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

P. Pravec, P. Scheirich, P. Kušnirák, K. Hornoch, A. GaládAstronomical Institute AS CR, Ondřejov, Czech Republic

The 3rd Workshop on Binaries in the Solar SystemHawaii, the Big Island, 2013 June 30 – July 2

Spin-up fission asteroid systemsCurrent 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 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 systemsPrimary 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.

Spin-up fission asteroid systemsSecondary 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.

Spin-up fission asteroid systemsDistances 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 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•many asteroid pairs

Small telescopes, but a lot of timeNEOSource project,1.54-m Danish telescope, La Silla

Study of non-gravitational asteroid evolution processes via photometric observationsPI 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:

•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 theavailable 4-apparition data (2007, 2009, 2010 and 2012) with an orbit model with apsidalprecession only – suspect non-zero inclination of the orbit wrt the primary’s equator, hencenodal 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 relativelylong 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 withsuper-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 ≥ 0.5P1 = 5.15 hP2 = 18.22 hPorb = 117.0 h

Assuming P1 belongs to the primaryand 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 asynchronous orbit with Porb ≈ 18 h before it was moved to its current distant orbit??

(4951) Iwamoto

D2/D1 = 0.88 ± 0.1P1 = Porb = 117.9 ± 0.2 h(at least one component is synchronous)αL = 2.25 (unc. 25%)

No way how αL could beclose to 1.

(32039) 2000 JO23

D2/D1 ≥ 0.58P1 = 3.30 or 6.60 hP2 = 11.10 hPorb = 360 h

αL ≥ 2.3

Again, no way how αL could be close to 1.

Semi-wide binaries with super-critical angular momentum

(Pravec and Harris 2007)Present update

A: (semi-)asynchronous, “KW4-like” binaries

B: fully synchronous, near equal-sized binaries (“double asteroids”)

Semi-wide binaries with super-critical angular momentum

3. Binaries with a second, non-synchronous 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 componentdoesn’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 berelatively 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