Complex asteroid systems Asteroid pairs containing binaries and triples Petr Pravec Co-Is: P. Scheirich, P. Kušnirák, K. Hornoch, A. Galád Astronomical Institute AS CR, Ondřejov, Czech Republic IAU Symposium 318 Hawai’i, 2015 August 5

Asteroid pairs Vokrouhlický and Nesvorný (2008) found a population of pairs of asteroids residing on very similar orbits. They showed that the pairs cannot be random, but they must be genetically related. Pravec and Vokrouhlický (2009) extended the analysis and found numerous significant pairs. Backward integrations and spectral observations (e.g., Polishook et al., Moskovitz et al.) of pair components confirmed their relation.

Asteroid pairs – products of spin-up fission Pravec et al. (2010) studied a sample of 32 asteroid pairs and they found a correlation between the primary rotational period and the secondary-to-primary mass ratio. They interpreted it as a result of transfer of the rotational angular momentum to the orbital angular momentum of the finally ejected secondary, following a theory of spin-up fission of cohesionless asteroids proposed by Scheeres (2007). Our calculations assumed that there were only *one* secondary formed on a transient orbit around the primary after the fission and before secondary ejection. However, now we know that some members of asteroid pairs are actually binary or triple systems. (Pravec et al. 2010)

Multiple system of (3749) Balam A distant satellite of the main-belt asteroid (3749) Balam was discovered by Merline et al. (2002). A close satellite of Balam was discovered by Marchis et al. (2008). An asteroid paired with Balam, now designated (312497) 2009 BR60, was identified by Vokrouhlický (2009). 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 e = 0.06 Distant satellite, D 3 /D 1 ≈ 0.22, P orb = h, e = (Vachier et al. 2012) Unbound secondary (312497), D unb /D 1 = 0.15 (from ΔH), separated from (3749) Balam about 300 kyr ago (Vokrouhlický 2009)

Survey for binaries among paired asteroids We run a photometric survey for binaries among primaries of asteroid pairs with the 1.54-m telescope on La Silla since October Supporting observations are taken with smaller telescopes at Ondřejov and collaborating stations. A proper observational strategy for resolving binarity (by detecting mutual events superimposed to the primary rotational lightcurve) is used.

Asteroid pairs with bound secondaries We know 10 now (in a sample of 72 pairs) Paired binary/ternaryPairDiscoveryRefs (3749) Balam Merline et al. (2002), Marchis et al. (2008), Vokr. (2009) (6369) 1983 UC UY Pravec et al. (this work) (8306) Shoko SR Pravec et al. (2013) (9783) Tensho-kan Pravec et al. (this work) (10123) Fideoja Pravec et al. (this work) (21436) Chaoyichi YK392014Pravec et al. (this work) (26416) 1999 XM susp. by Polishook (2014) confirmed by Pravec et al. (2015) (43008) 1999 UD TM682014Pravec et al. (this work) (44620) 1999 RS Pravec et al. (this work) (80218) 1999 VO Pravec et al. (this work)

Trends in properties of paired binaries/triples

Primary rotations Primaries of the 10 paired binaries/triples have P 1 from 2.40 to 3.35 h. They are on the high end of the distribution of spin rates of primaries of asteroid pairs. They are also in the upper half of the distribution of spin rates of primaries of similar binary asteroids in the MBA and NEA background population, which have P 1 from 2.2 up to 5 hours. Asteroid systems with paired binaries/triples tend to have a higher-than-average (for ordinary asteroid pairs as well as for binaries in the background asteroid population) total angular momentum content.

Primary shapes Low amplitudes of the their rotational lightcurves indicate nearly spheroidal shapes of the primaries of paired binaries/ternaries – the same feature as observed for primaries of binaries in the background MBA/NEA population. However, it is significant that most, if not all pairs with P 1 < 3.5 h and primary amplitudes ≤ 0.12 mag (a 1 /b 1 ≤ 1.12) have bound secondaries around their primaries.

