Binaries among small main-belt asteroids

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

2. Binary population Porb vs D1 Porb lower limit of 11 h both for NEAs and MBAs. (Could be there closer, fully synchronous systems?) Porb 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 D1 < 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 D1 = 10 km) Data from Pravec and Harris, Icarus, 190 (2007) 250-253. Updates available on URL given in the paper.

3. Primary rotation rates for NEAs and small MB/MCs (Pravec et al. 2008, in press) Distribution of f1 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. Concentration at fast spin rates with a peak at f1 = 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.

4. But there are more similarities than differences between NEA + small close MBA binaries Similarities: • Total angular momentum close to critical. • Size ratio distribution (D2/ D1 < 0.5 mostly). • Primaries have low equatorial elongations. • Secondaries mostly synchronous and having a broader distribution of eq. elongations.

5. NEA/MBA binary similarities:1. Angular momentum content αL = Ltot/Lcritsph where Ltot is a total angular momentum of the system, Lcritsph is angular momentum of an equivalent (i.e., the same total mass and volume), critically spinning sphere. Binaries with D1≤ 10 km have αLbetween 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)

6. NEA/MBA binary similarities:2. Size ratio

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

8. 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 (Porb, P1, P2) ambiguous – may be an unsynchronous rotation of the secondary, or a rotation of a third body.

9. 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.

10. Photometrically observed binaries - examples NEA binaries: 31 + 1 ternary; 10 of them with both radar+lc, 13 of them with radar only, 9 of them with photometry only. MBA binaries with D1≤ 10 km and Porb< 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)

11. (5481) Kiuchi - a typical photometric binary MBA detection Porb = 20.90 ± 0.01 h D2/D1 = 0.33 ± 0.02 P1 = 3.6196 ± 0.0002 h A1 = 0.10 mag Secondary rotation unresolved (may have a low amplitude). Eccentricity low.

12. (7225) Huntress - a low attenuation depth detection Porb = 14.67 ± 0.01 h D2/D1 = 0.21 ± 0.02 P1 = 2.4400 ± 0.0001 h A1 = 0.11 mag

13. (3073) Kursk - usual parameters, but primary lc Porb = 44.96 ± 0.02 h D2/D1 = 0.25 ± 0.02 P1 = 3.4468 ± 0.0001 h A1 = 0.21 mag Primary’s lc shape more irregular than usual.

14. (16635) 1993 QO - a three-period case Porb = 32.25 ± 0.03 h D2/D1 ≥ 0.27 P1 = 2.2083 ± 0.0002 h, A1 = 0.17 mag P2 = 7.622 ± 0.002 h, A2 = 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.

15. (2486) Metsahovi - a two-period case (no events) P1 = 4.4518 h, A1 = 0.12 mag P2 = 2.6404 h, A2 = 0.04 mag

16. (1717) Arlon - a three-period case, longish P_orb P1 = 5.148 h P2 = 18.23 h Porb = 117.0 h D2/D1 ≥ 0.5 D1~ 9 km αL = 1.8 (unc. factor 1.25)

17. (2478) Tokai - a fully synchronous system Porb = 25.89 h D2/D1 ≥ 0.72 P1 or P2 = Porb A = 0.41 mag D1 = 8 km (±30%) αL = 1.40 (±10%)

18. (4851) Iwamoto - a (relatively) wide synchronous system Porb = 118.0 ± 0.2 h D2/D1 ≥ 0.76 P1 or P2 = Porb A = 0.34 mag D1 = 4.0 km (assuming pV = 0.20 ± 0.07 for its S-type classification) αL = 2.25 (±10%)

19. Primaries of small wide binaries detected with AO (1509) Esclangona, (3749) Balam, and (4674) Pauling have all fast rotating primaries with P1 = 3.25, 2.80, and 2.53 h, respectively, and amplitudes 0.06-0.13 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 Porb = 33.38 h has been found from photometry (Marchis et al. 2008).

20. 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 Porb ~ 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.