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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2025
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| author | Rahatgaonkar, Rohan Carvajal, Juan Pablo Puzia, Thomas H. Luco, Baltasar Jehin, Emmanuel Hutsemékers, Damien Opitom, Cyrielle Manfroid, Jean Aravind, K. Marsset, Michaël Yang, Bin Buchanan, Laura Fraser, Wesley C. Forbes, John Bannister, Michele Bodewits, Dennis Bolin, Bryce T. Belyakov, Matthew Knight, Matthew M. Snodgrass, Colin Bufanda, Erica Dorsey, Rosemary Ferellec, Léa La Forgia, Fiorangela Lippi, Manuela Murphy, Brian Nayak, Prasanta K. Donckt, Mathieu Vander |
| author_facet | Rahatgaonkar, Rohan Carvajal, Juan Pablo Puzia, Thomas H. Luco, Baltasar Jehin, Emmanuel Hutsemékers, Damien Opitom, Cyrielle Manfroid, Jean Aravind, K. Marsset, Michaël Yang, Bin Buchanan, Laura Fraser, Wesley C. Forbes, John Bannister, Michele Bodewits, Dennis Bolin, Bryce T. Belyakov, Matthew Knight, Matthew M. Snodgrass, Colin Bufanda, Erica Dorsey, Rosemary Ferellec, Léa La Forgia, Fiorangela Lippi, Manuela Murphy, Brian Nayak, Prasanta K. Donckt, Mathieu Vander |
| contents | We report VLT spectroscopy of the interstellar comet 3I/ATLAS (C/2025~N1) from $r_{\rm h}\!\simeq\!4.4$ to $2.85$~au using X-shooter (300--550\,nm, $R\!\simeq\!3000$) and UVES (optical, $R\!\simeq\!35k-80k$). The coma is dust-dominated with a fairly constant red optical continuum slope ($\sim$21--22\%/1000Å). We report detection of CN emission and also detect numerous Ni\,\textsc{ii}~lines while Fe\,\textsc{i}~remains undetected, potentially implying efficiently released gas-phase Ni. At $r_{\rm h}\!\simeq\!3.14$~au we derive $3σ$ limits of $Q({\rm OH})<{1.48\times10^{26}}\ {\rm s^{-1}}$, but find no indications for [O\,\textsc{i}], C$_2$, C$_3$ or NH$_2$. From our latest X-shooter measurements conducted on 2025-08-21 ($r_{\rm h} = 2.85$\,au) we measure production rates of $\log~Q(\mathrm{CN}) = {24.81\pm 0.01}$ molecules s$^{-1}$ and $\log~Q$(Ni) $= {23.30\pm0.07}$ atoms s$^{-1}$, and characterize their evolution as the comet approaches perihelion.~We observe a steep heliocentric-distance scaling for the production rates $Q(\mathrm{Ni}) \propto r_h^{-7.7 \pm 1.0}$ and for $Q(\mathrm{CN}) \propto r_h^{-6.7 \pm 0.2}$, and predict a Ni--CO$_{(2)}$ correlation if the Ni\,\textsc{ii}\ emission is driven by the carbonyl formation channel.~Energetic considerations of activation barriers show that this behavior is inconsistent with 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 are testable with future coordinated ground-based and space-based monitoring as 3I becomes more active during its continued passage through the solar system. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2508_18382 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | 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* Rahatgaonkar, Rohan Carvajal, Juan Pablo Puzia, Thomas H. Luco, Baltasar Jehin, Emmanuel Hutsemékers, Damien Opitom, Cyrielle Manfroid, Jean Aravind, K. Marsset, Michaël Yang, Bin Buchanan, Laura Fraser, Wesley C. Forbes, John Bannister, Michele Bodewits, Dennis Bolin, Bryce T. Belyakov, Matthew Knight, Matthew M. Snodgrass, Colin Bufanda, Erica Dorsey, Rosemary Ferellec, Léa La Forgia, Fiorangela Lippi, Manuela Murphy, Brian Nayak, Prasanta K. Donckt, Mathieu Vander Solar and Stellar Astrophysics Earth and Planetary Astrophysics We report VLT spectroscopy of the interstellar comet 3I/ATLAS (C/2025~N1) from $r_{\rm h}\!\simeq\!4.4$ to $2.85$~au using X-shooter (300--550\,nm, $R\!\simeq\!3000$) and UVES (optical, $R\!\simeq\!35k-80k$). The coma is dust-dominated with a fairly constant red optical continuum slope ($\sim$21--22\%/1000Å). We report detection of CN emission and also detect numerous Ni\,\textsc{ii}~lines while Fe\,\textsc{i}~remains undetected, potentially implying efficiently released gas-phase Ni. At $r_{\rm h}\!\simeq\!3.14$~au we derive $3σ$ limits of $Q({\rm OH})<{1.48\times10^{26}}\ {\rm s^{-1}}$, but find no indications for [O\,\textsc{i}], C$_2$, C$_3$ or NH$_2$. From our latest X-shooter measurements conducted on 2025-08-21 ($r_{\rm h} = 2.85$\,au) we measure production rates of $\log~Q(\mathrm{CN}) = {24.81\pm 0.01}$ molecules s$^{-1}$ and $\log~Q$(Ni) $= {23.30\pm0.07}$ atoms s$^{-1}$, and characterize their evolution as the comet approaches perihelion.~We observe a steep heliocentric-distance scaling for the production rates $Q(\mathrm{Ni}) \propto r_h^{-7.7 \pm 1.0}$ and for $Q(\mathrm{CN}) \propto r_h^{-6.7 \pm 0.2}$, and predict a Ni--CO$_{(2)}$ correlation if the Ni\,\textsc{ii}\ emission is driven by the carbonyl formation channel.~Energetic considerations of activation barriers show that this behavior is inconsistent with 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 are testable with future coordinated ground-based and space-based monitoring as 3I becomes more active during its continued passage through the solar system. |
| title | 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* |
| topic | Solar and Stellar Astrophysics Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2508.18382 |