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 3 rd 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): 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 D 1 ~ 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 D 1 ~ 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 D 1 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 D 1 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 D 2 /D 1 close to 1 (“Double Asteroids”) (69230) Hermes, (809) Lundia, (854) Frostia, (1089) Tama, (1139) Atami, (1313) Berna, (2478) Tokai, (4492) Debussy, (4951) Iwamoto – all D 2 /D 1 between 0.8 and 1 Smallest D 2 /D 1 (observational sensitivity-limited) (1862) Apollo: D 2 /D 1 ~ 0.04 (Ostro et al. 2005, unc. factor 2) Systems with D 2 /D 1 < ~ abundant. Decrease at D 2 /D 1 < 0.3 and especially below 0.2 maybe observational bias.

Spin-up fission asteroid systems Distances between components: Shortest P orb ~ 11.9 h (65803) Didymos: ± 0.02 h (Pravec et al. 2006) 2006 GY2: 11.7 ± 0.2 h (Brooks 2006) Corresponds to a/D 1 = 1.5 ± 0.2. Consistent with the Roche’s limit for strengthless satellites at a/D 1 = 1.27 (for same densities of the two bodies) that corresponds to P orb ~ 9.5 h for the bulk density of 2 g/cm 3. Decreasing number density at P orb > 1 day - a real decrease plus observational selection effect. Largest separation = infinity 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 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: D 1 = 1 to 6 km D 2 /D 1 = 0.23 to 0.45 P 1 = 2.40 to 3.15 h P orb = 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): D sec /D 1 = 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, D 1 = 4.2 km (from WISE data, unc. ~10%), P 1 = 2.80 h, nearly spheroidal (A = 0.10 mag) Close satellite, D 2 /D 1 = 0.45, P orb = 33.4 h (Marchis et al. 2008), moderate eccentricity Distant satellite, D 3 /D 1 ≈ 0.22, P orb = h, e = (Vachier et al. 2012) Unbound secondary, D sec /D 1 = 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 ( α L = 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 d ϖ /dt = deg/day. Note that d ϖ /dt = 1 deg/day corresponds to J 2 = 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 P orb = 56.5 h, could perhaps it be at a closer (synchronous) orbit with P orb ≈ 38.8 h before the asteroid pair 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: α L = 1.8, 2.25 and ~2.9 (uncertainties ± ). Common feature: Large satellite D 2 /D 1 = 0.6 to 0.9 ( ± 0.1) and distant, of course (with large fraction of the angular momentum being in the orbital): P orb = 117, 118, and 360 h

(1717) Arlon D 2 /D 1 ≥ 0.5 P 1 = 5.15 h P 2 = h P orb = h Assuming P 1 belongs to the primary and P 2 belongs to the secondary: α L = 1.82 (unc. 25%) Is the assumption right? And, again, we may speculate: Couldn’t the satellite be at a synchronous orbit with P orb ≈ 18 h before it was moved to its current distant orbit??

(4951) Iwamoto D 2 /D 1 = 0.88 ± 0.1 P 1 = P orb = ± 0.2 h (at least one component is synchronous) α L = 2.25 (unc. 25%) No way how α L could be close to 1.

(32039) 2000 JO23 D 2 /D 1 ≥ 0.58 P 1 = 3.30 or 6.60 h P 2 = h P orb = 360 h α L ≥ 2.3 Again, no way how α L 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 P orb, the (even much shorter) P 2 may actually belong to another, probably more distant satellite (i.e., the system is ternary); the P 2 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