High water D/H ratio of the interstellar object 3I/ATLAS is consistent with a low-metallicity origin

@misc{furuya2026high,
  title = {{High water D/H ratio of the interstellar object 3I/ATLAS is consistent with a low-metallicity origin}},
  author = {Kenji Furuya and Martin Cordiner and Dominique Bockelée-Morvan and Dennis Bodewits and Colin Orion Chandler and Maria N. Drozdovskaya and Nathan X. Roth and Geronimo Villanueva},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.12370},
  eprint = {2609.12370},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.12370},
}

TY  - UNPB
TI  - High water D/H ratio of the interstellar object 3I/ATLAS is consistent with a low-metallicity origin
AU  - Kenji Furuya
AU  - Martin Cordiner
AU  - Dominique Bockelée-Morvan
AU  - Dennis Bodewits
AU  - Colin Orion Chandler
AU  - Maria N. Drozdovskaya
AU  - Nathan X. Roth
AU  - Geronimo Villanueva
PY  - 2026
DA  - 2026/09/14
PB  - arXiv
DO  - 10.48550/arxiv.2609.12370
UR  - https://arxiv.org/abs/2609.12370
AB  - Recent JWST observations have revealed unusually high ^12C/^13C ratios in carbon-bearing molecules of the interstellar object 3I/ATLAS, consistent with formation in a lower-metallicity environment than the present-day local interstellar medium (ISM). 3I/ATLAS also exhibits an exceptionally high water D/H ratio, exceeding those in Solar System comets and nearby low-mass star-forming regions. Here we investigate whether this high water D/H ratio can be reproduced in a low-metallicity formation scenario, using gas-ice astrochemical models. Assuming that the water observed in 3I/ATLAS was inherited from the parent molecular cloud and core, we perform a grid of astrochemical models covering the cloud to core stages, varying the gas density, ultraviolet radiation field (χ), cosmic-ray ionization rate (ζ), and metallicity, while solving thermal balance for the gas temperature. We find that lower metallicity enhances H_3^+ deuteration and, more importantly, its transfer to water ice. In contrast, water D/H ratio depends non-monotonically on χ and ζ, because of competing chemical and thermal effects. In our models, the observed water D/H ratio is most readily reproduced at subsolar metallicities, ≲0.5Z_⊙, and relatively high cloud densities of ∼10^4 cm^−3 without strong constraints on either χ or ζ, as long as ζ<10^−15 s^−1. The D/H ratio of methane normalized by that of water is not sensitive to the metallicity, being consistent with the similar values observed in 67P/Churyumov-Gerasimenko and 3I/ATLAS. These results suggest that water deuteration may provide a complementary probe of the metallicity and physical condition of the parent molecular cloud and dense core of interstellar objects.
ER  -