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*
Rohan Rahatgaonkar, Juan Pablo Carvajal, Thomas H. Puzia, Baltasar Luco, Emmanuël Jehin, Damien Hutsemékers, Cyrielle Opitom, Jean Manfroid, K. Aravind, Michaël Marsset, Bin Yang, Laura E. Buchanan, Wesley C. Fraser, John C. Forbes, Michele T. Bannister, Dennis Bodewits, Bryce Bolin, Matthew Belyakov, Matthew M. Knight, C. Snodgrass, Erica Bufanda, Rosemary C. Dorsey, Léa Ferellec, Fiorangela La Forgia, Manuela Lippi, Brian Murphy, Prasanta K. Nayak, Mathieu Vander Donckt
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) < 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.