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Population of small asteroid systems - We are still in a survey phase

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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

Current sample:

Our binary asteroid parameters database (Pravec and Harris 2007, update June 2013):

- 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 knownbinariesappear to be “KW4-like” systems, but wehavefound several unusualcases:

- Primaries of asteroid pairs being binary (or ternary)
- Semi-wide binaries withsuper-critical angular momentum
- Binaries with a second, non-synchronous rotational component

Primary sizes:

Largest D1 ~ 10 km

- (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

- 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.

Secondary relative sizes:

Largest D2/D1 close to 1 (“Double Asteroids”)

- (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)

- (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.

Distances between components:

Shortest Porb ~ 11.9 h

- (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 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

- many asteroid pairs

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

converge)

Another (fourth) component –distant satellite– present in (3749) Balam.

- Hierarchy:
- Primary, D1 = 4.2 km (from WISE data, unc. ~10%), P1 = 2.80 h, nearly spheroidal (A = 0.10 mag)
- Close satellite, D2/D1 = 0.45, Porb = 33.4 h (Marchis et al. 2008), moderate eccentricity
- Distant satellite, D3/D1 ≈ 0.22, Porb = 1300-3900 h, e = 0.3-0.8 (Vachier et al. 2012)
- 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.

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:

(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

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

is synchronous)

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

αL≥ 2.3

Again, no way how αL could

be close to 1.

A: (semi-)asynchronous,

“KW4-like” binaries

B: fully synchronous,

near equal-sized binaries

(“double asteroids”)

(Pravec and Harris 2007)

Present update

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).

Thank you