![Temporal evolution of selected species and D/H ratios during the cloud (top panels) and core stages (lower panels). Panels (a) and (e) show the abundances of selected species during the cloud and core stages, respectively. Solid, dashed, and dotted lines denote models with Z = 1, 0.5, and 0.2, respectively. The remaining panels show the temporal evolution of the D/H ratios and the H2 OPR for Z = 1 (b, f), Z = 0.5 (c, g), and Z = 0.2 (d, h). In all panels, time is normalized by τfreeze for the corresponding density and metallicity. -- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/09/High-water.png?fit=1200%2C675&ssl=1)
[astro-ph.EP] 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 ∼104 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.
Kenji Furuya, Martin Cordiner, Dominique Bockelée-Morvan, Dennis Bodewits, Colin Orion Chandler, Maria N. Drozdovskaya, Nathan X. Roth, Geronimo Villanueva
Comments: Accepted for publication in ApJL. The typo in the abstract has been corrected
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2609.12370 [astro-ph.EP] (or arXiv:2609.12370v2 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.12370
Focus to learn more
Submission history
From: Kenji Furuya
[v1] Fri, 11 Sep 2026 02:40:46 UTC (358 KB)
[v2] Mon, 14 Sep 2026 02:10:29 UTC (358 KB)
https://arxiv.org/abs/2609.12370
Astrobiology, Astrochemistry, Astrogeology, Interstellar,
