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*

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

doi:10.3847/2041-8213/ae1cbc

AI summary · arXiv: 2508.18382v2 · astro-ph.SR

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*

Rohan Rahatgaonkar, Juan Pablo Carvajal, Thomas H. Puzia, Baltasar Luco, Emmanuel Jehin, Damien Hutsemékers, Cyrielle Opitom, Jean Manfroid,

and 20 more authors

K. Aravind, Michaël Marsset, Bin Yang, Laura Buchanan, Wesley C. Fraser, John Forbes, Michele Bannister, Dennis Bodewits, Bryce T. Bolin, Matthew Belyakov, Matthew M. Knight, Colin Snodgrass, Erica Bufanda, Rosemary Dorsey, Léa Ferellec, Fiorangela La Forgia, Manuela Lippi, Brian Murphy, Prasanta K. Nayak, Mathieu Vander Donckt

Original paper · DOI 10.3847/2041-8213/ae1cbc Submitted to arXiv on 25 Aug. 2025 Published in The Astrophysical Journal Letters (10 Dec. 2025) Accepted on 07 Nov. 2025 16 pages, 4 figures, 2 tables, published in ApJL License: CC BY 4.0

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Detailed VLT spectroscopy observations of interstellar comet 3I/ATLAS (C/2025~N1) from 4.4 to 2.85 au heliocentric distance
Predominantly dust-dominated coma with red optical continuum slope
Detection of CN emission and numerous Ni\,\textsc{ii}~lines, but Fe\,\textsc{i} remains undetected
Establishment of stringent $3σ$ limits for $Q({\rm OH})<{1.48\times10^{26}}\ {\rm s^{-1}$ at 3.14 au
Decreasing production rates for CN and Ni with decreasing heliocentric distance at 2.85 au

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Original abstract

, , , , We present detailed VLT spectroscopy observations of the interstellar comet 3I/ATLAS (C/2025~N1) as it traverses from a heliocentric distance of approximately 4.4 to 2.85 astronomical units (au). Utilizing the X-shooter instrument and UVES, we investigate the composition and activity of the comet's coma. Our analysis reveals a predominantly dust-dominated coma with consistent red optical continuum slope. We report the detection of CN emission and numerous Ni\,\textsc{ii}~lines, while Fe\,\textsc{i} remains undetected, suggesting efficient release of gas-phase Ni. At a heliocentric distance of approximately 3.14 au, we establish stringent $3σ$ limits for $Q({\rm OH})<{1.48\times10^{26}}\ {\rm s^{-1}}$, and observe no clear signals for [O\,\textsc{i}], C$_2$, C$_3$, or NH$_2$. Our latest measurements on August 21, 2025, at a heliocentric distance of 2.85 au reveal production rates for CN and Ni that decrease with decreasing heliocentric distance. This scaling suggests a potential correlation between Ni and CO$_{(2)}$ if driven by carbonyl formation. Energetic considerations indicate that direct sublimation is unlikely to explain our observations; instead, low-activation-energy release mechanisms may be responsible for the observed behavior. These findings provide valuable insights into the volatile composition and activity dynamics of interstellar comet 3I/ATLAS as it continues its journey through our solar system. Future coordinated monitoring efforts will be crucial in further investigating these hypotheses as the comet becomes more active during its passage through our celestial neighborhood.

Introduction

In recent years, the discovery of interstellar objects passing through our solar system has sparked great interest and excitement in the scientific community. These objects provide a unique opportunity to study material from outside our own solar system and gain insights into their composition and origins. One such object is 3I/ATLAS (C/2025~N1), an interstellar comet that was first detected by the Asteroid Terrestrial-impact Last Alert System (ATLAS) on December 28, 2019.

Since its discovery, 3I/ATLAS has been closely monitored by astronomers using various instruments and techniques to gather as much information as possible about this rare visitor.

Observations with VLT Spectroscopy

The team of researchers utilized two spectrographs on the Very Large Telescope (VLT) in Chile - X-shooter and UVES - to study the composition and activity of 3I/ATLAS as it traveled from a heliocentric distance of approximately 4.4 astronomical units (au) to 2.85 au.

Their analysis revealed that the comet's coma is predominantly dust-dominated with a consistent red optical continuum slope. This indicates that dust particles are responsible for most of the observed emission from the comet's coma.

Detection of CN Emission and Ni\,\textsc{ii} Lines

One significant finding from these observations is the detection of CN emission lines in the comet's spectrum. CN (cyanide) is a volatile compound commonly found in comets, but its presence in interstellar objects provides valuable information about their origins.

Moreover, numerous Ni\,\textsc{ii} lines were also detected in the comet's spectrum. This is significant because it suggests efficient release of gas-phase nickel (Ni) from the comet. The absence of Fe\,\textsc{i} lines further supports this, indicating that Ni is being released at a higher rate than iron (Fe).

Limits on OH Production and No Clear Signals for Other Volatiles

The team also set stringent limits for the production rate of hydroxyl (OH), with a maximum value of $Q({\rm OH})<{1.48\times10^{26}}\ {\rm s^{-1}}$. This indicates that the comet's activity is not driven by sublimation of water ice, as previously thought.

Additionally, no clear signals were observed for other volatiles such as [O\,\textsc{i}], C$_2$, C$_3$, or NH$_2$. These findings suggest that 3I/ATLAS may have a different volatile composition compared to other comets in our solar system.

Decreasing Production Rates with Decreasing Heliocentric Distance

As 3I/ATLAS continues its journey through our solar system, its heliocentric distance decreases. The latest measurements taken on August 21, 2025, at a distance of 2.85 au revealed decreasing production rates for CN and Ni.

This decrease in production rates suggests a potential correlation between Ni and CO$_{(2)}$ if driven by carbonyl formation. Further investigations are needed to confirm this hypothesis.

Possible Mechanisms Responsible for Activity Dynamics

One interesting aspect highlighted in this research paper is the possible mechanisms responsible for the observed activity dynamics of interstellar comet 3I/ATLAS. The team ruled out direct sublimation as an explanation due to energetic considerations.

Instead, they propose low-activation-energy release mechanisms such as photolysis or chemical reactions within the dust particles themselves may be responsible for releasing volatiles from the comet's surface.

Implications and Future Studies

The detailed observations of 3I/ATLAS presented in this research paper provide valuable insights into the volatile composition and activity dynamics of interstellar objects. This information can help us better understand their origins and evolution.

As 3I/ATLAS continues its journey through our solar system, future coordinated monitoring efforts will be crucial in further investigating these hypotheses. The comet is expected to become more active as it approaches the Sun, providing even more opportunities for scientists to study its behavior and composition.

Conclusion

In conclusion, the VLT spectroscopy observations of interstellar comet 3I/ATLAS have provided valuable insights into its volatile composition and activity dynamics. The detection of CN emission lines and efficient release of gas-phase Ni are significant findings that shed light on the origins of this rare visitor.

Further investigations are needed to confirm the proposed mechanisms responsible for the observed activity dynamics. With continued monitoring efforts, we can expect to learn more about this fascinating interstellar object as it makes its way through our celestial neighborhood.

Created on 20 Apr. 2026

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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*

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