Very Large Telescope 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*

Abstract

We report VLT spectroscopy of the interstellar comet 3I/ATLAS (C/2025~N1) from $r_{\mathrm{h}}\simeq 4.4$ to $2.85$~au using X-shooter (300--550\,nm, $R\simeq 3000$) and UVES (optical, $R\simeq 35k-80k$). The coma is dust-dominated with a fairly constant red optical continuum slope ($\sim$21--22\%/1000Å). We report detection of CN emission and also detect numerous Ni\,\textsc{ii}~lines while Fe\,\textsc{i}~remains undetected, potentially implying efficiently released gas-phase Ni. At $r_{\mathrm{h}}\simeq 3.14$~au we derive $3\sigma$ limits of $Q(\mathrm{OH})<1.48\times 10^{26}{\mathrm{s}}^{-1}$, but find no indications for [O\,\textsc{i}], C${}_2$, C${}_3$ or NH${}_2$. From our latest X-shooter measurements conducted on 2025-08-21 ($r_{\mathrm{h}}=2.85$\,au) we measure production rates of $\log Q(\mathrm{CN})=24.81\pm 0.01$ molecules s${}^{-1}$ and $\log Q$(Ni) $=23.30\pm 0.07$ atoms s${}^{-1}$, and characterize their evolution as the comet approaches perihelion.~We observe a steep heliocentric-distance scaling for the production rates $Q(\mathrm{Ni})\propto r_h^{-7.7\pm 1.0}$ and for $Q(\mathrm{CN})\propto r_h^{-6.7\pm 0.2}$, and predict a Ni--CO${}_{(2)}$ correlation if the Ni\,\textsc{ii}\ 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.

arXiv comment16 pages, 4 figures, 2 tables, published in ApJL

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• Pre-perihelion evolution of the NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS. From extreme to normal
• The Volatile Inventory of 3I/ATLAS as seen with JWST/MIRI
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• Ion abundances in the plasma tail of 3I/ATLAS show that it is N2-rich
• Dust Activity Evolution and Constraints on Icy Grains in the Interstellar Comet 3I/ATLAS