XUE: Protoplanetary Disks in Extreme UV

• XUE is a program examining disks exposed to strong external UV irradiation from massive stars, highlighting disk truncation and inner chemical richness.
• JWST/MIRI observations show that extreme UV fields truncate or deplete outer disk regions while preserving water and silicate-rich inner disks critical for rocky planet formation.
• Thermochemical models indicate that elevated gas temperatures and modified molecular abundances under intense UV flux influence both disk structure and subsequent planet formation.

eXtreme UV Environments (XUE) is a James Webb Space Telescope program devoted to the study of protoplanetary disks embedded in strongly irradiated, massive star-forming regions. Its central aim is to determine how intense external ultraviolet fields affect the inner physical structure, chemistry, and planet-forming potential of disks, especially in environments more representative of where most stars and planetary systems form than the nearby, relatively isolated regions that dominated pre-JWST disk chemistry studies. In the published XUE results, the dominant empirical pattern is that external irradiation appears to truncate or deplete the outer disk while often leaving the inner few astronomical units chemically rich, including water-bearing gas and silicate dust relevant to rocky planet formation (Ramírez-Tannus et al., 9 May 2025).

1. Definition, nomenclature, and physical scope

In current disk studies, XUE stands specifically for eXtreme UV Environments and refers to disks exposed to strong external ultraviolet irradiation from nearby massive stars. The program’s operational environmental parameter is the external far-ultraviolet (FUV) field, estimated by integrating PHOENIX atmosphere models over 912–2000 Å and scaling by projected distance to the ionizing stars, under the assumption of no intervening extinction between the massive stars and the disks. The final XUE sample spans roughly $10^3$ to $10^6\,G_0$, where $G_0$ is the Habing field (Ramírez-Tannus et al., 9 May 2025).

A point of terminology is that the program name uses “UV” in a broad environmental sense, not as a claim that the observations directly cover the stellar extreme-ultraviolet band. In the broader ultraviolet-instrumentation convention, EUV is commonly taken as 100–912 Å and UV as 912–3000 Å, with the hydrogen Lyman absorption edge at 912 Å providing the natural boundary because the interstellar medium becomes strongly absorbing below it (Werner, 2010). This suggests that, in the XUE literature, “extreme UV environments” is best understood as shorthand for exceptionally intense external ultraviolet irradiation in clustered star formation, rather than as a direct observational program in the interstellar-attenuated EUV band.

2. Observational setting: NGC 6357 and the XUE sample

The XUE program is centered on NGC 6357, a very young, massive star-forming complex at a distance of about 1.69 kpc and an age of roughly 1–1.6 Myr. The region contains more than 20 O stars and one of the most massive stars in the Galaxy, so the external FUV field is both strong and spatially variable. XUE selected disks in three sub-clusters—Pismis 24, G353.1+0.6, and G353.1+0.7—to control for age and natal cloud environment while sampling a wide range of irradiation levels (Ramírez-Tannus et al., 9 May 2025).

The initial program description presented 15 disks in NGC 6357, motivated by the view that most stars and planetary systems form in clustered, UV-rich environments rather than in nearby quiet regions such as Taurus or Lupus (Ramirez-Tannus et al., 2023). The later program paper reports a final sample of 12 disks after removal of one failed observation and two foreground contaminants. The targets were drawn from MYStIX-selected cluster members with KMOS-based spectral types and Spitzer mid-IR excesses; the chosen disks were Class II systems spanning roughly G to A spectral types and placed as close as possible to the massive ionizing stars (Ramírez-Tannus et al., 9 May 2025).

The observations were obtained in Cycle 1 program GO-1759 with JWST/MIRI-MRS in all three spectral settings, SHORT, MEDIUM, and LONG. The strategy used a four-point dither optimized for point sources, FASTR1 readout, 40 groups per integration, and two integrations per dither position. No dedicated off-source background was taken because the nebular background is highly structured; instead, the reduction used custom nod subtraction in addition to JWST pipeline v1.14.0 standard processing (Ramírez-Tannus et al., 9 May 2025).

3. Continuum classes, spectral indices, and silicate diagnostics

A major part of the XUE analysis is classification of the mid-infrared spectral energy distributions by the Meeus Group I/II scheme. In this framework, Group I disks have strong mid- to far-IR excesses and are generally interpreted as flared disks or disks with gaps or inner holes, while Group II disks have more modest IR excesses and are considered self-shadowed or more compact. Within the XUE sample, XUE 1, 3, 5, and 10 resemble Group I, while XUE 2, 4, 6, 7, 8, 9, 11, and 12 resemble Group II. Because the 10 $\mu$m silicate feature is present in all of them, all XUE disks are described as Group Ia or IIa; XUE 3 and 5 in particular show evidence for inner holes or reduced inner dust emission (Ramírez-Tannus et al., 9 May 2025).

