JWST Observations of Young protoStars (JOYS+): Detecting icy complex organic molecules and ions

Abstract

Complex organic molecules (COMs) are ubiquitously detected in the gas phase and thought to be mostly formed on icy grains. Nevertheless, there have not been any unambiguous detections of COMs larger than CH$_3$OH in ices reported thus far. Exploring this matter in greater detail has now 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$-$10 mu m range, the fingerprint region of oxygen-bearing COMs. In the JWST Observations of Young protoStars (JOYS+) program, more than 30 protostars are undergoing observation with the Medium Resolution Spectrograph (MRS) of the Mid-IR Instrument (MIRI). The goal of this study is to comprehensively explore the COMs ice signatures in one low- and one high-mass protostar: NGC 1333 IRAS 2A and IRAS 23385+6053, respectively. We performed global continuum and silicate subtractions of the MIRI-MRS spectra, followed by a local continuum subtraction in optical depth scale in the range around 6.8 and 8.6 mu m, the ice COM fingerprint region. We explored different choices for the local continuum and silicate subtraction. Next, we fit the observational data with a large sample of available IR laboratory ice spectra. We used the ENIIGMA fitting tool, a genetic algorithm-based code that not only finds the best fit between the lab data and the observations, but also performs a statistical analysis of the solutions, such as deriving the confidence intervals and quantifying fit degeneracy. We report the best fits for the spectral ranges between 6.8 and 8.6 mu m in NGC 1333 IRAS 2A and IRAS 23385+6053, originating from simple molecules and COMs, as well as negative ions. Overall, we find that ten chemical species are needed to reproduce the astronomical data. The strongest feature in this range (7.7 mu m) is dominated by CH$_4$, with contributions from SO$_2$ and OCN$^-$. Our results indicate that the 7.2 and 7.4 mu m bands are mostly dominated by HCOO$^-$. We also find statistically robust detections of COMs based on multiple bands, most notably, CH$_3$CHO, CH$_3$CH$_2$OH, and CH$_3$OCHO. We also report a likely detection of CH$_3$COOH. Based on the ice column density ratios between CH$_3$CH$_2$OH and CH$_3$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. Based on the high-quality JWST (MIRI-MRS) spectra, we conclude that COMs are present in interstellar ices, thus providing additional proof for the 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 may be inherited from the early protostellar phases to a significant extent.

JOYS+: Mid-infrared detection of gas-phase SO2 emission in a low-mass protostar

“…These studies have shown that the SO 2 in these high-mass systems resides at typical temperatures of ∼100−300 K, although higher temperatures of up to 700 K have also been reported (Keane et al 2001). The average abundances with respect to H 2 are >10 −7 , which is consistent with the SO 2 abundance of interstellar ices (Boogert et al 1997(Boogert et al , 2015Zasowski et al 2009;McClure et al 2023;Rocha et al 2024) and cometary ices (Altwegg et al 2019;Rubin et al 2019), suggesting that gaseous SO 2 may originate from ice sublimation in the inner hot cores of these high-mass protostellar systems. A MIR detection of gaseous SO 2 , either in absorption or emission, toward low-mass protostellar systems is, to the best of our knowledge, still lacking.…”

“…In order to fit the SO 2 emission in the ν 3 band, the local continuum had to be subtracted. However, the spectral region surrounding the emission features is dominated by absorption of ices such as the 7.25 and 7.4 µm ice bands that are typically ascribed to complex organics including ethanol (CH 3 CH 2 OH) and acetaldehyde (CH 3 CHO) or the formate ion (HCOO − ; e.g., Schutte et al 1999;Öberg et al 2011;Boogert et al 2015;Terwisscha van Scheltinga et al 2018;Rocha et al 2024). A fourth-order polynomial was fitted through obvious linefree channels (i.e., 7.015, 7.113, 7.162, 7.442, 7.489, 7.545, and 7.635 µm; see the top panel of Fig.…”

“…B.1). Furthermore, several ice absorption features, including those of SO 2 ice, are detected toward the bright continuum source and have been analyzed by Rocha et al (2024). Any gas emission and absorption features other than SO 2 in this source, including the CO 2 emission in the outflow, will also be presented in future papers.…”

“…B.3. Similar to the analysis of the ν 3 band, both models are corrected for an extinction of A V = 55 mag (Rocha et al 2024) using a modified version of the McClure (2009) extinction law (see Appendix C). No obvious SO 2 emission features are present in the data whereas the best-fit ν 3 LTE model clearly predicts that we should have seen the SO 2 at these wavelengths.…”

“…It is therefore important to constrain the physical conditions in which the volatile sulfur-bearing species reside in order to understand their chemistry and constrain the main sulfur reservoirs. JWST has proven that it can detect sulfurbearing species both in interstellar ices (e.g., SO 2 and OCS; Yang et al 2022;McClure et al 2023;Rocha et al 2024) and in exoplanetary atmospheres (e.g., SO 2 ; Tsai et al 2023). Here, we present one of the first detected medium-resolution midinfrared (MIR) spectra of a low-mass Class 0 protostellar system, NGC 1333 IRAS 2A, containing the first detection of gaseous SO 2 in emission at MIR wavelengths.…”