Binaries among small main-belt asteroids

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Binaries among small main-belt asteroids

• Petr Pravec
• Astronomical Institute AS CR, Czech Republic
• Workshop on Binaries
• Paris-Meudon, 2008 May 19-22

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Binary population Porb vs D1
Porb lower limit of 11 h both for NEAs and
MBAs. (Could be there closer, fully synchronous
systems?)
Porb has a tail into the range gt100 h for small
MBAs (the low observed number there is an
observational selection effect) but not for NEAs.
High abundance (fraction) of binaries in D1 lt 10
km, but much lower above. Binary fraction 15 4
among NEAs (Pravec et al. 2006), similar or
maybe even higher fraction among MBAs (up to D1
10 km)
Data from Pravec and Harris, Icarus, 190 (2007)
250-253. Updates available on URL given in the
paper.
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Primary rotation rates for NEAs and small MB/MCs
(Pravec et al. 2008, in press)
Distribution of f1 much broader, most of them in
the range 6-10 d-1. If both NEA and small MBA
binaries formed at the spin barrier, then MBA
binaries are more evolved than NEA binaries.
Concentration at fast spin rates with a peak at
f1 9-10 d-1, coincides with an excess of
rotation rates seen in the f-distribution for all
NEAs the excess appears to be due to binaries.
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But there are more similarities than differences
between NEA small close MBA binaries

• Similarities
• Total angular momentum close to critical.
• Size ratio distribution (D2/ D1 lt 0.5 mostly).
• Primaries have low equatorial elongations.
• Secondaries mostly synchronous and having a
  broader distribution of eq. elongations.

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NEA/MBA binary similarities1. 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 large
  amount of angular momentum was added or removed
  since formation of the system.
• (Pravec and Harris 2007)

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NEA/MBA binary similarities2. Size ratio
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NEA/MBA binary similarities 3. Primary
component shapes
Model of the primary of 1999 KW4 (Ostro et al.
2006)

• Primaries of asynchronous binaries have low
  equatorial elongations both among NEAs and small
  MBAs.

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NEA/MBA binary similarities4. Secondaries

• Broader range of equatorial elongations a/b 11
  to 21.
• Some synchronous, but some may not be
  interpretation of a third period (Porb, P1, P2)
  ambiguous may be an unsynchronous rotation of
  the secondary, or a rotation of a third body.

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Orbit poles few data so far

• Good data covering long enough arc (range of
  geometries) for a few NEA binaries only
  (Scheirich 2008, PhD thesis).
• Observations of binaries in their return
  apparitions needed to constrain orbit pole
  distribution.

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Photometrically observed binaries - examples

• NEA binaries 31 1 ternary 10 of them with
  both radarlc, 13 of them with radar only, 9 of
  them with photometry only.
• MBA binaries with D1 10 km and Porb lt 20 d 45
  (all from LCs, one of them marginally resolved
  with radar) 1 detection of a close satellite in
  asteroid (3749 Balam) with a distant satellite
  discovered in 2002 with AO (i.e., ternary system)

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(5481) Kiuchi - a typical photometric binary MBA
detection
Porb 20.90 0.01 h D2/D1 0.33 0.02 P1
3.6196 0.0002 h A1 0.10 mag
Secondary rotation unresolved (may have a
low amplitude). Eccentricity low.
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(7225) Huntress - a low attenuation depth
detection
Porb 14.67 0.01 h D2/D1 0.21 0.02 P1
2.4400 0.0001 h A1 0.11 mag
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(3073) Kursk - usual parameters, but primary lc
Porb 44.96 0.02 h D2/D1 0.25 0.02 P1
3.4468 0.0001 h A1 0.21 mag
Primarys lc shape more irregular than usual.
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(16635) 1993 QO - a three-period case
Porb 32.25 0.03 h D2/D1 0.27 P1 2.2083
0.0002 h, A1 0.17 mag P2 7.622 0.002 h, A2
0.05 mag
The 7.6-h period is assumed to be a rotation
period of the secondary, but it might be also a
rotation of a third body.
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(2486) Metsahovi - a two-period case (no events)
P1 4.4518 h, A1 0.12 mag P2 2.6404 h, A2
0.04 mag
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(1717) Arlon - a three-period case, longish P_orb
P1 5.148 h P2 18.23 h Porb 117.0 h D2/D1
0.5
D1 9 km aL 1.8 (unc. factor 1.25)
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(2478) Tokai - a fully synchronous system
Porb 25.89 h D2/D1 0.72 P1 or P2 Porb A
0.41 mag
D1 8 km (30) aL 1.40 (10)
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(4851) Iwamoto - a (relatively) wide synchronous
system
Porb 118.0 0.2 h D2/D1 0.76 P1 or P2
Porb A 0.34 mag
D1 4.0 km (assuming pV 0.20 0.07 for
its S-type classification) aL 2.25 (10)
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Primaries of small wide binaries detected with AO

• (1509) Esclangona, (3749) Balam, and (4674)
  Pauling have all fast rotating primaries with P1
  3.25, 2.80, and 2.53 h, respectively, and
  amplitudes 0.06-0.13 mag (Warner 2005, Marchis et
  al. 2008, Warner et al. 2006). Their distant
  satellites have orbital periods on an order of
  100 days. In (3749), another, close satellite
  with Porb 33.38 h has been found from
  photometry (Marchis et al. 2008).

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Conclusions

• NEA and small close MBA binaries are suggested to
  be formed by same or similar mechanism(s) causing
  fission of critically spinning asteroids at the
  spin barrier.
• Differences between NEA and small close MBA
  binaries suggest that small close MBA binaries
  are more evolved than NEA binaries.
• Systems with orbital periods shorter than 60
  hours have a total angular momentum content close
  to the critical limit for a single body in a
  gravity regime, but a couple systems with Porb
  118 h have a higher total angular momentum.
• Small wide binaries detected with AO have
  primaries pretty similar (and one has even
  another, close satellite) to primaries of close
  binary systems.