Trends in characteristics of small NEA and MBA binaries Petr Pravec Astronomical Institute AS CR, Czech Republic Workshop on Binaries in the Solar System Steamboat Springs, CO, 2007 August 20-23

Binary systems among small asteroids Abundant binary population (15 ± 4% for NEAs, Pravec et al ) observed among asteroids with D ≤ 10 km everywhere we have looked thoroughly enough. NEA binaries – photometry since 1997 ( Pravec et al.,....) and radar since 2000 ( Margot et al., Ostro et al., Benner et al.,....). Small close MBA binaries – photometry since 2002 ( Ryan et al., Warner et al., Pravec et al.,....); P orb < 5 days, a/D 1 < 10; detection probability nearly zero for wider systems. Small wide MBA binaries – AO/HST since 2002 ( Merline et al., Tamblyn et al.); P orb > 10 days, a/D 1 > 15; lower detection limit at ang. sep. ~0.2” for typical size (brightness) ratio between components.

Binary asteroid photometric surveys Photometric surveys have produced nearly half of NEA binaries, and most small MB binaries. NEA binary survey ( ) and the BinAstPhotSurvey (since 2004): Pravec et al., Pray et al., Warner et al., Higgins et al., Reddy et al., Cooney et al., Kusnirak et al., Jakubik et al., Gajdos et al. Additional surveys: Ryan et al., Behrend et al., Krugly et al., Kryszczynska et al. Now we have data on periods -rotation and orbital- plus additional information (H, rough shape information, taxonomy) for 52 small binary systems, major part of them from photometric measurements. Data published in Pravec and Harris, Icarus, 190 (2007) Available on-line on URL given in the paper.

Photometric binary detection Full (regular) binary detection – mutual events (occultations/eclipses) detected and a solution for P orb, P 1, (P 2 ), and D 2 /D 1 obtained (plus additional information, e.g., H, rough estimates of equatorial elongations). Any other case (without mutual events solved) is NOT a regular binary detection. Probable (but not full) binary detection – two additive rotational components detected; we can assume that they belong to primary and secondary, but without events seen, we cannot tell much on parameters of the system. Binary non-detection – lightcurve fitted with a single period.

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. Differences: 1.NEA binaries concentrate at D 1 < 2 km, while MBA binaries are abundant up to ~10 km. 2.MBA binaries have a broader distribution of periods; NEA binaries are less evolved, or wider systems among them have been eliminated.

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. Mostly synchronous, but some not. Resolved rotation periods of non-synchronous secondaries are in the range 4–18 h.

NEA/MBA binary differences: 1. Size limit for NEAs Sisyphus with D ~ 9 km, if indeed binary, should have size ratio <0.1, if the apparent limit on secondary sizes applies.

NEA/MBA binary differences: 2. Period distribution NEA primaries concentrate in the pile up at f around 9-10 d -1 (P of 2-3 h) in front of the spin barrier. MBA primaries have a considerably broader distribution of spin rates, with a lower concentration at fast spin rates and a more pronounced tail (correlated with D). MBA binaries may be more evolved than NEA binaries, if all have formed near the spin barrier. NEAs: MBAs:

NEA + small close MBA binaries Similarities: 1.Total angular momentum close to critical – suggests binary formation from single bodies at the spin barrier, but no prominent change of angular momentum after formation; is YORP significantly limited after binary formation? 2.Size ratio distribution (D 2 / D 1 < 0.5 mostly). 3.Primaries have low equatorial elongations – is it a product of formation, or a condition for long term stability of a close asynchronous binary with relatively large secondary? 4.Secondaries mostly synchronous and having a broader distribution of eq. elongations – a relatively rapid synchronization mechanism required; classical tides are not fast enough if NEA binaries are young. Differences: 1.NEA binaries concentrate at D 1 < 2 km, while MBA binaries are abundant up to ~10 km – explained if NEA binaries have short lifetimes (1-2 Myr, limited by disruptions during close approaches to Earth and Venus, Walsh and Richardson 2006); binary MBAs transferred to near-Earth orbits don’t survive long, and only smaller NEAs have short enough YORP spin up time scales wrt lifetime of NEAs (~10 Myr, Gladman et al. 2000). 2.MBA binaries have a broader distribution of periods; NEA binaries are less evolved, or wider systems among them have been eliminated - is it a result of “shaping” the NEA binary population with close approaches to terrestrial planets (strong dependence of lifetime of NEA binaries on relative separation), or are longer living MBA binaries more evolved?

Intermediate MB binaries 1717 Arlon: D 1 ~ 9 km, D 2 /D 1 ≥ 0.5, P orb = 117 h, P 1 = h, P 2 = h, α L ~ 1.8 (unc. factor 1.25) (Cooney et al. 2006) 4951 Iwamoto: D 1 = 4 km (assuming p V = 0.20 ± 0.07 for its S type classification), D 2 /D 1 = 0.88 ± 0.1, P 1 = P 2 = P orb = 118 h, α L = 2.25 (±10%) (Reddy et al. 2007)

Conclusions NEA and small close MBA binaries are suggested to belong to the same population formed by a mechanism causing fission of critically spinning asteroids at the spin barrier. Differences between the NEA and small close MBA binaries are consistent with the NEA binary population being controlled by close approaches to Earth and Venus. Binaries with intermediate separations (1998 ST27, 1717 Arlon, 4951 Iwamoto) show distinct characteristics, they may be “excited” systems originated in the population of NEA/small close MBA binaries, but excitation mechanism unknown. Systems with P orb = 5-10 d are effectively undetectable with current techniques.

Thank you.

(Additional slides for possible discussion follow)

Time scales for small binaries Lifetimes of asteroids: NEA: ~10 Myr (Gladman et al. 2000) MBA: ~300 Myr (1-km asteroid) YORP spin up time scale: NEA: ~1*D 2 [*Myr/km 2 ] -> ~1 Myr (1-km asteroid) MBA: ~3*D 2 [*Myr/km 2 ] -> ~30 Myr (3-km asteroid) Lifetime of NEA binary: 1-2 Myr (limited by disruptions during close approaches to Earth and Venus; Walsh and Richardson 2006) Lifetime of MB binary: ~300 Myr (= lifetime of an MBA of size of the secondary, if it is controlled by asteroidal collisions in the main belt). The estimated short lifetime of NEA binaries suggests that few MB binaries survived since transfer to NEA orbits; most NEA binaries may have formed after transfer to near-Earth space. It may explain the observation that NEA binaries concentrate at sizes <2 km (Pravec et al. 2006); larger NEAs may not have enough time to form binaries. The strong dependence of lifetime of NEA binaries on relative separation of components may be an explanation (alternative to that NEA binaries may be less evolved by a “tidal mechanism”) for the observation that they have a narrower distribution of periods, concentrating at faster spin rates in front of the spin barrier.

Primary rotation vs size

Photometric detection of Asynchronous Binary