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 Very Large Telescope spectroscopy of the interstellar comet 3I/Asteroid Terrestrial-impact Last Alert System (C/2025 N1), from rh ≃ 4.4 to 2.85 au, using X-Shooter (300─550 nm, R ≃ 3000) and the Ultraviolet and Visual Echelle Spectrograph (optical, R ≃ 35─80 k). The coma is dust-dominated, with a fairly constant red optical continuum slope (∼21%─22%/1000 Å). We report the detection of CN emission and also detect numerous Ni I lines, while Fe I remains undetected, potentially implying efficiently released gas-phase Ni. At rh ≃ 3.14 au, we derive 3σ limits of Q(OH) < 1.48 × 10^26 s^−1 but find no indications for [O I], C2, C3, or NH2. From our latest X-Shooter measurements, conducted on 2025 August 21 (rh = 2.85 au), we measure production rates of logQ(CN)=24.81±0.01 molecules s^−1 and logQ(Ni) = 23.30 ± 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)∝rh−7.7±1.0 and Q(CN)∝rh−6.7±0.2, and we predict an Ni─CO2)</SUB> 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 the 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 will be testable with future coordinated ground-based and space-based monitoring, as 3I becomes more active during its continued passage through the solar system.