Mathematics – Probability
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dda....40.0609f&link_type=abstract
American Astronomical Society, DDA meeting #40, #6.09; Bulletin of the American Astronomical Society, Vol. 41, p.900
Mathematics
Probability
Scientific paper
Kuiper-belt binaries (KBBs) provide an invaluable window into conditions in the primordial solar system. Several mechanisms for the formation of KBBs have been proposed including; two-body collisions inside the Hill sphere of a larger body (Weidenschilling, Icarus, 160, 212, 2002); strong dynamical friction (Goldreich, et al., Nature, 420, 643, 2002); exchange reactions (Funato, et al., Nature, 427, 518, 2004) and chaos assisted capture (CAC, Astakhov, et al., MNRAS, 360, 401, 2005). The recently characterized mutual orbit of the symmetric (i.e., roughly equal mass) KBB 2001 QW322 is an outlier even among the already unusual population of Kuiper-belt binaries (Petit, et al., Science, 322, 432, 2008): the orbit is extremely large (≈ 105 km or about 30% of the Hill sphere radius), retrograde, inclined ≈ 120° from the ecliptic and with eccentricity e ≤ 0.4 (and possibly e ≤ 0.05). The large, almost circular, orbit probably precludes orbit reduction through tidal dissipation and likely also rules out strong dynamical friction (which predicts much tighter orbits), exchange (which predicts high eccentricities) and CAC (which produces non-circular orbits). While a collisional mechanism is possible, the probability of forming such a large, near-Keplerian, orbit in this way seems unlikely.
A study of non-linear dynamics of the system suggests a hybrid formation mechanism as follows: (i) CAC into a long-living orbit close to a periodic orbit in Hill's problem; (ii) stabilization by gravitational scattering; (iii) weak dynamical friction then switches the original orbit ``adiabatically'' into a large, almost circular, retrograde orbit similar to that actually observed. The sense of the orbital angular momentum behaves as an adiabatic invariant (Astakhov, et al., Nature, 423, 264, 2003).
Binaries like QW322 may be rare because they are hard to detect. Our calculations suggest that this object may be the harbinger of a larger population of such extreme binaries.
Farrelly David
Gamboa Suárez Antonio
Hestroffer Daniel
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