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Determine the heliocentric-distance dependence of CN production in 3I/ATLAS

Determine the functional dependence on heliocentric distance, including an accurate power-law index, of the CN production rate Q(CN) in interstellar comet 3I/ATLAS, which could not be robustly constrained from the three X-shooter detections reported here.

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Background

CN (B-X) emission in 3I/ATLAS was first detected at r_h = 3.07 au, and production rates were derived using a Haser model from both X-shooter and UVES spectra.

However, only three X-shooter visits showed detectable CN, limiting the ability to fit and firmly establish the heliocentric-distance scaling of Q(CN). A tentative steep power-law index (~r_h-9.38) is suggested by the limited data, but the authors explicitly state they cannot robustly constrain the dependence.

Establishing this scaling is important for interpreting the activation mechanisms and comparing 3I/ATLAS with Solar System comets and 2I/Borisov, where production-rate dependencies on heliocentric distance inform volatile release processes.

References

With only three X-shooter visits showing detectable CN emission, we cannot robustly constrain the heliocentric distance dependence of the Q(CN) production rate.

— VLT observations of interstellar comet 3I/ATLAS II. From quiescence to glow: Dramatic rise of Ni I emission and incipient CN outgassing at large heliocentric distances (2508.18382 - Rahatgaonkar et al., 25 Aug 2025) in Section 4.2 (Time evolution of CN emission)