The continuum comparison to nearby disks uses the spectral indices

$$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$

The XUE analysis evaluates $n_{6-13}$, $n_{5-12}$, and $n_{12-20}$, and finds no significant correlation with external FUV flux. Statistically, the XUE values lie within the distribution of Orion disks, and Kolmogorov-Smirnov tests do not reject the hypothesis that the samples are drawn from the same parent population (Ramírez-Tannus et al., 9 May 2025).

The 10 $\mu$m silicate feature is treated with the normalized form

$$S_\nu = 1 + \frac{F_{\nu,\mathrm{smooth}}-F_{\nu,\mathrm{cont}}} {\langle F_{\nu,\mathrm{cont}}\rangle},$$

from which the feature strength and shape are defined as

$10^6\,G_0$0

and

$10^6\,G_0$1

The XUE disks show the familiar anti-correlation between silicate strength and shape, but at a given $10^6\,G_0$2 they tend to have lower $10^6\,G_0$3 than nearby T Tauri and Herbig disks. The authors explicitly caution that present uncertainties in extinction corrections prevent firm conclusions about inner-disk grain properties (Ramírez-Tannus et al., 9 May 2025).

4. Molecular inventory and inner-disk chemistry

The XUE molecular inventory is one of the program’s defining results. Across the sample, the MIRI spectra show emission from CO, H$10^6\,G_0$4O, HCN, C$10^6\,G_0$5H$10^6\,G_0$6, OH, CO$10^6\,G_0$7, and in one source CO$10^6\,G_0$8 isotopologues. Despite the harsh external environment, the program paper concludes that the XUE disks are molecularly rich, and that the disks around these more massive stars have molecular richness comparable to isolated T Tauri systems (Ramírez-Tannus et al., 9 May 2025).

Water is analyzed in three thermal regimes using the temperature-dependent flux ratio near 23.85 $10^6\,G_0$9m. The XUE sample contains hot water at roughly $G_0$0 K, warm water at roughly $G_0$1 K, and cold water at roughly $G_0$2 K as diagnostic categories. Most XUE disks show a hot-water component, only three show warm water, and none show cold water. The absence of cold water, together with the continued presence of hot inner-disk water in most systems, implies that external irradiation has not erased volatile-rich inner chemistry but has affected the thermal and structural conditions under which different water reservoirs are observable (Ramírez-Tannus et al., 9 May 2025).

The same sample is notable for the near absence of strong ultraviolet-surface tracers expected from large externally irradiated disk atmospheres. Only XUE 10 shows PAH emission at 6.2 $G_0$3m and 11.3 $G_0$4m; all other XUE disks are essentially PAH-poor. The water line luminosity at 17 $G_0$5m is described as high but not extraordinary, and the usual increase of water luminosity with $G_0$6 or $G_0$7 flattens above about $G_0$8 (Ramírez-Tannus et al., 9 May 2025).

A common oversimplification is that sufficiently strong external irradiation should monotonically impoverish the chemistry of the inner disk. The XUE sample does not support that one-parameter picture. The measured inner-disk spectra remain chemically rich, yet the same objects typically lack the strong line fluxes and PAH behavior expected from large UV-heated outer surfaces. This combination is central to the program’s interpretation (Ramírez-Tannus et al., 9 May 2025).

5. Disk structure, truncation, and thermochemical interpretation

The structural interpretation of XUE is driven less by direct imaging than by the mismatch between theoretical expectations for large irradiated disks and the observed spectral properties. Radiation-thermochemical models predict that large flared disks with extended UV-heated surface layers should display strong molecular line emission and bright PAHs from the disk surface. In the XUE sample, that is generally not observed: line fluxes are not strongly enhanced relative to nearby disks, only one disk shows PAHs, and many continua are more consistent with compact, truncated, or self-shadowed structures. The program paper interprets this as evidence that external UV photons have photoevaporatively truncated the disks, reducing the emitting surface area of the outer disk while leaving the inner region relatively intact (Ramírez-Tannus et al., 9 May 2025).

A detailed thermochemical study of XUE 1 makes this interpretation explicit. Using ProDiMo, the disk surface density is modeled as

$G_0$9

with an external ultraviolet field defined relative to the Draine field. The preferred solution is a disk truncated at $\mu$0 au with a gas-to-dust ratio of unity in the outskirts, while the molecular features detected by MIRI arise within the first $\mu$1 au. In model comparisons using the same disk structure but different irradiation levels, strong external irradiation raises the gas temperature by about an order of magnitude and boosts the gas-phase water reservoir beyond 10 au by a factor of 100 relative to a non-irradiated analogue (Portilla-Revelo et al., 1 Apr 2025).

The XUE 1 modeling also introduces an important caveat: there is a degeneracy between disk size and external irradiation strength. A larger disk exposed to a weaker FUV field could reproduce some observables, so the compact, strongly irradiated solution is favored rather than absolutely unique. The need for better three-dimensional constraints on shielding and O-star geometry remains an open modeling issue (Portilla-Revelo et al., 1 Apr 2025).

