Asteroid Rotations and Binaries

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Asteroid Rotations and Binaries

• Petr Pravec1 and Alan W. Harris2
• 1Astronomical Institute AS CR, Czech Republic
• 2Space Science Institute
• VII Workshop on Catastrophic Disruptions in the
  Solar System
• 2007 June 29

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Three major asteroid size ranges
Asteroid population splits according to
properties related to their rotations into three
major ranges at D60 km and 0.2 km

1. Large asteroids, D gt 60 km
2. Small asteroids, D 0.2 60 km
3. Very small asteroids, D lt 0.2 km

• Large asteroids rotations collisionally evolved
• Small asteroids rotations driven by YORP
• Spin barrier at sizes D0.2 to 10 km suggesting
  cohesionless structure from 0.2 up to 3 km
• Superfast rotators below D0.2 km cohesion
  implied
• Binary population among asteroids with D0.3-10
  km related to critical spins near the spin
  barrier

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Large asteroids collisionally evolved rotations

• Variation of ltfgt with D reduced for Normalized
  spin rate f/ltfgt
• Minimum of ltfgt at D about 100 km perhaps due to
  an effect called angular momentum drain/splash
  (Dobrovolskis and Burns 1984 Cellino et al.
  1990).

• Spin frequency distribution is Maxwellian
  (Pravec and Harris 2000, Pravec et al. 2002)

Asteroids larger than 60 km have spin rates with
the distribution predicted for a collisionally
evolved system.
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Small asteroids - rotations driven by YORP

• YORP detected in 2000 PH5 and Apollo (Lowry et
  al., Taylor et al., Kaasalainen et al. 2007)

Period change caused by YORP in 2000 PH5
Acceleration of 2000 PH5 rotation by YORP
(Lowry et al. 2007)
(Taylor et al. 2007)
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Small asteroids - rotations driven by YORP

• YORP detected in 2000 PH5 and Apollo (Lowry et
  al., Taylor et al., Kaasalainen et al. 2007)
• Excess of both slow and fast rotators among small
  asteroids (e.g., Pravec and Harris 2000)

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Small asteroids - rotations driven by YORP

• YORP detected in 2000 PH5 and Apollo (Lowry et
  al., Taylor et al., Kaasalainen et al. 2007)
• Excess of both slow and fast rotators among small
  asteroids (e.g., Pravec and Harris 2000)
• Alignment of spin axes of members of the Koronis
  family (Slivan et al., Vokrouhlický et al.03)

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Small asteroids - rotations driven by YORP

• YORP detected in 2000 PH5 and Apollo (Lowry et
  al., Taylor et al., Kaasalainen et al. 2007)
• Excess of both slow and fast rotators among small
  asteroids (e.g., Pravec and Harris 2000)
• Alignment of spin axes of members of the Koronis
  family (Slivan et al., Vokrouhlický et al. 03)
• Close binary systems among small asteroids with a
  total angular momentum
• near critical (Pravec and Harris 2007)

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Spin barrier
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Spin barrier in 2nd dimension

• Limiting curves for bulk densities 1, 2, 3, 4, 5
  g/cm3 for cohesionless elastic-plastic solid
  bodies. (Holsapple 2001, 2004)
• gt99 of measured asteroids larger than 200 m
  rotate slower than the limit for bulk density of
  3 g/cm3 (Harris 1996 Pravec and Harris 2000).
• Most NEAs smaller than 200 m rotate too fast to
  be held together by self-gravitation, some
  cohesion implied.

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Scaled tensile strength
Above D3 km, an upper limit on the tensile
strength given by the spin barrier is higher than
a scaled tensile strength of cracked but coherent
rocks, so the existence of the spin barrier does
not constrain whether asteroids in the size range
3-10 km are strengthless objects or just cracked
but coherent bodies. Below D3 km, the maximum
possible tensile strength allowed by the spin
barrier for a majority of asteroids in the size
range is too low for them to be cracked but
coherent bodies (Holsapple 2007) this implies
that a cohesionless structure is predominant
among asteroids with D0.2 to 3 km.
The area above the spin barrier is unpopulated at
sizes Dgt0.2 km (except the single point 2001
OE84 at D0.7 km, P29 min).
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Binary systems among small asteroids

• NEA binaries since 1997 by photometry, since
  2000 by radar

• Small MBA binaries (D 10 km) since 2004
  binary Vestoid
• 3782 Celle (Ryan et al. 2004), many more since
  then (see Pravec et al. 2006,
• Warner et al. 2005, Pravec and Harris 2007)
• We are sampling small binaries from NEAs to the
  inner main belt mostly, but
• occassionally sampling the central main belt too.

