H18 2 O in the terrestrial planet-forming regions of protoplanetary disks

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 isotopeselective 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 H18 2 O in 22 disks from the JDISC Survey with evidence for substantial water vapor reservoirs, with the goal of measuring H16 2 O/H18 2 O ratios, and potentially revealing the predicted enhancement of H18 2 O caused by isotope-selective photodissociation. We find marginal detections of H18 2 O in six disks, and a more significant detection of H18 2 O in the disk around WSB 52. Modeling of the detected H18 2 O lines assuming an ISM ratio of H16 2 O/H18 2 O predicts H18 2 O features consistent with observations for four of the modeled disks, but stronger H 18 2 O features than are observed in three of the modeled disks, which includes WSB 52. Therefore, these latter three disks require a higher H16 2 O/H18 2 O ratio than the ISM in the water-emitting region, in contrast to long-standing theoretical expectations. We suggest that either the H18 2 O-rich water has been removed from the emitting region and replaced by H18 2 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.