Orbital periods (of inner satellites) There is a possible tendency of paired binaries to have longer orbital periods than the median (or the mode) for binaries in the background population of MB asteroids, but this needs to be confirmed on a larger sample.

D 2 / D 1 (bound secondary-to-primary size ratios) The bound secondaries of paired binaries (or the inner satellites of paired ternaries) have D 2 / D 1 in a range of 0.35 ± The lack of bound secondaries with D 2 / D 1 < 0.25 may be an observational bias; bound secondaries of asteroid pairs may have a similar relative size distribution as those of binaries in the background MB asteroid population.

Bound vs unbound secondary sizes The unbound secondaries tend to be of the same size or smaller than the bound secondaries. Does it suggest that when there were two secondaries around the primary at some time in the past, the smaller one was typically ejected? An exception is the pair that has an anomalously large unbound secondary with D unb / D 1 = 0.93 ± It required an additional source or supply of angular momentum than provided by rotational fission of a cohesionless rubble-pile original asteroid to be ejected.

Additional properties Following parameters and characteristics were obtained for some of the paired binaries only. Three of the ten paired binaries have synchronous secondary rotations: (8306) Shoko, (44620) 1999 RS43 and (80218) 1999 VO123. Secondary rotations of the other seven have not been constrained. Two of the ten paired binaries have a non-zero eccentricity of : (3749) Balam and (21436) Chayoichi. The other eight are consistent with circular orbits (though they could have small eccentricities too, just unresolved yet). One to three of the ten paired binaries are triple systems: (3749) Balam is a confirmed triple, having a larger close and a smaller distant satellite, and (8306) Shoko and (10123) Fideoja are suspect triples as they show additional rotational lightcurve components.

Ages of the bound and unbound secondaries

Unbound secondary ages Backward integrations by J. Žižka and D. Vokrouhlický suggest following ages: Paired binary/ternaryPairTime (kyr) since separation (unc. factor 1.2-2) (3749) Balam (6369) 1983 UC UY (8306) Shoko SR (9783) Tensho-kan (10123) Fideoja (21436) Chaoyichi YK39 70 (26416) 1999 XM (43008) 1999 UD TM (44620) 1999 RS (80218) 1999 VO

Bound secondary ages Constraints obtained from the observation that the bound secondaries of (8306) Shoko, (44620) 1999 RS43 and (80218) 1999 VO123 are in synchronous rotation. The tidal synchronization time scale (from Goldreich and Sari 2009): For the three synchronous binaries, we estimate a/R 1 = 6.6, 6.2 and 6.2. We assume ω d = 7.5*10 -4 s (for bulk density 2 g cm -3 ). We assume Q = 10 1 to For the Love number, Goldreich and Sari (2009) give k rubble <~ R/km. For the three synchronous binaries, we estimate R 2 = 0.55, 0.33 and 0.14 km, which gives k 2 <~ 1*10 -6 to 5*10 -6 ; we assume k 2 = for all the three. This gives an estimated τ sync ~ 2*10 7 yr. The three bound secondaries are in their orbits for longer times.

Bound secondaries older than the unbound ones? For the three systems, the unbound secondaries separated ~1-8*10 5 yr ago, while the observed synchronous rotations of the bound secondaries suggest that they are in their orbits for >~ 2*10 7 yr. The bound secondaries might be formed in an earlier fission event (but could their orbits remain unchanged during the process of secondary ejection after a recent fission event? Note their D unb /D 2 = 0.6, 1.1 and 2.9!), OR their tidal synchronization was much faster than thought so far (Q/k lower by at least 1-2 orders of magnitude than suggested by the theory), OR the unbound secondaries were not ejected quickly after fission, but they separated from the system after spending a longer time in orbit around the primary. Data on the complex asteroid systems containing both bound and unbound secondaries are going to provide important constraints on the processes of spin- up fission and subsequent evolution of rubble pile asteroids.

Thank you

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 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).