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 × 1026 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 log Q ( CN ) = 24.81 ± 0.01 molecules s−1 and log Q (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-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 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.