Synchronous Rotation in the (120347) Salacia-Actaea System
Abstract
We report on roughly 16 years of photometric monitoring of the transneptunian binary system (120347) Salacia-Actaea which provides significant evidence that Salacia and Actaea are tidally locked to the mutual orbital period in a fully synchronous configuration. The orbit of Actaea is updated, followed by a Lomb-Scargle periodogram analysis of the ground-based photometry which reveals a synodic period similar to the orbital period and a peak-to-peak lightcurve amplitude of {\delta}m = 0.0900 {\pm} 0.0036 mag (1{\sigma} uncertainty). Incorporating archival HST photometry that resolves each component, we argue that the periodicity in the unresolved data is driven by a longitudinally varying surface morphology on Salacia, and derive a sidereal rotation period that is within 1{\sigma} of the mutual orbital period. A rudimentary tidal evolution model is invoked that suggests synchronization occurred within 1.1 Gyr after Actaea was captured/formed.
Keywords
Cite
@article{arxiv.2509.02734,
title = {Synchronous Rotation in the (120347) Salacia-Actaea System},
author = {Cameron Collyer and Estela Fernandez-Valenzuela and Jose Luis Ortiz and Bryan J. Holler and Benjamin Proudfoot and Nicolas Morales and Rafael Morales and Susan Benecchi and Flavia L. Rommel and Will Grundy and Darin Ragozzine},
journal= {arXiv preprint arXiv:2509.02734},
year = {2025}
}