2026ApJ...999...92S

Isotopologues play an important role in solar system cosmochemistry studies, revealing details of early planet formation physics and chemistry. Oxygen isotopes, as measured in solar system materials, reveal evidence for both mass-dependent fractionation processes and a mass-independent process commonly attributed to isotope-selective photodissociation of CO in the solar nebula. The sensitivity of JWST's MIRI-MRS enables studies of isotopologues in the terrestrial planet-forming regions around nearby young stars. We report here on a search for H${}_{2}^{18}$O in 22 disks from the JDISC Survey with evidence for substantial water vapor reservoirs, with the goal of measuring H${}_{2}^{16}$O/H${}_{2}^{18}$O ratios, and potentially revealing the predicted enhancement of H${}_{2}^{18}$O caused by isotope-selective photodissociation. We find marginal detections of H${}_{2}^{18}$O in six disks, and a more significant detection of H${}_{2}^{18}$O in the disk around WSB 52. Modeling of the detected H${}_{2}^{18}$O lines assuming an interstellar medium (ISM) ratio of H${}_{2}^{16}$O/H${}_{2}^{18}$O predicts H${}_{2}^{18}$O features consistent with observations for four of the modeled disks, but stronger H${}_{2}^{18}$O features than are observed in three of the modeled disks, which includes WSB 52. Therefore, these latter three disks require a higher H${}_{2}^{16}$O/H${}_{2}^{18}$O ratio than the ISM in the water-emitting region, in contrast to long-standing theoretical expectations. We suggest that either the H${}_{2}^{18}$O-rich water has been removed from the emitting region and replaced by H${}_{2}^{18}$O-poor water formed by reactions with ¹⁸O-poor CO, or that the gas-phase water is depleted in ¹⁸O via mass-dependent fractionation processes at the water snowline.