H$_2^{18}$O in the terrestrial planet-forming regions of protoplanetary disks

TL;DR

JWST/MIRI-MRS observations of 22 protoplanetary disks (JDISCS) search for $H_2^{18}$O in terrestrial planet-forming zones to test isotope-fractionation theories. The authors fit $H_2^{16}$O with a three-temperature slab to characterize the water reservoir, then predict $H_2^{18}$O for the ISM ratio and search for detections in the 22–28 μm region, finally constraining the isotopologue ratio by fitting the strongest $H_2^{18}$O lines. Most disks yield $H_2^{16}O/H_2^{18}O$ ratios at or above the ISM value ($ ext{557}$), with Sz 129 and TW Cha hinting at $^{18}$O enrichment; the disk WSB 52 shows a robust $H_2^{18}$O detection but with a ratio above ISM, challenging the simple isotope-selective photodissociation model. The results motivate alternative explanations, such as removal of $^{18}$O-rich water or mass-dependent fractionation near the snowline, and highlight the need for far-IR observations to probe colder, possibly transport-dominated reservoirs. Overall, the study demonstrates JWST’s capability to constrain disk water isotopologues and informs theories of planet-forming disk chemistry and oxygen isotope evolution.

Abstract

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 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 $^{18}$O-poor CO, or that the gas-phase water is depleted in $^{18}$O via mass-dependent fractionation processes at the water snowline.

Figures (12)

• Figure 1: Spectra of targets meeting our selection criteria for further analysis (P/C>0.2 and H$_2^{18}$O SNR>2), between 22.7 and 24.2 $\mu$m, along with best-fit 3-component water models (blue; see Table \ref{['table:disks']}), 3-component H$_2^{18}$O models (green) assuming an ISM $^{16}$O/$^{18}$O ratio of 557, and 3-component H$_2^{18}$O models with a best-fit $^{16}$O/$^{18}$O ratio (orange). Vertical dashed lines show H$_2^{18}$O lines from Table \ref{['table:stronglines']}.
• Figure 2: Same as Figure \ref{['fig:allspectra_short']} but for the wavelength range of 25.2 to 27.1 $\mu$m. Gray vertical dash-dot lines mark the location used for determination of the noise level. Green stars mark the H$_2^{18}$O lines used to fit the H$_2^{16}$O/H$_2^{18}$O ratio. A zoomed-in portion of these features is provided in Figure \ref{['fig:allspectra_long_zoom']}.
• Figure 3: Portions of the nominal H$_2^{18}$O emission model for WSB 52, assuming the emission arises in the same 3-component reservoirs as given in Table \ref{['table:disks']}, and that H$_2^{16}$O/ H$_2^{18}$O=557. Blue and orange shading shows the contributions from the hot and warm reservoirs, respectively. The cold contribution is not sufficient to be visible on this scale. The dotted curve shows the sum of the three components. Dashed vertical lines indicate the emission lines selected for Table \ref{['table:stronglines']}; green stars mark the two features used while fitting the H$_2^{16}$O/H$_2^{18}$O ratio.
• Figure 4: HITRAN data for strong H$_2^{18}$O lines identified in our modeling.
• Figure 5: Left: H$_2^{18}$O signal-to-noise ratio vs. H$_2$O peak-to-continuum ratio from Arulanantham25. Dashed lines mark the cutoffs of 2 and 0.2 for these two parameters, respectively, to select for further analysis. Here and in the other two panels, filled circles mark sources that meet the cutoff criteria, WSB 52 is highlighted in orange, Sz 129 in green, and TW Cha in purple. Center: Emitting areas of warm vs. hot H$_2^{16}$O models fit to spectra --- see Table \ref{['table:disks']}. Right: Emitting areas of cold vs. hot H$_2^{16}$O models.