Observing Co-Located Neutral and Ionized Gas-Phase Iron Depletion in the Magellanic Clouds

The paper Observing Co-Located Neutral and Ionized Gas-Phase Iron Depletion in the Magellanic Clouds by Yun Qi Li et al. measures neutral gas-phase elemental abundances (S, Fe) in the Magellanic Clouds along 33 sightlines using high-resolution UV spectroscopy (HST/COS and HST/STIS), and compares them to ionized gas-phase abundances (S, Fe) adopted from the literature for six co-located H ii regions (with the furthest separation of ≲ 3′, 50 pc).

The abstract states: "Comparing S abundances show that S is minimally depleted in the H ii regions and surrounding diffuse ISM. However, we find that the gas-phase Fe abundances in H ii regions can be lower than those of the neighboring neutral ISM by 0.3 to 2 dex. This difference is likely an offset in the amount of Fe depleted into dust grains. As accretion of gas-phase Fe is likely not effective at the temperatures of the H ii regions, Fe depletion into solid form would have occurred in the dense atomic or molecular clouds prior to star formation. Stronger depletion in the H ii regions shows that Fe-bearing grains survive destruction in the first few million years following ionization. Our observations highlight that Fe depletion in H ii regions can be a useful tracer of Fe depletion in dense molecular clouds, which are challenging to observe directly via UV absorption."

The paper's key findings are summarized in the conclusion:

1. "We measured the neutral gas elemental abundances in sightlines near six SMC and LMC H ii regions. As shown in Figure 5, four out of the six H ii regions have lower observed gas-phase 12 + log(Fe/H) compared to neutral gas in vicinity (∼ 0.5 dex). Two of the Fe-deficient H ii regions, LMC N11B and SMC N66A, have 9 and 13 neutral ISM abundance tracers respectively that cover all directions surrounding the H ii regions. Given the proximity of the compared neutral gas, this result further substantiates that Fe is depleted in H ii regions. We show that Fe depletion within the H ii regions can be stronger than surrounding neutral gas."

2. "Determining Fe abundances in H ii regions relies heavily on ionization corrections. Current photoionization modeling-based ionization correction methods carry systematic uncertainties, but have generally been shown to overestimate the total Fe abundance in ionized gas (Rodríguez & Rubin 2005; Méndez-Delgado et al. 2024). Using Cloudy C25 (Gunasekera et al. 2025), we find that lower intrinsic metallicity may cause a further overestimation of the total Fe abundance (Figure 10). Therefore, errors in ionization corrections are unlikely to account for the observed underabundance of gas-phase Fe in H ii regions. Nonetheless, the impact of intrinsic metallicity on Fe ionization corrections in H ii regions should be revisited."

3. "Compared to the Fe gas-phase abundances in the neighboring neutral ISM, the deficiency of gas-phase Fe in the H ii regions is likely an effect of dust depletion. As illustrated in Figure 11, large amounts of gas-phase Fe is accreted into dust in the dense atomic or molecular clouds prior to the formation of the H ii regions, while those Fe-bearing grains survive subsequent destruction. We show that observing H ii regions allows probing Fe depletion within dense molecular clouds that are difficult to observe via absorption spectroscopy; comparing the Fe depletion in the H ii regions with that of the surrounding diffuse ISM allows characterizing both the mixing timescales of neutral and ionized gas and grain growth timescales within the precursor molecular clouds. While our current constraints are broad due to uncertainties in the local environment evolutionary history, future studies utilizing assumptions based on local observations will provide the tighter constraints to these timescales."

4. "The observed sulfur (S) abundances in neighboring neutral and ionized patches of the ISM shown in Figure 4 are as expected. In the neutral gas, we observe super-intrinsic S abundances due to sightlines intersecting H ii regions. In the ionized gas, the lower S abundances are likely not an effect of depletion into dust."

The paper also discusses that "Our comparison shows that the offset in Fe depletion is greater in the SMC than in the LMC, driven by a lower baseline Fe depletion in the low-metallicity SMC neutral ISM. However, three of the four SMC H ii regions correspond to only a single neutral gas sightline each. Expanding this sample by observing additional neutral gas sightlines near SMC N81, SMC N88A, and SMC N90 would be highly valuable. SMC N88A is a particularly compelling case, with Fe iv directly detected, providing Fe abundances that are unaffected by ionization correction uncertainties."