6. Representative sources: XUE 1 and XUE 10

Two XUE sources define especially clear empirical endpoints. XUE 1 is the first published XUE case and demonstrates that a highly irradiated disk can still display a chemically rich inner few au. XUE 10 shows the opposite chemical extreme: a CO$\mu$2-rich, water-poor terrestrial-planet-forming region with unusual isotopic structure.

Source	Key observational result	Interpretation
XUE 1	H$\mu$3O, CO, CO$\mu$4, HCN, and C$\mu$5H$\mu$6 detected; small, partially crystalline silicate dust present	Inner-disk chemistry and dust resemble nearby low-mass disks despite strong irradiation
XUE 10	First simultaneous $\mu$7 detection of four CO$\mu$8 isotopologues in a protoplanetary disk; strict upper limit on H$\mu$9O content	CO$$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$0-rich, water-poor inner disk, plausibly linked to water removal and outer-disk truncation

For XUE 1, the discovery paper reports abundant water, CO, CO$$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$1, HCN, and C$$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$2H$$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$3 in the inner few au, together with small, partially crystalline silicate dust at the disk surface. The derived column densities, the oxygen-dominated gas-phase chemistry, and the silicate dust are described as surprisingly similar to disks in nearby, relatively isolated low-mass star-forming regions. A later thermochemical analysis concludes that the source is best described by a compact disk truncated at 10 au, with the MIRI molecular emission confined largely to the inner $$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$4 au (Ramirez-Tannus et al., 2023, Portilla-Revelo et al., 1 Apr 2025).

For XUE 10, the dedicated follow-up paper identifies an F-type source in NGC 6357 exposed to an external FUV field of about $$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$5. The mid-infrared spectrum shows the first simultaneous $$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$6 detection in a protoplanetary disk of four CO$$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$7 isotopologues—$$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$8CO$$n_{\lambda_1-\lambda_2}= \frac{\log_{10}(\lambda_1 F_1)-\log_{10}(\lambda_2 F_2)} {\log_{10}(\lambda_1)-\log_{10}(\lambda_2)}.$$9, $n_{6-13}$0CO$n_{6-13}$1, $n_{6-13}$2O$n_{6-13}$3C$n_{6-13}$4O, and $n_{6-13}$5O$n_{6-13}$6C$n_{6-13}$7O—together with faint CO emission at $n_{6-13}$8 and H I Pf$n_{6-13}$9 at $n_{5-12}$0. The water content is constrained to a total column density $n_{5-12}$1. The CO$n_{5-12}$2 species trace gas temperatures of roughly 300–370 K, with column densities from $n_{5-12}$3 to $n_{5-12}$4 and an equivalent emitting radius of 1.15 au. The paper proposes that the spectrum reflects H$n_{5-12}$5O removal through advection or strong stellar-UV photodissociation, together with enhanced local CO$n_{5-12}$6 gas-phase production; outer disk truncation is argued to support the observed CO$n_{5-12}$7–H$n_{5-12}$8O dichotomy (Frediani et al., 18 Jul 2025).

The XUE 10 study also raises the possibility of isotopically anomalous oxygen in CO$n_{5-12}$9, while stressing that line optical depth effects, especially in $n_{12-20}$0CO$n_{12-20}$1, may account for part of the apparent anomaly. The authors explicitly call for a 2D isotope-sensitive thermo-chemical model to distinguish radiative transfer effects from true abundance anomalies (Frediani et al., 18 Jul 2025).

7. Significance for planet formation and relation to broader XUV-environment studies

The broad conclusion of the XUE program is that externally irradiated inner disks can remain chemically rich even when their outer reservoirs are likely being eroded. The program paper states that most disks display water emission from the inner disk, suggesting that rocky planets can form in the presence of water even in these extreme environments. At the same time, the apparent absence of large outer disks implies that wide-orbit giant planets are unlikely, because the outer reservoirs needed for giant-planet formation may be removed early by photoevaporation or may never assemble (Ramírez-Tannus et al., 9 May 2025).

This places XUE at an earlier stage of the same irradiation-driven evolutionary sequence addressed in exoplanet XUV studies. For close-in exoplanets, X-ray and EUV irradiation are treated as the principal drivers of atmospheric heating and escape, while UV bands regulate photochemistry; small volatile-rich planets are expected to lose a larger fraction of their mass than hot Jupiters under comparable irradiation histories (King et al., 2018). Work on young exoplanet hosts similarly emphasizes that X-ray through UV irradiance histories determine atmospheric heating, photoionization, photodissociation, and long-term atmospheric survival, and that direct UV data are essential because the EUV band is largely unobservable and must be reconstructed (Rockcliffe et al., 27 May 2026).

A plausible implication is that XUE constrains the initial disk-stage boundary conditions that later feed into XUV-driven planetary atmospheric evolution. In that reading, XUE is not only a study of externally irradiated disks in NGC 6357; it is also a contribution to the larger problem of how ultraviolet environments shape planetary systems from the disk phase through atmospheric escape and volatile loss.