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Title: The colour-magnitude distribution of Hilda asteroids: Comparison with Jupiter Trojans
Author: Ian Wong, Michael E. Brown

Current models of Solar System evolution posit that the asteroid populations in resonance with Jupiter are comprised of objects scattered inward from the outer Solar System during a period of dynamical instability. In this paper, we present a new analysis of the absolute magnitude and optical colour distribution of Hilda asteroids, which lie in the 3:2 mean motion resonance with Jupiter, with the goal of comparing the bulk properties with previously published results from an analogous study of Jupiter Trojans. We report an updated power law fit of the Hilda magnitude distribution through H=14. Using photometric data listed in the Sloan Moving Object Catalogue, we confirm the previously-reported strong bimodality in visible spectral slope distribution, indicative of two sub-populations with differing surface compositions. When considering collisional families separately, we find that collisional fragments follow a unimodal colour distribution with spectral slope values consistent with the bluer of the two sub-populations. The colour distributions of Hildas and Trojans are comparable and consistent with a scenario in which the colour bimodality in both populations developed prior to emplacement into their present-day locations. We propose that the shallower magnitude distribution of the Hildas is a result of an initially much larger Hilda population, which was subsequently depleted as smaller bodies were preferentially ejected from the narrow 3:2 resonance via collisions. Altogether, these observations provide a strong case supporting a common origin for Hildas and Trojans as predicted by current dynamical instability theories of Solar System evolution.

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Title: Did the Hilda collisional family form during the late heavy bombardment?
Authors: M. Bro, D. Vokrouhlický, A. Morbidelli, D. Nesvorný, W.F. Bottke

We model the long-term evolution of the Hilda collisional family located in the 3/2 mean-motion resonance with Jupiter. Its eccentricity distribution evolves mostly due to the Yarkovsky/YORP effect and assuming that: (i) impact disruption was isotropic, and (ii) albedo distribution of small asteroids is the same as for large ones, we can estimate the age of the Hilda family to be 4_{-1}^{+0} Gyr. We also calculate collisional activity in the J3/2 region. Our results indicate that current collisional rates are very low for a 200km parent body such that the number of expected events over Gyrs is much smaller than one.
The large age and the low probability of the collisional disruption lead us to the conclusion that the Hilda family might have been created during the Late Heavy Bombardment when the collisions were much more frequent. The Hilda family may thus serve as a test of orbital behaviour of planets during the LHB. We tested the influence of the giant-planet migration on the distribution of the family members. The scenarios that are consistent with the observed Hilda family are those with fast migration time scales \simeq 0.3 Myr to 3 Myr, because longer time scales produce a family that is depleted and too much spread in eccentricity. Moreover, there is an indication that Jupiter and Saturn were no longer in a compact configuration (with period ratio P_{S}/P_{J} > 2.09) at the time when the Hilda family was created.

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Title: Albedos of Small Hilda Group Asteroids as Revealed by Spitzer
Authors: Erin Lee Ryan, Charles E. Woodward

We present thermal 24 \mu m observations from the Spitzer Space Telescope of 62 Hilda asteroid group members with diameters ranging from 3 to 12 kilometres. Measurements of the thermal emission when combined with reported absolute magnitudes allow us to constrain the albedo and diameter of each object. From our Spitzer sample, we find the mean geometric albedo, p_{V} = 0.07 ± 0.05 for small (D < 10 km) Hilda group asteroids. This value of p_{V} is greater than and spans a larger range in albedo space than the mean albedo of large (D \gtrsim 10 km) Hilda group asteroids which is p_{V} = 0.04 ± 0.01. Though this difference may be attributed to space weathering, the small Hilda group population reportedly displays greater taxonomic range from C-, D- and X-type whose albedo distributions are commensurate with the range of determined albedos. We discuss the derived Hilda size-frequency distribution, colour-colour space, and geometric albedo for our survey sample in the context of the expected migration induced "seeding" of the Hilda asteroid group with outer solar system proto-planetesimals as outlined in the "Nice" formalism.

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