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*

We report VLT spectroscopy of the interstellar comet 3I/ATLAS (C/2025~N1) from $r_{\mathrm{h}}\text{\hspace{0.17em}⁣}\simeq \text{\hspace{0.17em}⁣}4.4$ to $2.85$~au using X-shooter (300–550,nm, $R\text{\hspace{0.17em}⁣}\simeq \text{\hspace{0.17em}⁣}3000$) and UVES (optical, $R\text{\hspace{0.17em}⁣}\simeq \text{\hspace{0.17em}⁣}35k-80k$). The coma is dust-dominated with a fairly constant red optical continuum slope ($\sim$21–22%/1000Å). We report detection of CN emission and also detect numerous Ni,$\text{textsc}ii$~lines while Fe,$\text{textsc}i$~remains undetected, potentially implying efficiently released gas-phase Ni. At $r_{\mathrm{h}}\text{\hspace{0.17em}⁣}\simeq \text{\hspace{0.17em}⁣}3.14$~au we derive $3\sigma$ limits of $Q(\mathrm{O}\mathrm{H})<1.48\times {10}^{26}{\mathrm{s}}^{-1}$, but find no indications for [O,$\text{textsc}i$], 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 $\log Q(\mathrm{C}\mathrm{N})=24.81\pm 0.01$ 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 $Q(\mathrm{N}\mathrm{i})\propto r_h^{-7.7\pm 1.0}$ and for $Q(\mathrm{C}\mathrm{N})\propto r_h^{-6.7\pm 0.2}$, and predict a Ni–CO${}_{(2)}$ correlation if the Ni,$\text{textsc}ii$\ 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.