The paper concludes with a recommendation: "Given this result indicates stronger Fe depletion in the H ii regions, we may be able to observe additional dust signatures in absorption or emission within these environments. One potential future experiment is to search for enhanced silicate extinction in the H ii regions. A fraction of the depleted gas-phase Fe may reside within silicates, which can be detected via the 9.7 μm and the 18 μm features in absorption spectra."

Improvements to AI Systems:

1. Dust-Depletion-Aware Abundance Modeling: Enhance AI models used for interstellar medium (ISM) abundance analysis to incorporate spatially resolved, multi-phase (neutral vs. ionized) dust depletion corrections. Specifically, train models to predict Fe depletion offsets (0.3–2 dex) between H ii regions and adjacent neutral gas, using sightline proximity (≤50 pc) and metallicity (SMC vs. LMC) as input features. This would prevent AI-driven spectral fitting from assuming uniform depletion across phases.

2. Ionization Correction Uncertainty Quantification: Improve AI-based photoionization modeling (e.g., Cloudy emulators) to explicitly flag systematic overestimation of total Fe abundance in low-metallicity H ii regions. The improved system would output confidence intervals that widen with decreasing intrinsic metallicity, based on the paper’s finding that lower Z worsens Fe overestimation. This would make AI-recovered abundances more reliable for metal-poor galaxies.

3. Dense Cloud Tracer Inference: Build an AI system that uses H ii region Fe depletion as a proxy for dense molecular cloud depletion (which is UV-absorption-inaccessible). The system would infer grain growth timescales and mixing timescales by comparing H ii region depletion to surrounding diffuse ISM depletion, using the paper’s framework (Figure 11). This enables AI to predict dust properties in star-forming regions without direct molecular cloud observations.

4. Spectral Feature Prediction for Silicate Dust: Train a generative AI model to predict silicate extinction signatures (9.7 μm and 18 μm absorption features) in H ii regions where Fe depletion is observed to be stronger than in neutral gas. The model would correlate depletion magnitude with expected silicate optical depth, guiding future JWST or SOFIA observations. This turns abundance measurements into actionable dust-detection predictions.

5. Sightline Sampling Optimization: Develop an AI planning tool that recommends optimal new UV absorption sightlines (e.g., near SMC N81, N88A, N90) to maximize statistical power for detecting Fe depletion offsets. The tool would use the paper’s finding that single-sightline cases are insufficient, and prioritize targets with direct Fe iv detections (like N88A) to minimize ionization correction biases. This improves observational efficiency for future HST/COS campaigns.

6. Metallicity-Dependent Depletion Baseline Calibration: Update AI-driven ISM evolution models to incorporate the paper’s result that SMC neutral ISM has a lower baseline Fe depletion than the LMC. The improved system would adjust dust-to-gas ratio predictions and grain growth models as a function of host galaxy metallicity, preventing overestimation of dust content in low-Z environments.

Depletion is the observed phenomenon where gas-phase elemental abundances are reduced through accretion onto dust grains. We measure neutral gas-phase elemental abundances (S, Fe) in the Magellanic Clouds along 33 sightlines using high-resolution UV spectroscopy (HST/COS and HST/STIS), and compare them to ionized gas-phase abundances (S, Fe) adopted from the literature for six co-located H, ii regions (with the furthest separation of 3', 50 pc). Comparing S abundances show that S is minimally depleted in the H, ii regions and surrounding diffuse ISM. However, we find that the gas-phase Fe abundances in H, ii regions can be lower than those of the neighboring neutral ISM by 0.3 to 2 dex. This difference is likely an offset in the amount of Fe depleted into dust grains. As accretion of gas-phase Fe is likely not effective at the temperatures of the H, ii regions, Fe depletion into solid form would have occurred in the dense atomic or molecular clouds prior to star formation. Stronger depletion in the H, ii regions shows that Fe-bearing grains survive destruction in the first few million years following ionization. Our observations highlight that Fe depletion in H, ii regions can be a useful tracer of Fe depletion in dense molecular clouds, which are challenging to observe directly via UV absorption.