Photometric Confirmation and Characterization of the Ennomos Collisional Family in the Jupiter Trojans

TL;DR

This study confirms and characterizes the Ennomos collisional family among Jupiter Trojans using multiband photometry, revealing unique spectral properties that inform theories of surface evolution and solar system history.

Contribution

It provides the first detailed photometric analysis of the Ennomos family, identifying its members and their spectral characteristics, and discusses implications for surface processes and Trojan origin models.

Findings

Ennomos family is confined to specific orbital parameters.

Majority of members have near-solar spectral slopes.

Distinct neutral colors suggest different surface processing mechanisms.

Abstract

Collisional families offer a unique window into the interior composition of asteroid populations. Previous dynamical studies of the Jupiter Trojans have uncovered a handful of potential collisional families, two of which have been subsequently confirmed through spectral characterization. In this paper, we present new multiband photometric observations of the proposed Ennomos family and derive precise $g-i$ colors of 75 candidate family members. While the majority of the targets have visible colors that are indistinguishable from background objects, we identify 13 objects with closely grouped dynamical properties that have significantly bluer colors. We determine that the true Ennomos collisional family is tightly confined to $a'_{p} > 5.29$ au and $0.45 < \sin{i_{p}} < 0.47$, and the majority of its confirmed members have near-solar spectral slopes, including some of the bluest objects hitherto discovered in the Trojan population. The property of distinctly neutral colors that is shared by both the Ennomos family and the previously characterized Eurybates family indicates that the spectral properties of freshly exposed surfaces in the Jupiter region are markedly different than the surfaces of uncollided Trojans. This implies that the processes of ice sublimation and space weathering at 5.2 au yield a distinct regolith chemistry from the primordial environment within which the Trojans were initially accreted. It also suggests that the Trojans were emplaced in their present-day location from elsewhere sometime after the initial population formed, which is a key prediction of recent dynamical instability models of solar system evolution.