Title: $0.7-2.5~μ$m spectra of Hilda asteroids,
Abstract: The Hilda asteroids are primitive bodies in resonance with Jupiter whose
origin and physical properties are not well understood. Current models posit
that these asteroids formed in the outer Solar System and were scattered along
with the Jupiter Trojans into their present-day positions during a chaotic
episode of dynamical restructuring. In order to explore the surface composition
of these enigmatic objects in comparison with an analogous study of Trojans
(Emery et al. 2011), we present new near-infrared spectra (0.7-2.5 $\mu$m) of
25 Hilda asteroids. No discernible absorption features are apparent in the
data. Synthesizing the bimodalities in optical color and infrared reflectivity
reported in previous studies, we classify 26 of the 28 Hildas in our spectral
sample into the so-called less-red and red sub-populations and find that the
two sub-populations have distinct average spectral shapes. Combining our
results with visible spectra, we find that Trojans and Hildas possess similar
overall spectral shapes, suggesting that the two minor body populations share a
common progenitor population. A more detailed examination reveals that while
the red Trojans and Hildas have nearly identical spectra, less-red Hildas are
systematically bluer in the visible and redder in the near-infrared than
less-red Trojans, indicating a putative broad, shallow absorption feature
between 0.5 and 1.0 $\mu$m. We argue that the less-red and red objects found in
both Hildas and Trojans represent two distinct surface chemistries and
attribute the small discrepancy between less-red Hildas and Trojans to the
difference in surface temperatures between the two regions.