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

TL;DR

This study uses the 7-Dimensional Telescope to observe the emergence of CN gas in interstellar object 3I/ATLAS, revealing its transition from dust to gas activity as it approaches the Sun, with implications for understanding interstellar comet behavior.

Contribution

First spatially and temporally resolved observations of CN gas emergence in 3I/ATLAS using a novel 7D telescope, highlighting its volatile activation at ~2-3 au.

Findings

CN emission appears at r_h < 3 au

Gas activity transitions from dust-dominated to volatile-driven

Activation timing resembles that of 2I/Borisov

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.