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*
Bibliographic record
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.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".