A Chemistry-First Centered Icy Chemical Inventory of Protostellar Sources with JWST

The chemical evolution in star forming regions is driven by the interplay between gas and ice mantles. Identifying the ice compositions at the early stage of star formation thus provides constraints on the chemical processes inaccessible from gas-phase characterizations. As part of the CORINOS program, spectra from the James Webb Space Telescope (JWST) MIRI MRS were taken toward four Class 0 protostars: IRAS 15398-3359, Ser-emb7, L483, and B335. The spectra were processed with simultaneous fitting of a continuum and silicate absorption to produce optical depth mid-infrared spectra of the ices at 5-28 $μ$m (360-2000 cm$^{-1}$) toward these four sources. Simple molecules such as water (H$_2$O), carbon dioxide (CO$_2$), methanol (CH$_3$OH), formic acid/formate (HCOOH/HCOO$^-$), ammonia/ammonium (NH$_3$/NH$_4$$^+$), and formaldehyde (H$_2$CO) are the most abundant features in these ices, while complex organic molecules (COMs) represent a smaller contribution. Likely COMs include hydroxylamine (NH$_2$OH), methylamine (CH$_3$NH$_2$), and ethanol (CH$_3$CH$_2$OH). Absorption features belonging to functional groups such as -CH$_3$ and -OH suggest that additional COMs are present, but these cannot be unambiguously assigned due to overlapping bands. Formation pathways toward these COMs utilizing radical-radical combination reactions based on laboratory simulation experiments is presented. By extension, COMs predicted by these reactions, but absent from the spectra, are discussed. The results provide insight into the chemical environment of these ices and also highlight the critical need for caution and sufficient evidence in order to confidently identify COMs in ice.

💡 Research Summary

The paper presents a detailed JWST MIRI‑MRS study of ice absorption features toward four Class 0 protostars (IRAS 15398‑3359, Ser‑emb7, L483, and B335) as part of the CORINOS program. Observations cover the 5–28 µm (360–2000 cm⁻¹) range with high signal‑to‑noise, allowing the authors to extract reliable optical‑depth spectra after simultaneous fitting of a fourth‑order polynomial continuum and synthetic silicate (pyroxene and olivine) absorption. The resulting τ(λ) spectra were decomposed using Gaussian components and a Levenberg‑Marquardt optimizer (LMFit) until residuals matched the noise level.

Across all sources, the dominant ice constituents are water (H₂O), carbon dioxide (CO₂), methanol (CH₃OH), formic acid/formate (HCOOH/HCOO⁻), ammonia/ammonium (NH₃/NH₄⁺), and formaldehyde (H₂CO). Their column densities vary, with L483 and B335 showing the strongest water and methanol bands, indicative of colder, more ice‑rich envelopes.

The authors also search for complex organic molecules (COMs). Based on weak absorptions in the 6–10 µm region and the presence of functional‑group bands (–CH₃, –OH), they propose hydroxylamine (NH₂OH), methylamine (CH₃NH₂), and ethanol (CH₃CH₂OH) as plausible COM candidates. However, these features overlap with stronger simple‑ice bands, making definitive identification impossible with the current data. The paper emphasizes that functional‑group signatures suggest additional, as yet unassigned COMs, but stresses the need for caution.

To interpret the possible COM formation pathways, the authors draw on laboratory ice irradiation experiments that demonstrate radical–radical combination reactions at low temperatures (10–30 K). Examples include NH₂· + OH → NH₂OH, CH₃· + NH₂· → CH₃NH₂, and CH₃· + CH₂· → C₂H₆. These mechanisms are consistent with the tentative COM detections and explain how complex species can arise in the icy mantles before thermal desorption. The paper also discusses COMs predicted by these pathways (e.g., acetic acid, acetaldehyde, dimethyl ether) that are not observed, attributing their absence to limited S/N, band blending, or insufficient laboratory reference spectra.

Methodologically, the study contrasts traditional local baseline fitting with statistical global‑optimization tools such as ENIIGMA. While the latter can efficiently explore large ice libraries, it lacks built‑in chemical feasibility checks, requiring expert validation. The authors adopt a “chemistry‑first” strategy: they first use laboratory spectra to guide peak assignments, then verify that the resulting ice mixture is astrochemically plausible.

The paper concludes that JWST MIRI‑MRS provides unprecedented access to the mid‑infrared ice inventory of deeply embedded protostars, revealing both the ubiquity of simple ices and the tentative presence of a few COMs. Nonetheless, COM identification remains challenging due to overlapping bands, modest spectral resolution, and incomplete laboratory databases. Future work should combine higher‑resolution JWST modes (e.g., NIRSpec, MIRI high‑resolution), complementary millimeter observations (ALMA), and expanded laboratory ice spectra to confirm the tentative COMs and uncover additional species. The study underscores that icy mantles are fertile sites for prebiotic chemistry and that rigorous, evidence‑based identification is essential for advancing our understanding of chemical evolution in star‑forming regions.