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

We report VLT spectroscopy of the interstellar comet 3I/ATLAS (C/2025 N1) from r_h≃4.4 to 2.85 au using X-shooter (300-550 nm, R≃3000) and UVES (optical, R≃35k−80k).

The coma is dust-dominated with a fairly constant red optical continuum slope (∼21-22%/1000Å). At r_h≃3.17 au we derive 3σ limits of Q(OH)<7.76×1023 s⁻¹, but find no indications for [O I], C₂, C₃ or NH₂.

We report detection of CN emission and also detect numerous Ni I lines while Fe I remains undetected, potentially implying efficiently released gas-phase Ni. From our latest X-shooter measurements conducted on 2025-08-21 (rh=2.85,au) we measure production rates of log Q(CN)=23.61±0.05 molecules s−1 and log Q(Ni) =22.67±0.07 atoms s−1, and characterize their evolution as the comet approaches perihelion.

We observe a steep heliocentric-distance scaling for the production rates Q(Ni)∝r−8.43±0.79h and for Q(CN)∝r−9.38±1.2h, and predict a Ni-CO(2) correlation if the Ni I emission is driven by the carbonyl formation channel.

Energetic considerations of activation barriers show that this behavior is inconsistent with direct sublimation of canonical metal/sulfide phases and instead favors low-activation-energy release from dust, e.g. photon-stimulated desorption or mild thermolysis of metalated organics or Ni-rich nanophases, possibly including Ni-carbonyl-like complexes.

These hypotheses are testable with future coordinated ground-based and space-based monitoring as 3I becomes more active during its continued passage through the solar system.

Rohan Rahatgaonkar, Juan Pablo Carvajal, Thomas H. Puzia, Baltasar Luco, Emmanuel Jehin, Damien Hutsemékers, Cyrielle Opitom, Jean Manfroid, Michaël Marsset, Bin Yang, Laura Buchanan, Wesley C. Fraser, John Forbes, Michele Bannister, Dennis Bodewits, Bryce T. Bolin, Matthew Belyakov, Matthew M. Knight, Colin Snodgrass, Erica Bufanda, Rosemary Dorsey, Léa Ferellec, Fiorangela La Forgia, Manuela Lippi, Brian Murphy, Prasanta K. Nayak, Mathieu Vander Donckt

Comments: 12 pages, 3 figures, 2 tables, submitted to ApJL
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2508.18382 [astro-ph.SR] (or arXiv:2508.18382v1 [astro-ph.SR] for this version)
https://doi.org/10.48550/arXiv.2508.18382
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Submission history
From: Thomas H. Puzia
[v1] Mon, 25 Aug 2025 18:15:44 UTC (2,178 KB)
https://arxiv.org/abs/2508.18382

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