Transcript Photometric Survey of Binary Near

Photometric Survey for
Asynchronous Binary Asteroids
Petr Pravec
Astronomical 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/Dprim0.18):
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 2004Small 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 Marscrossing 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 in
2-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!