Albedos of Small Hilda Group Asteroids as Revealed by Spitzer

Abstract: We present thermal 24 $\mu$m observations from the \textit{Spitzer Space Telescope} of 62 Hilda asteroid group members with diameters ranging from 3 to 12 kilometers. Measurements of the thermal emission when combined with reported absolute magnitudes allow us to constrain the albedo and diameter of each object. From our \textit{Spitzer} sample, we find the mean geometric albedo, $p_{V} =$ 0.07 $\pm$ 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 $\pm$ 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, color-color 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.