Transcript Photometric Survey of Binary Near

Binaries among small main-belt
asteroids
Petr Pravec
Astronomical Institute AS CR, Czech Republic
Workshop on Binaries
Paris-Meudon, 2008 May 19-22
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
>100 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 < 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.
Primary rotation rates for NEAs and small MB/MCs
(Pravec et al. 2008, in press)
Concentration at fast spin rates with a
peak at f1 = 9-10 d-1, coincides with an
excess of rotation rates seen in the fdistribution for all NEAs – the excess
appears to be due to binaries.
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.
But there are more similarities than differences
between NEA + small close MBA binaries
Similarities:
1.
Total angular momentum close to critical.
2.
Size ratio distribution (D2/ D1 < 0.5 mostly).
3.
Primaries have low equatorial elongations.
4.
Secondaries mostly synchronous and having a broader distribution
of eq. elongations.
NEA/MBA binary similarities:
1. 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
large amount of angular momentum
was added or removed since
formation of the system.
(Pravec and Harris 2007)
NEA/MBA binary similarities:
2. Size ratio
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.
NEA/MBA binary similarities:
4. Secondaries
Broader range of equatorial
elongations: a/b= 1:1 to 2:1.
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.
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.
Photometrically observed binaries examples
NEA binaries: 31 + 1 ternary; 10 of them with both radar+lc, 13 of
them with radar only, 9 of them with photometry only.
MBA binaries with D1 ≤ 10 km and Porb < 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)
(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.
(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
(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
Primary’s lc shape more irregular than usual.
(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.
(2486) Metsahovi
- a two-period case (no events)
P1 = 4.4518 h, A1 = 0.12 mag
P2 = 2.6404 h, A2 = 0.04 mag
(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
αL = 1.8 (unc. factor 1.25)
(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%)
αL = 1.40 (±10%)
(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)
αL = 2.25 (±10%)
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).
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.