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

Recent JWST observations have revealed unusually high ${}^{12}$C/${}^{13}$C 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 ($\chi$), cosmic-ray ionization rate ($\zeta$), 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 $\chi$ and $\zeta$, because of competing chemical and thermal effects. In our models, the observed water D/H ratio is most readily reproduced at subsolar metallicities, $\lesssim 0.5Z_{\odot }$, and relatively high cloud densities of $\sim$10${}^4$ cm${}^{-3}$ without strong constraints on either $\chi$ or $\zeta$, as long as $\zeta <{10}^{-15}$ 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.