di
Page 47 and 48: Figure 2.4: Normalized probability
Page 49 and 50: Figure 2.5: Normalized probability
Page 51 and 52: Figure 2.6: (a) Satellite eccentric
Page 53: various complications of measuring
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
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which means it should have a critic
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Figure 4.4: The total number of bin
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the original eccentricity distribut
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Figure 4.6: Properties of the binar
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Figure 4.8: Binary percentage for m
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Figure 4.9: Effects of tidal evolut
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quickly altered, or involved in bin
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tidal disruption. However, the impl
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Chapter 5ConclusionsA general concl
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small additions to the angular mome
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cently this large pool of observers
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differences in results when a lower
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Table A.1.Number of binaries produc
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Thus the reaccumulated bodies in th
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of 2.2 h observed is significantly
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Benner, L. A. M., Nolan, M. C., Ost
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V7.0:LCREF TAB, 35, 4Harris, A. W.,
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Storrs, A. D., Close, L. M., & Mena
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122Pravec, P., Kušnirá, P., Hicks
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Shepard, M. K., Schlieder, J., Nola
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Forming Binary Near-Earth Asteroids From Tidal Disruptions

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