---
title: 'XUE: Protoplanetary Disks in Extreme UV'
url: https://www.emergentmind.com/topics/extreme-uv-environments-xue
type: topic
---

# XUE: Protoplanetary Disks in Extreme UV

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 [2505.06093].

## 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 [2505.06093].

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 [1001.0294]. 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 [2505.06093].

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 [2310.11074]. 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 [2505.06093].

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 [2505.06093].

## 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 [2505.06093].

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 [2505.06093].

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

\[
F_{\mathrm{peak}}=\max(S_\nu)\quad \text{between 6 and 14 }\mu\mathrm{m},
\]

and

\[
F_{11.3}/F_{9.8}=\frac{S_\nu(11.3\,\mu\mathrm{m})}{S_\nu(9.8\,\mu\mathrm{m})}.
\]

The XUE disks show the familiar anti-correlation between silicate strength and shape, but at a given \(F_{\mathrm{peak}}\) they tend to have **lower \(F_{11.3}/F_{9.8}\)** 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 [2505.06093].

## 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\(_2\)O, HCN, C\(_2\)H\(_2\), OH, CO\(_2\)**, and in one source **CO\(_2\) 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 [2505.06093].

Water is analyzed in three thermal regimes using the temperature-dependent flux ratio near **23.85 \(\mu\)m**. The XUE sample contains **hot water** at roughly **\(900\) K**, **warm water** at roughly **\(400\) K**, and **cold water** at roughly **\(200\) 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 [2505.06093].

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 \(\mu\)m** and **11.3 \(\mu\)m**; all other XUE disks are essentially **PAH-poor**. The water line luminosity at **17 \(\mu\)m** is described as **high but not extraordinary**, and the usual increase of water luminosity with \(M_\star\) or \(L_\star\) flattens above about **\(2\,M_\odot\)** [2505.06093].

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 [2505.06093].

## 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 [2505.06093].

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

\[
\Sigma(r)=\Sigma_0\left(\frac{R_{\mathrm{tap}}}{r}\right)^\epsilon
\exp\!\left[-\left(\frac{r}{R_{\mathrm{tap}}}\right)^{2-\gamma}\right],
\qquad \epsilon=\gamma=1,
\]

with an external ultraviolet field defined relative to the Draine field. The preferred solution is a disk **truncated at \(10\) au** with a **gas-to-dust ratio of unity in the outskirts**, while the molecular features detected by MIRI arise within the first **\(\sim 1\) 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 [2504.00841].

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 [2504.00841].

## 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\(_2\)-rich, water-poor terrestrial-planet-forming region with unusual isotopic structure.

| Source | Key observational result | Interpretation |
|---|---|---|
| XUE 1 | H\(_2\)O, CO, CO\(_2\), HCN, and C\(_2\)H\(_2\) detected; small, partially crystalline silicate dust present | Inner-disk chemistry and dust resemble nearby low-mass disks despite strong irradiation |
| XUE 10 | First simultaneous \(>5\sigma\) detection of four CO\(_2\) isotopologues in a protoplanetary disk; strict upper limit on H\(_2\)O content | CO\(_2\)-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\(_2\), HCN, and C\(_2\)H\(_2\)** 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 **\(\sim 1\) au** [2310.11074; 2504.00841].

For **XUE 10**, the dedicated follow-up paper identifies an **F-type** source in NGC 6357 exposed to an external FUV field of about **\(5.2\times10^3\,G_0\)**. The mid-infrared spectrum shows the first simultaneous **\(>5\sigma\)** detection in a protoplanetary disk of four CO\(_2\) isotopologues—\(^{12}\)CO\(_2\), \(^{13}\)CO\(_2\), \(^{16}\)O\(^{12}\)C\(^{18}\)O, and \(^{16}\)O\(^{12}\)C\(^{17}\)O—together with faint CO emission at **\(2\sigma\)** and **H I Pf\(\alpha\)** at **\(8\sigma\)**. The water content is constrained to a total column density **\(\lesssim 10^{18}\,\mathrm{cm^{-2}}\)**. The CO\(_2\) species trace gas temperatures of roughly **300–370 K**, with column densities from **\(7.4\times10^{17}\,\mathrm{cm^{-2}}\)** to **\(1.3\times10^{20}\,\mathrm{cm^{-2}}\)** and an equivalent emitting radius of **1.15 au**. The paper proposes that the spectrum reflects **H\(_2\)O removal** through advection or strong stellar-UV photodissociation, together with enhanced local CO\(_2\) gas-phase production; **outer disk truncation** is argued to support the observed CO\(_2\)–H\(_2\)O dichotomy [2507.13921].

The XUE 10 study also raises the possibility of isotopically anomalous oxygen in CO\(_2\), while stressing that **line optical depth effects**, especially in \(^{13}\)CO\(_2\), 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 [2507.13921].

## 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 [2505.06093].

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 [1804.11124]. 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 [2605.28768].

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.

Source: https://www.emergentmind.com/topics/extreme-uv-environments-xue