Astrochemistry

H182O In The Terrestrial Planet-forming Regions Of Protoplanetary Disks

By Keith Cowing

Status Report

astro-ph.EP

January 29, 2026

Filed under astro-ph.EP, astrochemistry, exoplanet, Interstellar, interstellar medium, isotopologues, JWST, Protoplanetary Disk, solar nebula

H182O In The Terrestrial Planet-forming Regions Of Protoplanetary Disks — [L] Spectra of targets meeting our selection criteria for further analysis (P/C>0.2 and H₂¹⁸O SNR>2), between 22.7 and 24.2 µm, along with best-fit 3-component water models (blue; see Table 1), 3-component H₂¹⁸O models (green) assuming an ISM ¹⁶O/¹⁸O ratio of 557, and 3-component H₂¹⁸O models with a best-fit ¹⁶O/¹⁸ ratio (orange). Vertical dashed lines show H₂¹⁸O lines from Table 2. [R] Same as Figure 1 but for the wavelength range of 25.2 to 27.1 µm. Gray vertical dash-dot lines mark the location used for determination of the noise level. Green stars mark the H₂¹⁸O lines used to fit the H₂¹⁶O/H₂¹⁸O ratio. A zoomed-in portion of these features is provided in Figure 11. — astro-ph.EP

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 H₂¹⁸O in 22 disks from the JDISC Survey with evidence for substantial water vapor reservoirs, with the goal of measuring H₂¹⁶O/H₂¹⁸O ratios, and potentially revealing the predicted enhancement of H₂¹⁸O caused by isotope-selective photodissociation.

We find marginal detections of H₂¹⁸O in six disks, and a more significant detection of H₂¹⁸O in the disk around WSB 52. Modeling of the detected H₂¹⁸O lines assuming an ISM ratio of H₂¹⁶O/H₂¹⁸O predicts H₂¹⁸O features consistent with observations for four of the modeled disks, but stronger H₂¹⁸O features than are observed in three of the modeled disks, which includes WSB 52. Therefore, these latter three disks require a higher H₂¹⁶O/H₂¹⁸O ratio than the ISM in the water-emitting region, in contrast to long-standing theoretical expectations.

We suggest that either the H₂¹⁸O-rich water has been removed from the emitting region and replaced by H₂¹⁸O-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.

Colette Salyk, Klaus M. Pontoppidan, Ke Zhang, Sophie Heinzen, Jenny K. Calahan, Andrea Banzatti, D. Annie Dickson-Vandervelde, Edwin A. Bergin, Geoffrey A. Blake, Nicole Arulanantham, Sebastiaan Krijt, John Carr, Joan Najita, Joel Green, Carlos Romero-Mirza

Comments: 20 pages, 11 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2601.20096 [astro-ph.EP] (or arXiv:2601.20096v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2601.20096
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Related DOI:
https://doi.org/10.3847/1538-4357/ae3db1
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Submission history
From: Colette Salyk
[v1] Tue, 27 Jan 2026 22:30:41 UTC (9,665 KB)
https://arxiv.org/abs/2601.20096
Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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