Observational confirmation and characterization of the Ennomos collisional family in the Jupiter Trojans

The Jupiter Trojans hold the key to understanding fundamental aspects of solar system evolution. Recent models posit that the Trojans initially formed in the outer solar system before being scattered inward during a period of dynamical instability. Constraining the surface composition of Trojans serves as a powerful empirical test of these theories. Studying collisional families in particular and comparing their properties to the background population can provide crucial insights into the chemical makeup of these objects and the effects of solar irradiation on their pristine surfaces. While the overall Trojan population is known to contain two distinct color classes – the red and less-red subpopulations – analyses of the color distribution of the only hitherto confirmed Trojan collisional family – the Eurybates family – have revealed significantly bluer colors than the background population, suggesting that the present-day space weathering environment of the Trojans is markedly different than that of their primordial formation region. While these observations offer tentative support to dynamical instability models, more families must be studied before concrete conclusions about the surface colors of Trojan collisional fragments can be made. To this end, we obtained photometric observations of the purported Ennomos family. By combining multiband observations from 2015 and 2021, we measured high-precision visible colors of more than 80 out of the 104 candidate family members. Our analysis of the color distribution reveals a cluster of objects with significantly bluer characteristic spectral slope values than the background population, similar to the Eurybates family members. We also identify a number of objects that have colors more consistent with the background less-red and red Trojans and are likely interlopers. Having confirmed the Ennomos collisional family, we characterize the size distribution and dynamical age of the family members and place our findings in the broader context of Trojan formation and composition. Further study of this collisional family will greatly impact the interpretation of data from the upcoming NASA Lucy mission, which is slated to fly by Eurybates in 2027.