2025ApJ...992L..30S

We present the results of James Webb Space Telescope observations of the protostar ST6 in the Large Magellanic Cloud (LMC) with the Medium Resolution Spectrograph of the Mid-Infrared Instrument (4.9-27.9 μm). Characterized by one-third to half-solar metallicity and strong UV radiation fields, the environment of the LMC allows us to study the physics and chemistry of star-forming regions under conditions similar to those at earlier cosmological epochs. We detected five icy complex organic molecules (COMs): methanol (CH₃OH), acetaldehyde (CH₃CHO), ethanol (CH₃CH₂OH), methyl formate (HCOOCH₃), and acetic acid (CH₃COOH). This is the first conclusive detection of CH₃COOH ice in an astrophysical context, and CH₃CHO, CH₃CH₂OH, and HCOOCH₃ ices are the first secure detections outside the Galaxy and in a low-metallicity environment. We address the presence of glycolaldehyde (HOCH₂CHO, a precursor of biomolecules), an isomer of HCOOCH₃ and CH₃COOH, but its detection is inconclusive. ST6's spectrum is also rich in simple ices: H₂O, CO₂, CH₄, SO₂, H₂CO, HCOOH, OCN^–, HCOO^–, NH₃, and NH₄⁺. We obtain the composition and molecular abundances in the icy dust mantles by fitting the spectrum in the 6.8-8.4 μm range with a large sample of laboratory ice spectra using the ENIIGMA fitting tool or the local continuum method. We found differences in the simple and COM ice abundances with respect to H₂O ice between ST6 and Galactic protostars that likely reflect differences in metallicity and UV flux. More laboratory ice spectra of COMs are needed to better reconstruct the observed infrared spectra of protostars.