Binaries have been found numerous among small
asteroids (D 10 km) everywhere we looked
thoroughly enough.
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Binary fraction among small asteroids

• NEAs
• 15 ? 4 of NEAs are binary
• (Pravec et al. 2006)
• Inner MB asteroids
• Debiasing their distribution more sensitive to
  assumptions on orbit pole distribution awaiting
  more data to constrain it. Rough numbers similar
  to the NEA binary fraction.

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Binary population among small asteroids
Data on periods -rotation and orbital- plus
limited shape information for 51 small binary
systems, major part of them from photometric
measurements.
Data published in Pravec and Harris,
2007, Icarus, in press. Available on-line on URL
given in the paper.
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Binary primaries spin rates
NEAs
MBAs

• NEA primaries concentrate in the pile up at f
• around 9-10 d-1 (P of 2-3 h) in front of the spin
  barrier.

MBA primaries have a considerably broader
distribution of spin rates, with a lower
concentration at fast spin rates and a more
pronounced tail (correlated with D). MBA
binaries may be more evolved than NEA binaries,
if all have formed near the spin barrier.
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Binary primaries shapes
Model of the primary of 1999 KW4 (Ostro et al.
2006)

• Primaries of asynchronous binaries,
• both among NEAs and MBAs, have
• shapes with low equatorial elongation.
• The model of 1999 KW4 shows an
• equatorial belt that appears like it was
• paved by some process.

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Angular momentum content

• aL Ltot/Lcritsph
• where Ltot is a total angular momentum of the
  system, Lcritsph is angular momentum of an
  equivalent (i.e., the same total mass and
  volume), critically spinning sphere.
• Binaries with D1 10 km have aL between 0.9 and
  1.3, as expected for systems originating from
  critically spinning rubble piles, if no angular
  momentum was added or removed since formation of
  the system.
• (Pravec and Harris 2007)

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Size ratio vs primary size
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Proposed binary formation theories

• Ejecta from large asteroidal impacts (e.g., Durda
  et al. 2004) may work for small satellites of
  large asteroids or for wide binary systems among
  small asteroids, but it does not predict a
  relation to critical spin for small close binary
  systems.
• Tidal disruptions during close encounters with
  terrestrial planets (Bottke et al. 1996
  Richardson and Walsh) does not work in the main
  belt, so, it cannot be a formation mechanism for
  MB binaries, but it may contribute to and shape
  the population of NEA binaries.
• Fission of critically spinning parent bodies spun
  up by YORP (e.g., Bottke et al. 2006) seems to
  be a primary formation mechanism for small close
  binaries.

(Walsh and Richardson 2006)
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Fission of critically spinning parent bodies spun
up by YORP

• The critical content of angular momentum in small
  close binaries may be consistent with this
  mechanism.
• YORP may be slowed down after formation of the
  binary if the primarys figure is shaped to a
  more symmetric shape the total angular momentum
  is not changed significantly from the critical
  amount.
• Further evolution after YORP is slowed down may
  be driven by a mechanism transferring angular
  moment from the primarys rotation to the orbital
  motion resulting in a slow down of primarys
  rotation and moving the orbit outward (longer
  periods). Longer periods of MB binaries are
  consistent with them being more evolved (being
  older, or more rapidly evolving) than smaller NEA
  binaries.

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Time scales for small binaries

• Lifetimes of asteroids
• NEA 10 Myr (Gladman et al. 2000)
• MBA 300 Myr (1-km asteroid)
• YORP spin up time scale
• NEA 1D2 Myr/km2 -gt 1 Myr (1-km asteroid)
• MBA 3D2 Myr/km2 -gt 30 Myr (3-km asteroid)
• Lifetime of NEA binary 1-2 Myr (limited by
  disruptions during close approaches to Earth and
  Venus Walsh and Richardson 2006)
• Lifetime of MB binary 300 Myr ( lifetime of an
  MBA of size of the secondary, if it is controlled
  by asteroidal collisions in the main belt).

• The estimated short lifetime of NEA binaries
  suggests that few MB binaries survived since
  transfer to NEA
• orbits most NEA binaries may have formed after
  transfer to near-Earth space. It may explain
• the observation that NEA binaries concentrate at
  sizes lt2 km (Pravec et al. 2006) larger NEAs
• may not have enough time to form binaries.

• The strong dependence of lifetime of NEA
  binaries on relative separation of components
• may be an explanation (alternative to that NEA
  binaries may be less evolved by a tidal
• mechanism) for the observation that they have a
  narrower distribution of periods, concentrating
• at faster spin rates in front of the spin barrier.

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Conclusions

• Rotations bring important (though sometimes only
  circumstantial) information on processes in the
  asteroid population.
• Collisions in the main belt most important
  effect for large asteroids with Dgt60 km.
• YORP and internal structure (strength)
  controlling spins
• (and shapes, binaries) of smaller asteroids.

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Fin! Gracias!
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(Additional slides for possible discussion follow)
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Primary rotation vs size
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Photometric detection of Asynchronous Binary