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arXiv · 2602.12930

Pre-perihelion Emergence of the CN Gas Coma in 3I/ATLAS Temporally and Spatially Resolved by the 7-Dimensional Telescope

Gregory S. H. Paek·Myungshin Im·Mankeun Jeong·Hyeonho Choi·Yoonsoo P. Bach·Masateru Ishiguro·Bumhoo Lim·Seo-Won Chang·Ji Hoon Kim·Jooyeon Geem·Willem B. Hoogendam
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Abstract

We present time-series medium-band (R~20-40) observations of the third interstellar object 3I/ATLAS (C/2025 N1) obtained with the 7-Dimensional Telescope (7DT), enabling spatially resolved monitoring of its gas and dust activity from 2025 July to September. The m400-band image (lambda_c = 400 nm, Delta lambda approx 25 nm) reveals the emergence of pronounced and spatially extended CN emission at heliocentric distances r_h < 3 au. This onset is consistently identified across multiple diagnostics, including a break in the light-curve evolution, excess reflectance, inward expansion of annular excess beyond 10,000-20,000 km, growth of the coma half-light radius from ~11,000 to ~19,000 km, and a rapid rise in the CN production rate Q_CN relative to Af rho. We further separate the CN-emitting and dust-scattered components through two-dimensional surface-brightness fitting into inner (dust) and outer (gas) components. The outer component preserves a nearly constant profile shape, varying only in normalization, implying relatively fast expansion of CN-bearing molecules. Together, these results reveal a transition in the optical from dust-dominated scattering at large heliocentric distances to volatile-driven, gas-dominated activity as 3I/ATLAS enters the inner Solar System. The timing and characteristics of the CN activation resemble the volatile enhancement observed in 2I/Borisov, suggesting that both known active interstellar objects exhibit comparable activation behavior at heliocentric distances of ~2-3 au.

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Gregory S. H. Paek, Myungshin Im, Mankeun Jeong, Hyeonho Choi, Yoonsoo P. Bach, Masateru Ishiguro, Bumhoo Lim, Seo-Won Chang, Ji Hoon Kim, Jooyeon Geem, Willem B. Hoogendam. 2026-02-13. Pre-perihelion Emergence of the CN Gas Coma in 3I/ATLAS Temporally and Spatially Resolved by the 7-Dimensional Telescope. https://arxiv.org/abs/2602.12930

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