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

Abstract We present time series medium-band ( R ∼ 20–40) observations of the third interstellar object (ISO) 3I/ATLAS (C/2025 N1) obtained with the 7-Dimensional Telescope, which enabled spatially resolved monitoring of its gas and dust activity from 2025 July to September. The m400-band image ( λ c = 400 nm, Δ λ ≈ 25 nm) reveals an emergence of a pronounced and spatially extended CN emission at a heliocentric distance of 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 Afρ . We further separate the CN-emitting and dust-scattered components via two-dimensional surface brightness fitting into inner (dust) and outer (gas) components. We find that the outer component preserves a nearly constant profile shape, varying only in normalization, implying a relatively fast expansion of CN-bearing molecules. Together, these results reveal how a transition occurs 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 ISOs exhibit comparable activation behavior at heliocentric distances of ∼2–3 au.