Transcript Photometric Survey of Binary Near

Asteroid Rotations and Binaries
Petr Pravec and Alan W. Harris
1
2
1Astronomical
Institute AS CR, Czech Republic
2Space Science Institute
VII Workshop on Catastrophic Disruptions in the Solar System
2007 June 29
Three major asteroid size ranges
Asteroid population splits according to properties related to
their rotations into three major ranges at D~60 km and 0.2 km:
1. Large asteroids, D > 60 km
2. Small asteroids, D = 0.2 – 60 km
3. Very small asteroids, D < 0.2 km
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Large asteroids – rotations collisionally evolved
Small asteroids – rotations driven by YORP
Spin barrier at sizes D=0.2 to 10 km – suggesting cohesionless
structure from 0.2 up to 3 km
Superfast rotators below D=0.2 km – cohesion implied
Binary population among asteroids with D=0.3-10 km – related
to critical spins near the spin barrier
Large asteroids – collisionally evolved rotations
• Variation of <f> with D reduced for: Normalized spin rate = f/<f>
Minimum of <f> 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.
Small asteroids - rotations driven by YORP
1.
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:
(Lowry et al. 2007)
Acceleration of 2000 PH5 rotation by YORP:
(Taylor et al. 2007)
Small asteroids - rotations driven by YORP
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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)
Small asteroids - rotations driven by YORP
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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)
Small asteroids - rotations driven by YORP
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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)
Spin barrier
Spin barrier in 2nd dimension
Limiting curves for bulk
densities 1, 2, 3, 4, 5 g/cm3
for cohesionless elasticplastic solid bodies.
(Holsapple 2001, 2004)
>99% 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.
Scaled tensile strength
Above D=3 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 D=3 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 D=0.2 to 3 km.
The area above the spin barrier is
unpopulated at sizes D>0.2 km
(except the single point 2001 OE84
at D=0.7 km, P=29 min).
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.
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.
Binary population among small asteroids
Data on periods
-rotation and orbitalplus 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.
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.
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.
Angular momentum content
αL = 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 αL
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)
Size ratio vs primary size
Proposed binary formation theories
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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.
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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.
(Walsh and Richardson 2006)
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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.
Fission of critically spinning parent bodies
spun up by YORP
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The critical content of angular momentum in small close binaries may
be consistent with this mechanism.
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YORP may be slowed down after formation of the binary if the primary’s
figure is shaped to a more symmetric shape; the total angular
momentum is not changed significantly from the critical amount.
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Further evolution after YORP is slowed down may be driven by a
mechanism transferring angular moment from the primary’s rotation to
the orbital motion – resulting in a slow down of primary’s 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.
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: ~1*D2 [*Myr/km2] -> ~1 Myr (1-km asteroid)
MBA: ~3*D2 [*Myr/km2] -> ~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 <2 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.
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 D>60 km.
• YORP and internal structure (strength) – controlling spins
(and shapes, binaries) of smaller asteroids.
¡Fin! ¡Gracias!
(Additional slides for possible discussion follow)
Primary rotation vs size
Photometric detection of Asynchronous Binary