High-resolution optical spectroscopy reveals the distinctive volatile composition of the interstellar comet…

Abstract

Comets preserve volatile material from the early stages of planetary-system formation. The discovery of the interstellar comet 3I/ATLAS enables a direct comparison between a comet formed around another star and comets in the Solar System. We characterise the coma composition of 3I/ATLAS across perihelion using homogeneous high-resolution near-UV and optical spectroscopy with UVES on the ESO Very Large Telescope, from August 2025 (r_h=3.14 au) pre-perihelion to February 2026 (r_h=4.34 au) post-perihelion. From flux-calibrated spectra, we measured OH, NH, CN, C₃, CH and C₂ emissions, derived production rates, examined abundance ratios and heliocentric-distance trends, constrained NH₂, and compared OH-based water production with estimates from forbidden oxygen lines. Gas production rates show broadly symmetric pre- and post-perihelion behaviour, with steeper heliocentric-distance dependences than typically observed in Solar System comets. NH and NH₂ are strongly depleted, indicating a severe deficiency of ammonia-related volatiles. The C₂/CN ratio indicates carbon-chain depletion and evolves with heliocentric distance, consistent with coma-driven effects such as hyperactivity rather than intrinsic compositional changes. Water production from forbidden oxygen lines exceeds OH-based estimates except near perihelion, suggesting varying contributions from H₂O and CO₂ to coma chemistry. Relative abundance correlations place 3I/ATLAS outside the canonical Solar System comet cluster and closest to strongly carbon-chain depleted comets, including G-Z-type comets, while showing strong depletion of NH- and C₃-bearing species relative to CN. 3I/ATLAS therefore extends the known diversity of cometary compositions, demonstrating that volatile reservoirs in other planetary systems can differ substantially from those sampled by Solar System comets.

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BibTeX RIS

BibTeX

@misc{aravind2026highresolution,
  title = {{High-resolution optical spectroscopy reveals the distinctive volatile composition of the interstellar comet 3I/ATLAS}},
  author = {K. Aravind and E. Jehin and C. Opitom and J. Manfroid and D. Hutsemékers and D. Bodewits and M. Bannister and R. C. Dorsey and L. Ferellec and H. Kawakita and M. Lippi and C. Snodgrass},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.26577},
  eprint = {2609.26577},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.26577},
}

RIS

TY  - UNPB
TI  - High-resolution optical spectroscopy reveals the distinctive volatile composition of the interstellar comet 3I/ATLAS
AU  - K. Aravind
AU  - E. Jehin
AU  - C. Opitom
AU  - J. Manfroid
AU  - D. Hutsemékers
AU  - D. Bodewits
AU  - M. Bannister
AU  - R. C. Dorsey
AU  - L. Ferellec
AU  - H. Kawakita
AU  - M. Lippi
AU  - C. Snodgrass
PY  - 2026
DA  - 2026/09/22
PB  - arXiv
DO  - 10.48550/arxiv.2609.26577
UR  - https://arxiv.org/abs/2609.26577
AB  - Comets preserve volatile material from the early stages of planetary-system formation. The discovery of the interstellar comet 3I/ATLAS enables a direct comparison between a comet formed around another star and comets in the Solar System. We characterise the coma composition of 3I/ATLAS across perihelion using homogeneous high-resolution near-UV and optical spectroscopy with UVES on the ESO Very Large Telescope, from August 2025 (r_h=3.14 au) pre-perihelion to February 2026 (r_h=4.34 au) post-perihelion. From flux-calibrated spectra, we measured OH, NH, CN, C_3, CH and C_2 emissions, derived production rates, examined abundance ratios and heliocentric-distance trends, constrained NH_2, and compared OH-based water production with estimates from forbidden oxygen lines. Gas production rates show broadly symmetric pre- and post-perihelion behaviour, with steeper heliocentric-distance dependences than typically observed in Solar System comets. NH and NH_2 are strongly depleted, indicating a severe deficiency of ammonia-related volatiles. The C_2/CN ratio indicates carbon-chain depletion and evolves with heliocentric distance, consistent with coma-driven effects such as hyperactivity rather than intrinsic compositional changes. Water production from forbidden oxygen lines exceeds OH-based estimates except near perihelion, suggesting varying contributions from H_2O and CO_2 to coma chemistry. Relative abundance correlations place 3I/ATLAS outside the canonical Solar System comet cluster and closest to strongly carbon-chain depleted comets, including G-Z-type comets, while showing strong depletion of NH- and C_3-bearing species relative to CN. 3I/ATLAS therefore extends the known diversity of cometary compositions, demonstrating that volatile reservoirs in other planetary systems can differ substantially from those sampled by Solar System comets.
ER  -

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