JWST Observations of Young protoStars (JOYS+): Detection of icy complex organic molecules and ions I. CH4 , SO2 , HCOO− , OCN−, H2 CO, HCOOH, CH3 CH2 OH, CH3 CHO, CH3 OCHO, CH3 COOH

Context. Complex organic molecules (COMs) are ubiquitously detected in the gas phase and are thought to be mostly formed on icy grains. Nevertheless, no unambiguous detection of COMs larger than CH3 OH in ices has been reported so far, but exploring this matter in more detail has become possible with the unprecedented possibilities offered by the James Webb Space Telescope (JWST) within the infrared (IR) spectral range with its very high sensitivity and spectral resolution in the critical 5−10 μm range, the fingerprint region of oxygen-bearing COMs. Aims. In the program JWST Observations of Young protoStars (JOYS+), more than 30 protostars are being observed with the Medium Resolution Spectrograph (MRS) of the Mid-IR Instrument (MIRI). The goal of this study is to comprehensively explore the COMs ice signatures in one low- and one high-mass protostar, NGC 1333 IRAS 2A and IRAS 23385+6053, respectively. Methods. We perform global continuum and silicate subtractions of the MIRI-MRS spectra, followed by a local continuum subtraction in optical depth scale in the range around 6.8 and 8.6 μm, the ice COM fingerprint region. Different choices of local continuum and silicate subtraction were explored. Next, we fit observational data with a large sample of available IR laboratory ice spectra. We use the ENIIGMA fitting tool, a genetic algorithm-based code that not only finds the best fit between the lab data and the observations but also performs statistical analysis of the solutions, such as deriving the confidence intervals and quantifying fit degeneracy. Results. We report the best fits for the spectral ranges between 6.8 and 8.6 μm in NGC 1333 IRAS 2A and IRAS 23385+6053, originating from simple molecules and COMs, as well as negative ions. In total, 10 chemical species are needed to reproduce the astronomical data. The strongest feature in this range (7.7 μm) is dominated by CH4 and has contributions of SO2 and OCN−. Our results indicate that the 7.2 and 7.4 μm bands are mostly dominated by HCOO−. We also find statistically robust detections of COMs based on multiple bands, most notably CH3CHO, CH3 CH2 OH, and CH3 OCHO. The likely detection of CH3 COOH is also reported. Based on the ice column density ratios between CH3CH2 OH and CH3 CHO of NGC 1333 IRAS 2A and IRAS 23385+6053, we find compelling evidence that these COMs are formed on icy grains. Finally, the derived ice abundances for NGC 1333 IRAS 2A correlate well with those in comet 67P/GC within a factor of 5. Conclusions. Based on the high-quality JWST (MIRI-MRS) spectra, we conclude that COMs are present in interstellar ices, thus providing additional proof for a solid-state origin of these species in star-forming regions. In addition, the good correlation between the ice abundances in comet 67P and NGC 1333 IRAS 2A is fully in line with the idea that cometary COMs can be significantly inherited from the early protostellar phases.