Binaries among small main-belt asteroids Petr Pravec Astronomical Institute AS CR, Czech Republic Workshop on Binaries Paris-Meudon, 2008 May 19-22

Binary population P orb vs D 1 Data from Pravec and Harris, Icarus, 190 (2007) Updates available on URL given in the paper. P orb lower limit of 11 h both for NEAs and MBAs. (Could be there closer, fully synchronous systems?) P orb has a tail into the range >100 h for small MBAs (the low observed number there is an observational selection effect) but not for NEAs. High abundance (fraction) of binaries in D 1 < 10 km, but much lower above. Binary fraction 15 ± 4 % among NEAs (Pravec et al. 2006), similar or maybe even higher fraction among MBAs (up to D 1 = 10 km)

Primary rotation rates for NEAs and small MB/MCs Concentration at fast spin rates with a peak at f 1 = 9-10 d -1, coincides with an excess of rotation rates seen in the f- distribution for all NEAs – the excess appears to be due to binaries. Distribution of f 1 much broader, most of them in the range 6-10 d -1. If both NEA and small MBA binaries formed at the spin barrier, then MBA binaries are more evolved than NEA binaries. (Pravec et al. 2008, in press)

But there are more similarities than differences between NEA + small close MBA binaries Similarities: 1.Total angular momentum close to critical. 2.Size ratio distribution (D 2 / D 1 < 0.5 mostly). 3.Primaries have low equatorial elongations. 4.Secondaries mostly synchronous and having a broader distribution of eq. elongations.

NEA/MBA binary similarities: 1. Angular momentum content α L = L tot /L critsph where L tot is a total angular momentum of the system, L critsph is angular momentum of an equivalent (i.e., the same total mass and volume), critically spinning sphere. Binaries with D 1 ≤ 10 km have α L between 0.9 and 1.3, as expected for systems originating from critically spinning rubble piles, if no large amount of angular momentum was added or removed since formation of the system. (Pravec and Harris 2007)

NEA/MBA binary similarities: 2. Size ratio

NEA/MBA binary similarities: 3. Primary component shapes Primaries of asynchronous binaries have low equatorial elongations both among NEAs and small MBAs. Model of the primary of 1999 KW4 (Ostro et al. 2006)

NEA/MBA binary similarities: 4. Secondaries Broader range of equatorial elongations: a/b= 1:1 to 2:1. Some synchronous, but some may not be; interpretation of a third period (P orb, P 1, P 2 ) ambiguous – may be an unsynchronous rotation of the secondary, or a rotation of a third body.

Orbit poles – few data so far Good data covering long enough “arc” (range of geometries) for a few NEA binaries only (Scheirich 2008, PhD thesis). Observations of binaries in their return apparitions needed to constrain orbit pole distribution.

Photometrically observed binaries - examples NEA binaries: ternary; 10 of them with both radar+lc, 13 of them with radar only, 9 of them with photometry only. MBA binaries with D 1 ≤ 10 km and P orb < 20 d: 45 (all from LCs, one of them marginally resolved with radar) + 1 detection of a close satellite in asteroid (3749 Balam) with a distant satellite discovered in 2002 with AO (i.e., ternary system)

(5481) Kiuchi - a typical photometric binary MBA detection P orb = ± 0.01 h D 2 /D 1 = 0.33 ± 0.02 P 1 = ± h A 1 = 0.10 mag Secondary rotation unresolved (may have a low amplitude). Eccentricity low.

(7225) Huntress - a low attenuation depth detection P orb = ± 0.01 h D 2 /D 1 = 0.21 ± 0.02 P 1 = ± h A 1 = 0.11 mag

(3073) Kursk - usual parameters, but primary lc P orb = ± 0.02 h D 2 /D 1 = 0.25 ± 0.02 P 1 = ± h A 1 = 0.21 mag Primary’s lc shape more irregular than usual.

(16635) 1993 QO - a three-period case P orb = ± 0.03 h D 2 /D 1 ≥ 0.27 P 1 = ± h, A 1 = 0.17 mag P 2 = ± h, A 2 = 0.05 mag The 7.6-h period is assumed to be a rotation period of the secondary, but it might be also a rotation of a third body.

(2486) Metsahovi - a two-period case (no events) P 1 = h, A 1 = 0.12 mag P 2 = h, A 2 = 0.04 mag

(1717) Arlon - a three-period case, longish P_orb P 1 = h P 2 = h P orb = h D 2 /D 1 ≥ 0.5 D 1 ~ 9 km α L = 1.8 (unc. factor 1.25)

(2478) Tokai - a fully synchronous system P orb = h D 2 /D 1 ≥ 0.72 P 1 or P 2 = P orb A = 0.41 mag D 1 = 8 km (±30%) α L = 1.40 (±10%)

(4851) Iwamoto - a (relatively) wide synchronous system P orb = ± 0.2 h D 2 /D 1 ≥ 0.76 P 1 or P 2 = P orb A = 0.34 mag D 1 = 4.0 km (assuming p V = 0.20 ± 0.07 for its S-type classification) α L = 2.25 (±10%)

Primaries of small wide binaries detected with AO (1509) Esclangona, (3749) Balam, and (4674) Pauling have all fast rotating primaries with P 1 = 3.25, 2.80, and 2.53 h, respectively, and amplitudes mag (Warner 2005, Marchis et al. 2008, Warner et al. 2006). Their distant satellites have orbital periods on an order of 100 days. In (3749), another, close satellite with P orb = h has been found from photometry (Marchis et al. 2008).

Conclusions NEA and small close MBA binaries are suggested to be formed by same or similar mechanism(s) causing fission of critically spinning asteroids at the spin barrier. Differences between NEA and small close MBA binaries suggest that small close MBA binaries are more evolved than NEA binaries. Systems with orbital periods shorter than 60 hours have a total angular momentum content close to the critical limit for a single body in a gravity regime, but a couple systems with P orb ~ 118 h have a higher total angular momentum. Small wide binaries detected with AO have primaries pretty similar (and one has even another, close satellite) to primaries of close binary systems.