Population of small asteroid systems
- We are still in a survey phase
P. Pravec, P. Scheirich, P. Kušnirák, K. Hornoch, A. Galád
Astronomical Institute AS CR, Ondřejov, Czech Republic
The 3rd Workshop on Binaries in the Solar System
Hawaii, the Big Island, 2013 June 30 – July 2
Spin-up fission asteroid systems
Current sample:
Our binary asteroid parameters database (Pravec and Harris 2007, update June 2013):
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39 NEA systems
79 MBA/MC systems (smaller than 20 km)
We have also identified 158 asteroid pairs (Vokrouhlický and Nesvorný 2008, Pravec and Vokrouhlický
2009, Pravec et al. 2010, plus others in prep.)
Many known binaries appear to be “KW4-like” systems, but we have found several
unusual cases:
1. Primaries of asteroid pairs being binary (or ternary)
2. Semi-wide binaries with super-critical angular momentum
3. Binaries with a second, non-synchronous rotational component
Spin-up fission asteroid systems
Primary sizes:
Largest D1 ~ 10 km
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(1052) Belgica: 10.3 ± 1.3 km (Franco et al. 2013)
(3868) Mendoza: 9.3 ± 1.0 km (Pravec et al. 2012)
Smallest D1 ~ 0.15 km
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2004 FG11: 0.15 ± 0.03 km (Taylor et al. 2012)
2003 SS84: 0.12 km (Nolan et al. 2003, no unc.)
This primary diameter range 0.15 to 10 km is the same range where we observe the spin barrier
(gravity dominated regime, predominantly cohesionless, ‘rubble-pile’ asteroid structure implied).
The upper limit on D1 seems to be because asteroids larger than ~10 km don’t get quite to the spin
barrier where they would fission; asteroid spin rates fall off from the spin barrier at D > 10 km. (Are
they too big to be spun up to the spin barrier by YORP during their lifetime? But see the talk by
Holsapple.)
The lower limit on D1 is likely because asteroids smaller than ~0.15 km are predominantly not “rubble
piles”. But the observational selection effect against detection of smaller binaries has to be checked.
Spin-up fission asteroid systems
Secondary relative sizes:
Largest D2/D1 close to 1 (“Double Asteroids”)
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(69230) Hermes, (809) Lundia, (854) Frostia, (1089) Tama, (1139) Atami, (1313) Berna, (2478)
Tokai, (4492) Debussy, (4951) Iwamoto – all D2 /D1 between 0.8 and 1
Smallest D2/D1 (observational sensitivity-limited)
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(1862) Apollo: D2/D1 ~ 0.04 (Ostro et al. 2005, unc. factor 2)
Systems with D2/D1 < ~0.4-0.5 abundant.
Decrease at D2/D1 < 0.3 and especially below 0.2
maybe observational bias.
Spin-up fission asteroid systems
Distances between components:
Shortest Porb ~ 11.9 h
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(65803) Didymos: 11.91 ± 0.02 h (Pravec et al. 2006)
2006 GY2:
11.7 ± 0.2 h (Brooks 2006)
Corresponds to a/D1 = 1.5 ± 0.2. Consistent with
the Roche’s limit for strengthless satellites at a/D1 =
1.27 (for same densities of the two bodies) that
corresponds to Porb ~ 9.5 h for the bulk density of 2
g/cm3.
Decreasing number density at Porb > 1 day
- a real decrease plus observational selection effect.
Largest separation = infinity
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many asteroid pairs
Small telescopes, but a lot of time
NEOSource project,1.54-m Danish telescope, La Silla
Study of non-gravitational asteroid evolution processes via photometric observations
PI Petr Pravec, Co-PI David Vokrouhlický
2012 October – 2016 December, remote observations on 80 nights/year with the
1.54-m telescope at La Silla
A number of other projects with 0.35-1 m telescopes.
1. Primaries of asteroid pairs being
binary (or ternary)
Primaries of asteroid pairs being
binary (or ternary)
Five cases so far:
(3749) Balam, (6369) 1983 UC, (9783) Tensho-kan, (10123) Fideoja, (80218) 1999 VO123
Similar to our other photometrically detected binaries in the main belt:
D1 = 1 to 6 km
D2/D1 = 0.23 to 0.45
P1 = 2.40 to 3.15 h
Porb = 29.5 to 56.5 h (possible lack of the closest
orbits with orbital periods < 1 day)
The unbound component (secondary of the asteroid pair):
Dsec/D1 = 0.15 to ~0.9 (four of them 0.15 to 0.35)
Age between 120 kyr and > 1 Myr (these are times before present when
geometric and Yarkovsky clones of the orbits of the two components
converge)
Another (fourth) component –distant satellite– present in (3749) Balam.
