Photometric Survey for Asynchronous Binary Asteroids

Petr PravecAstronomical Institute AS CR, Ondřejov

Presented at MACE 2006 in Vienna, Austria

2006 May 14

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 ...

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)

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 ....

Deconvolution of async binary asteroid LC

NEA binary population

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

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 (Dprim>0.3 km, Dsec/Dprim0.18):

39%

NEA binary population

15 4 % of NEAs are binary

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 1:1 to 2:1)• Dsec/Dprim 0.5

The characteristics hold for small binaries in MB+MC orbits as well.

NEA binaries – fast primary spins

NEA binaries – spheroidal primary shapes

NEA binaries – secondary sizes

NEA binaries characteristics (2)

Total specific angular momentum Primary is a fast rotator, additional angular momentum contained in the orbital

motion (and secondary’s 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 (NEA+MC+MB binaries) suggests that Ltot is close to Lcrit over the whole range of heliocentric distances ....

Binary population from NEAs to inner MBAs

Small NEA+MC+MB 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)

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 ....

Characteristics of binaries – multiparameter space

Models studied.Data have to be displayed in2-D projections.

Size ratio vs primary size

Primary spin vs size

Total Angular Momentum

αL = 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 αL ~ 1.3 (Harris and Pravec, in prep.)

Systems originating from critically spinning “rubble piles” are

expected to have αL between 1.0 and 1.3, if no angular

momentum was added or removed since creation of the system.

Angular momentum ratio vs size

Binary characteristics- concluding remarks

The population of small binaries (Dp <= 10 km) with Ltot~Lcrit 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.)

BinAstPhotSurvey- Opportunity for experienced photometrists

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

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 V>15 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.

Unrecognized (uneliminated) background stars affect measurements (1)

Unrecognized (uneliminated) background stars affect measurements (2)

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.

THE END

Thank you!