Photometric Survey for Asynchronous Binary Asteroids

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Photometric Survey for Asynchronous Binary
Asteroids

• Petr Pravec
• Astronomical Institute AS CR, Ondrejov
• Presented at MACE 2006 in Vienna, Austria
• 2006 May 14

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• There is an abundant population of small binary
  asteroids from near-Earth orbits to the main belt
  ....
• Studied with the photometric (lightcurve)
  technique
• 1994-2004 NEA binary population studied.
• 2004- The survey extended to the MB ...

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BinAstPhotSurvey

• Small Asynchronous binaries
• BinAstPhotSurvey (Photometric Survey for
  Asynchronous Binary Asteroids). A consortium of
  several experienced asteroid photometric stations
  in Europe, USA, Canada and Australia. See page
  and references therein.
• The key element of the Survey is a collaboration
  of the great asteroid photometrists
• D. Pray (Carbuncle Hill), B. Warner (PDO), D.
  Higgins (Hunters Hill), P. Kusnirak (Ondrejov),
  A. Galad et al. (Modra), W. Cooney et al. (SRO),
  V. Reddy and R. Dyvig (Badlands), P. Brown et al.
  (Elginfield), Yu. Krugly et al. (Kharkiv), R.
  Stephens (GMARS), R. Koff (Antelope Hills), N.
  Gaftonyuk (Simeiz), and others.
• Since December 2004 covered 100 asteroids with
  a proper strategy for detection of binaries and
  modeling of selection effects. Found 10
  binaries.
• Ryan et al., 2004 (Vestoids)
• Radar (limited to studies of NEAs)
• Small Synchronous binaries - Behrend et al.,
  2006 and others
• Large/wide binaries studied by AO technique,
  direct imaging (HST)

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What is an Asynchronous Binary Asteroid

• System of two bodies with at least one of them
  rotating non-synchronously with the orbital
  motion.
• Showing two (or three) lightcurve components that
  are linearly additive.
• The technique ....

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Deconvolution of async binary asteroid LC
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NEA binary population

• Studied 1994-2004
• (Pravec et al. 2006, Icarus 181, 63-93, and
  references therein)

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Fraction of NEA binaries - selection effects had
to be modeled

• Biases for
• Satellites larger than 0.18 Dprim
• Closer systems (shorter Porb)
• Resulting mean detection probability within the
  lightcurve survey for NEA binaries (Dprimgt0.3 km,
  Dsec/Dprim?0.18)
• 39

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NEA binary population

• 15 ? 4 of NEAs are binary

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NEA binaries characteristics (1)

• Main characteristics (for a bulk of the
  population)
• Primaries
• fast rotating, spheroidal
• Secondaries
• mostly synchronous (Psec Porb exceptions in
  wider systems)
• diverse shapes (eq. axial ratio range from 11 to
  21)
• Dsec/Dprim ? 0.5
• The characteristics hold for small binaries in
  MBMC orbits as well.

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NEA binaries fast primary spins
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NEA binaries spheroidal primary shapes
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NEA binaries secondary sizes
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NEA binaries characteristics (2)

• Total specific angular momentum
• Primary is a fast rotator, additional angular
  momentum contained in the orbital motion (and
  secondarys rotation).
• Ltot is very close to, but not generally
  exceeding, the critical limit for a single body
  (Ltot Lcrit)
• Suggests that
• NEA binaries formed at the critical spin rate
• little or no angular momentum has been added
  after the binary is formed.
• If precursors spun-up to fission by YORP, the
  increase in angular momentum must be shut off
  after the binary is formed.
• A study of Ltot on a larger sample (NEAMCMB
  binaries) suggests that Ltot is close to Lcrit
  over the whole range of heliocentric distances
  ....

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Binary population from NEAs to inner MBAs

• Small NEAMCMB binaries studied 2004-
• Small Asynchronous binaries BinAstPhotSurvey
  also Ryan et al., 2004
• Small Synchronous binaries - Behrend et al.,
  2006 and others
• Large/wide binaries studied by AO technique,
  direct imaging (HST)

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• The population of small asynchronous binary
  asteroids is not confined to near-Earth orbits
  only ....
• Small binaries in the Main Belt and Mars-crossing
  orbits are similar in most characteristics to NEA
  binaries. It suggests common origin/evolution
  mechanisms ....

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Characteristics of binaries multiparameter space
Models studied. Data have to be displayed in 2-D
projections.
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Size ratio vs primary size
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Primary spin vs size
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Total Angular Momentum

• aL Ltot/Lcrit.sp.
• where Ltot is a total angular momentum of the
  system, Lcrit is angular momentum of an
  equivalent (i.e., the same total mass and
  volume), critically spinning sphere.
• A rubble pile with only compressive strength
  can resist rotational disruption up to values of
  aL 1.3 (Harris and Pravec, in prep.)
• Systems originating from critically spinning
  rubble piles are expected to have aL between
  1.0 and 1.3, if no angular momentum was added or
  removed since creation of the system.

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Angular momentum ratio vs size
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Binary characteristics- concluding remarks

• The population of small binaries (Dp lt 10 km)
  with LtotLcrit extends from near-Earth to (at
  least) inner main belt.
• Suggests that planetary encounters are not the
  primary cause of their formation.
• If binaries were created by a spin-up mechanism,
  it must be shut off after creation of the
  binary no significant amount of angular momentum
  was added after creation.
• Is a gap in Ds /Dp between 0.5 and 0.8 real or
  just a selection effect? (May there be tidally
  evolved systems, e.g., fully synchronous, or
  distant secondaries? Such would be difficult to
  resolve with the current techniques.)

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BinAstPhotSurvey- Opportunity for experienced
photometrists

• Our consortium is open to photometrists able to
  keep a strategy of the Survey

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BinAstPhotSurvey- Opportunity for experienced
photometrists

• Interested to study asynchronous binaries?
• We welcome you to join us, if
• You are an experienced asteroid photometrist.
• You can measure asteroids with Vgt15 with errors
  0.03 mag.
• You can identify/eliminate error sources (e.g.,
  interferring background stars) down to a level of
  0.01 mag.
• You can take long series or linked data.
• You can reduce data fast (normally within 24 h
  since acquisition) so that observational programs
  on your and other collaborating stations within
  the Survey may be updated promptly.
• You will want to learn more on the strategy of
  the Survey in a course of your collaborative work
  within the consortium.
• Following the strategy of the Survey is necessary
  to get a high efficiency and reproducibility of
  the observations within the Survey.

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Unrecognized (uneliminated) background stars
affect measurements (1)
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Unrecognized (uneliminated) background stars
affect measurements (2)
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High-quality Flatfield another must for a
moving target

• Targeted asteroid (or comparison stars, if
  tracked on the asteroid) drift over the FOV
  during night. Flatfield imperfections (e.g., a
  slope in the flatfield) may cause observational
  artifacts in the measurements.
• Elimination of stray light is quite necessary as
  well it cannot be flatfielded in a normal way.
• Those problems occur at larger, professional
  telescopes as well.

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THE END

• Thank you!