Multi-year Ground-Based Survey Photometry of Active Comet 103P/Hartley 2 and Centaur (2060) Chiron: A Tale of Two Comets in the Pre-LSST Era

Abstract

Comets and Centaurs trace the evolution of trans-Neptunian objects (TNOs) into the inner solar system. Their activity reflects the interplay between volatile sublimation, dust dynamics, and ring scattering. Yet the long-term behavior of individual objects is less constrained. To probe this evolutionary transition, we use wide-field survey photometry from the Asteroid Terrestrial-impact Last Alert System, Zwicky Transient Facility, and Las Cumbres Observatory observations of the Jupiter-family comet (JFC) 103P/Hartley 2 during its 2023/24 apparition, and the Centaur (2060) Chiron across 2020-2025, including its 2021 outburst. For 103P, heliocentric activity slopes are asymmetric about perihelion, with a steep inbound index ($n_{r,\mathrm{pre}}=-3.48\pm 0.08$) and flatter outbound value ($n_{r,\mathrm{post}}=-1.16\pm 0.04$), consistent with enhanced relative dust contribution post-perihelion. Reduced brightness versus prior apparitions matches reported secular fading trends. Dust mass-loss rates are $\sim 4$-16 kg s${}^{-1}$ for assumed grain properties. Colors exhibit a blueward trend near perihelion, consistent with enhanced gas contamination of the $g$-band, with possible phase-dependent scattering. A periodogram recovers a $\sim 18.7$ hr activity-linked period near perihelion. For Chiron, subtracting a quiescent baseline reveals exponential decay from the 2021 outburst on a $\sim 1.4$ yr timescale. Seasonal phase curves flatten from ${\hat{I}}_o^2=0.150\pm 0.034$ mag deg${}^{-1}$ in 2021 to $\lesssim 0.09$ mag deg${}^{-1}$ by 2023-2025, converging with quiescent behavior. Broad-band colors remain unchanged at ATLAS ($c-o$)=0.22$\pm$0.09 mag. This extended activity suggests a new epoch of persistent, low-level activity and/or evolving ring-scattering. These objects bracket the TNO-to-JFC evolutionary sequence, with 103P near the volatile-depleted end, and Chiron still volatile-rich and capable of episodic activity.

32 pages, 12 figures, 8 tables, accepted for publication in PSJ