Very Large Telescope Observations of Interstellar Comet 3I/ATLAS. II. From Quiescence to Glow: Dramatic Rise of Ni <scp>i</scp> Emission and Incipient CN Outgassing at Large Heliocentric Distances*

Abstract

Abstract We report Very Large Telescope spectroscopy of the interstellar comet 3I/Asteroid Terrestrial-impact Last Alert System (C/2025 N1), from r h ≃ 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 r h ≃ 3.14 au, we derive 3 σ limits of Q (OH) &lt; 1.48 × 10 26 s −1 but find no indications for [O i ], C 2 , C 3 , or NH 2 . From our latest X-Shooter measurements, conducted on 2025 August 21 ( r h = 2.85 au), we measure production rates of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mspace width="0.25em"/> <mml:mi>Q</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>CN</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>24.81</mml:mn> <mml:mspace width="0.25em"/> <mml:mo>±</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>0.01</mml:mn> </mml:math> molecules s −1 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mspace width="1em"/> <mml:mi>Q</mml:mi> </mml:math> (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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>Q</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>Ni</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>r</mml:mi> <mml:mi>h</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>7.7</mml:mn> <mml:mspace width="0.25em"/> <mml:mo>±</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>1.0</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>Q</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>CN</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>r</mml:mi> <mml:mi>h</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6.7</mml:mn> <mml:mspace width="0.25em"/> <mml:mo>±</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>0.2</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , 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.

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