Multiple system (3749) Balam
Hierarchy:
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Primary, D1 = 4.2 km (from WISE data, unc. ~10%), P1 = 2.80 h, nearly spheroidal (A = 0.10 mag)
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Close satellite, D2/D1 = 0.45, Porb = 33.4 h (Marchis et al. 2008), moderate eccentricity
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Distant satellite, D3/D1 ≈ 0.22, Porb = 1300-3900 h, e = 0.3-0.8 (Vachier et al. 2012)
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Unbound secondary, Dsec/D1 = 0.15 (from ΔH), ~300-kyr old pair (Vokrouhlický 2009)
The inner couple (the primary + the close satellite) looks like a classical “KW4-type” binary,
also its angular momentum is close to critical (αL = 1.30 ± 0.14)
BUT
The orbit is moderately eccentric (e = 0.06) and we have not been able to fit the
available 4-apparition data (2007, 2009, 2010 and 2012) with an orbit model with apsidal
precession only – suspect non-zero inclination of the orbit wrt the primary’s equator, hence
nodal precession.
Multiple system (3749) Balam
e = 0.06 ± 0.02 (3 sigma), apsidal precession rate dϖ/dt = 0.7-1.2 deg/day.
Note that dϖ/dt = 1 deg/day corresponds to J2 = 0.10 (moderately flattened spheroid).
Paired binaries (6369) and (9783)
They look pretty much like classical (semi-)asynchronous binaries ---except for their relatively
long orbital periods--- with near-critical total angular momentum and nearly-spheroidal primary.
But we’ll look forward towards seeing more data from their return apparitions.
Paired binaries (10123) and (80218)
The second rotational period of 38.8 h in (10123) is
unusually long, probably slowed down by some process.
If it belongs to the secondary with Porb = 56.5 h, could
perhaps it be at a closer (synchronous) orbit with
Porb ≈ 38.8 h before the asteroid pair 10123-117306
formed some 1-2 Myr ago?? (But the secondary’s
spin rate might change during the pair formation too ….)
2. Semi-wide binaries with
super-critical angular momentum
Semi-wide binaries with
super-critical angular momentum
Three cases so far:
(1717) Arlon
(4951) Iwamoto
(32039) 2000 JO23
Total angular momentum content super-critical:
αL = 1.8, 2.25 and ~2.9 (uncertainties ± 0.2-0.6).
Common feature: Large satellite
D2/D1 = 0.6 to 0.9 (± 0.1)
and distant, of course (with large fraction of the angular momentum being in the orbital):
Porb = 117, 118, and 360 h
(1717) Arlon
D2/D1 ≥ 0.5
P1 = 5.15 h
P2 = 18.22 h
Porb = 117.0 h
Assuming P1 belongs to the primary
and P2 belongs to the secondary:
αL = 1.82 (unc. 25%)
Is the assumption right?
And, again, we may speculate:
Couldn’t the satellite be at a
synchronous orbit with
Porb ≈ 18 h before it was moved
to its current distant orbit??
(4951) Iwamoto
D2/D1 = 0.88 ± 0.1
P1 = Porb = 117.9 ± 0.2 h
(at least one component
is synchronous)
αL = 2.25 (unc. 25%)
No way how αL could be
close to 1.
(32039) 2000 JO23
D2/D1 ≥ 0.58
P1 = 3.30 or 6.60 h
P2 = 11.10 h
Porb = 360 h
αL ≥ 2.3
Again, no way how αL could
be close to 1.
Semi-wide binaries with
super-critical angular momentum
A: (semi-)asynchronous,
“KW4-like” binaries
B: fully synchronous,
near equal-sized binaries
(“double asteroids”)
(Pravec
Harris 2007)
Presentand
update
Semi-wide binaries with
super-critical angular momentum
3. Binaries with a second, nonsynchronous rotational component
Binaries with a second, non-synchronous
rotational component
We detected seven such cases so far:
(1830) Pogson
(Pravec et al. 2012)
(2006) Polonskaya
(Pravec et al. 2012)
(2577) Litva
(Warner et al. 2009)
(16635) 1993 QO
Binaries with a second, non-synchronous
rotational component
(2486) Metsahovi
(3982) Kastel’
(5474) Gingasen
(114319) 2002 XD58
Binaries with a second, non-synchronous
rotational component
The second, non-synchronous rotational lightcurve component observed in 7 of the
79 MBA binaries (9%) of our current binary sample.
In some cases with short Porb, the (even much shorter) P2 may actually belong to another,
probably more distant satellite (i.e., the system is ternary); the P2 lightcurve component
doesn’t disappear in total secondary events when the close satellite producing the
observed mutual events fully disappears behind the primary.
The four observed cases with two rotational components, but no mutual events, may be
relatively wide non-synchronous systems.
Conclusions
“Classical” close (semi-)asynchronous binaries (KW4-like) represent
only a, and actually the easiest observable, part of the population of
spin-up fission asteroid systems among 1-10 km sized MBAs.
Some systems apparently went formation/evolution paths leading to
more distant satellites or including ejection of a body from the system
(producing an asteroid pair with primary being binary).
Thank you