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, Ond řejov, Czech Republic The 3 rd Workshop on Binaries in the Solar System Hawaii, the Big Island, 2013 June 30 – July 2.
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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
Our binary asteroid parameters database (Pravec and Harris 2007, update June 2013):
We have also identified 158 asteroid pairs (Vokrouhlický and Nesvorný 2008, Pravec and Vokrouhlický 2009, Pravec et al. 2010, plus others in prep.)
Many knownbinariesappear to be “KW4-like” systems, but wehavefound several unusualcases:
Largest D1 ~ 10 km
Smallest D1~ 0.15 km
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.
Secondary relative sizes:
Largest D2/D1 close to 1 (“Double Asteroids”)
Smallest D2/D1 (observational sensitivity-limited)
Systems with D2/D1< ~0.4-0.5 abundant.
Decrease at D2/D1< 0.3 and especially below 0.2
maybe observational bias.
Distances between components:
Shortest Porb ~ 11.9 h
Corresponds to a/D1 = 1.5± 0.2. Consistent with the Roche’s limit for strengthlesssatellites 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
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)
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
Another (fourth) component –distant satellite– present in (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).
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.
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 withsuper-critical angular momentum
Three cases so far:
(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
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??
D2/D1 = 0.88 ± 0.1
P1 = Porb = 117.9 ± 0.2 h
(at least one component
αL= 2.25 (unc. 25%)
No way how αL could be
close to 1.
D2/D1 ≥ 0.58
P1 = 3.30 or 6.60 h
P2 = 11.10 h
Porb = 360 h
Again, no way how αL could
be close to 1.
B: fully synchronous,
near equal-sized binaries
(Pravec and Harris 2007)
3. Binaries with a second, non-synchronous rotational component
We detected seven such cases so far:
(Pravec et al. 2012)
(Pravec et al. 2012)
(Warner et al. 2009)
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.
“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).