Forming Binary Near-Earth Asteroids From Tidal Disruptions

More documents

Recommendations

Info

ondary body, which requires extensive observations at a viewing angle near the binaryorbital plane.Radar observations require the close approach of an asteroid to <strong>Earth</strong>, but can providedetailed physical and orbital information about the binary. The signal–to–noise ratio(SNR) of radar measurements is proportional to R −4tarand D3/2 tar , where R tar and D tar are thedistance to and diameter of the target body respectively. The SNR is also proportional toP 1/2 , the square root of the rotation period. Thus radar observations are more likely todiscover nearby, larger, slower-rotating secondaries (Ostro et al. 2002).The currently known NEA binaries share similar physical and orbital traits. All currentlyknown or suspected binaries have primary bodies with a diameter (D pri ) less than 4km, normal for NEAs but significantly smaller than large MBAs (see Table 1.1). All primarieswith measured rotations, with the exceptions of NEAs (69230) Hermes and 2000UG 11 , have rotation periods among the fastest observed between 2.2 and 3.6 h. Theserotation rates are very near the critical spin limit for a spherical strengthless body givenapproximately byP crit ≈ 3.3 √ ρ(h) (1.2)where ρ, the bulk density of the body, is in g cm −3 . For a body with ρ = 2.2 g cm −3 , P crit≈ 2.2 h, which defines the lower limit for primary spin rate currently observed (Pravec &Harris 2000). <strong>Asteroids</strong> spinning above the critical rate are not stable as material at theirequator feel more outward centrifugal acceleration than the acceleration from its own selfgravity.All the primaries have similar lightcurve amplitudes, typically below 0.2 magnitudes.The amplitude of a primary’s lightcurve (∆m) has a simple relationship with the body’sshape∆m ∼ 2.5log a b(1.3)where a and b are the long and intermediate length axes of a tri-axial ellipsoid. Thus3
the largest lightcurve amplitudes observed, ∼ 0.2 magnitudes, imply a 1.2:1.0 axis ratio.The entire population of NEAs has a much larger range of lightcurve amplitudes; 0.2 isrelatively close to spherical in comparison (Pravec et al. 2002).The secondaries typically have diameters between 0.2 and 0.6 times D pri . Again, theexception is Hermes which is a suspected synchronously rotating binary with equal-sizedcomponents, as well as the radar-discovered (1862) Apollo with a secondary 1/20th thesize of its primary 1 . All others are asynchronous systems, with the primary rotating muchfaster than the orbital period of the secondary (Pravec et al. 2004b). An observationallimit exists for bodies below 0.2 D pri , but between 0.6 and 1.0 D pri , where few systemsare observed, no biases are known. The secondaries are also consistent in their separationfrom the primary, with most being within 6 primary radii (R pri ). The exception is 1998ST 27 with a separation ∼ 10 R pri , which also has a relatively fast-spinning secondary(period < 6 h) and a high eccentricity (e > 0.3). Other than ST 27 , the few eccentricitiesthat are known are all below 0.1. Few rotations of secondaries are well known, thoughthey appear mostly synchronized with the orbital motion, with 1998 ST 27 again being anexception (Pravec et al. 2004b). No correlations are seen between properties and primarymass or asteroid spectral classification Pravec et al. (2006).The lightcurve survey of binary NEAs determined that 15±4% of NEAs larger than0.3 km are binaries with D sec /D pri ≥ 0.18 (Pravec et al. 2006). Among the fastest spinningNEAs with rotation rates between 2–3 h, the binary percentage is 66 +10−12%. These estimatesinclude the detection limitations of the lightcurve techniques, which are stronglybiased against discovering binaries with wide separations (Pravec et al. 2006).1 Synchronization timescales for a binary like Hermes are expected to be below 10 Myr, possibly evenbelow 1 Myr. See Section 2.3.6 for a detailed treatment of tidal evolution.4
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: of the system mass M by way of Kepl
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 and 54: various complications of measuring
Page 55 and 56: 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 and 108:
Figure 4.4: The total number of bin
Page 109 and 110:
the original eccentricity distribut
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
show all

Forming Binary Near-Earth Asteroids From Tidal Disruptions

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?