Probing the Ubiquity of Complex Ices in Protostars with JWST: The First Systematic Quantification of Weak Ice Bands between 6.8 and 7.9 μ m

Complex organic molecules (COMs) are the key to understanding the chemical evolution from simple interstellar molecules to potential prebiotic material. Although COMs have been extensively studied in the gas phase toward protostars, their counterparts in ices, where they are thought to form at earlier stages, remain far less constrained. A number of diagnostic features of complex ices lie between 6.8 and 8.8 μ m, a region known as the “COM ice fingerprint range,” but previous infrared facilities lacked the sensitivity and spectral resolution required to quantify the weak bands therein. With the unprecedented sensitivity and resolving power of the James Webb Space Telescope, these limitations can now be overcome. Here, we present the first large-sample quantitative study of the absorption features at 7.02, 7.24, 7.40, and 7.67 μ m, using mid-infrared spectra of 21 protostars observed by the Medium Resolution Spectroscopy mode of the Mid-Infrared Instrument. The CH 4 band at 7.67 μ m is the strongest band and shows remarkably uniform peak positions (7.67–7.68 μ m) and full widths at half-maximum (FWHMs; 0.06–0.08 μ m), suggesting CH 4 ice as its dominant carrier. The 7.24 and 7.40 μ m bands exhibit larger source-to-source variations in peak positions and FWHMs, but their occurrence and intensities are strongly correlated with each other. Comparisons with existing and new laboratory spectra suggest HCOO − as the most likely carrier of these two bands, yet HCOO − cannot fully reproduce their intensity ratios, implying additional contributions from other species such as C 2 H 5 OH, CH 3 CHO, and CH 3 COCH 3 . Our results reveal, for the first time, the potential ubiquity of weak features of complex ices in protostars, which have remained largely undetected due to observational limitations.