$0.7-2.5~\mu$m spectra of Hilda asteroids

TL;DR

This study presents near-infrared spectra of 25 Hilda asteroids, revealing two distinct sub-populations with different spectral characteristics, and suggests a shared origin with Jupiter Trojans despite surface chemistry differences.

Contribution

First comprehensive near-infrared spectral analysis of Hilda asteroids, identifying two sub-populations and comparing their properties with Jupiter Trojans to infer common origins.

Findings

Hilda asteroids have no discernible absorption features in 0.7-2.5 μm spectra.

Two sub-populations (less-red and red) have distinct spectral shapes.

Hildas and Trojans share similar overall spectral shapes, indicating a common progenitor.

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.