Abstract
JWST observations of the formation and evolution of ices at the earliest stages of the formation of new stars are presented and comparisons with our own young solar system are made. The detection of complex organic molecules in ices is highlighted and their relation to complex molecules in the gas discussed.
Background
Protostellar chemistry, while it may appear distant from that on Earth, has substantial implications for planet formation as well as the chemical environment in the early stages of the Solar system. Comets, which are thought to be a major reservoir of Earth’s oceans, likely inherit heavy water (deuterated water) from the formation stage of our Sun because of their similar abundance to that in young protostars, classified as Class 0 and I protostars (Cleeves et al. 2014; Tobin et al. 2023; Nomura et al. 2023). For more complex molecules, recent observations start to routinely detect gaseous organic molecules in protostars, some of which also have similar compositions as that in comet 67P/Churyumov-Gerasimenko, which is the most well-studied comet (Drozdovskaya et al. 2019), further emphasizing the need of understanding protostellar chemistry to realize the formation and evolution of our solar system.
Protostellar environments are cold (~10-20 K; ~250℃ below zero) and low-density (106-108 cm-3 compared to ~1019 cm-3 in Earth’s atmosphere at surface), making them uniquely different from terrestrial chemistry. In such environments, a substantial amount of gas freezes out onto dust grains, forming ice mantles where molecules have greater opportunities to react. Thus, the reactions that take place on the ice mantles and between gas and ice drive the chemical evolution at the protostellar stage. In the last decade, interferometric observations at sub-millimeter wavelengths enable a leap of discovery in the gas-phase protostellar chemistry, detecting rare isotopologues and complex organic molecules (so-called COMs; Jørgensen et al. 2020 and the references therein). Using these gas-phase measurements, we can infer the chemical reactions on ice mantles if the gas simply sublimates from ice at high temperature without any additional gas-phase reactions. However, this assumption greatly simplifies the reality and must be tested by direct observations of ice.
Measuring the absorption spectral features at infrared wavelengths is the most accessible way to characterize the ice compositions. Radiation coming from the central protostar penetrates the icy molecules in the cold dense gas around the protostar (the so-called envelope). In this process, parts of radiation could be absorbed due to the vibration of the icy molecules, leaving distinct absorption patterns on the observed IR spectra, which allow us to identify the species as well as quantify their abundances. Detecting ice absorption requires high-sensitivity IR observations, especially toward young protostars where the dusty envelope heavily attenuates the emission. Prior to the arrival of JWST, ice measurements relied on the Infrared Space Observatory (ISO), the AKARI Space Telescope, and the Spitzer Space Telescope combined with ground-based data (e.g., Whittet et al. 2001; Öberg et al. 2011; Kim et al. 2022). Because of the limited sensitivity, these observations detect mostly simple ice species, including H2O, CO, CO2, CH4, and CH3OH, toward the sources with sufficiently bright continuum. Thus, while we obtained a general picture of ice compositions and their evolution in protostars, further studies on ice mixtures in various environments and the search for rare isotopologues and complex organic ice species were on hold until JWST.
Protostellar Ice Chemistry in the Era of JWST
Since the start of JWST’s science operation, it has revolutionized our understanding of ice chemistry in protostars with its sensitivity, which is about two orders of magnitudes better than that of Spitzer, enabling ice measurements with exquisite details of the absorption features toward fainter sources. The first spectrum of an extremely young protostar, IRAS 15398-3359, showed exactly what we anticipated (Yang et al. 2022; Figure 1). Absorption features are clearly detected by the MIRI instrument (Rieke et al. 2015; Wright et al. 2015) with their shape unambiguously traced with high fidelity. We identified the commonly known ice features, including the H2O bending and libration modes, CH3OH, CH4, and NH3. Furthermore, we detected absorption features likely due to COMs at 7-8 μm, which were only hinted at in Spitzer observations of this source (Boogert et al. 2008). In tCycle 1 of JWST’s science operation, several other spectra of protostars with similar quality are also taken (Beuther et al. 2023; Federman et al. 2023), which will play a critical role in the first
stage of reshaping our understanding of ice chemistry in protostars.
The shape of ice absorption features is sensitive to the ice mixture as well as its structure (amorphous or crystalline). Changes from amorphous to crystalline and the mixing ratios often suggest ice processing in the past. Thus, constraining the ice morphology and their mixing ratio unveils the history of ice evolution. The pure CO2 ice feature is commonly considered as a signature of thermal processing (i.e., experiencing high temperature in the past), which can be easily identified by its narrow absorption feature. In a sample of five protostars, Brunken et al. (2024) decomposed the 13CO2 ice feature detected by JWST NIRSpec observations, detecting pure 13CO2 in the two most luminous protostars, which is consistent with the scenario of thermal processing. Previous studies of pure CO2 ice mostly rely on the 15 μm bending mode because the 4.27 μm stretching mode is usually saturated. But the bending mode is mixed with silicate and H2O ice features, introducing uncertainties to the analysis. Thus, the decomposition of 13CO2 demonstrated in Brunken et al. (2024) presents a new avenue to characterize the thermal history of protostars, which is only possible with JWST’s sensitivity.
Icy Complex Organic Molecules
In recent years, detection of gas-phase COMs has become common in embedded protostars. However, while some protostars have abundant gas-phase COMs (so-called COM-rich), many protostars show no sign of COM emission (so-called COM-poor). The contrast of their gas-phase chemical signatures begs the questions: Do these protostars go through distinctively different chemical evolution? And what processes govern the chemical evolution in the early phase of star formation? Gas-grain reactions on icy grains are the major pathways to form COMs; thus, it is imperative to directly probe the ice, searching for signatures of icy COMs. In the CORINOS program (PI: Y.-L. Yang), we targeted four protostars with and without abundant gas-phase COMs to determine whether the diversity in gaseous COMs originates from ice or is simply due to the efficiency of ice sublimation. As shown in Figure 1, detailed ice features are detected along with tentative detection of icy COMs. We also detect likely signatures of icy COMs in all four sources regardless of the presence of gaseous COMs. If these signatures represent icy COMs, our preliminary analysis suggests a similar abundance in ice- and gas-phase, hinting a direct connection between two phases (Kim et al. in prep.; Yang et al. in prep.). Absorption features of icy COMs are also detected by other programs. In the JOYS+ program, which combines two Guaranteed Time Programs led by E.F. van Dishoeck and M. Ressler, Rocha et al. (2023) present icy COM signatures toward a high-mass and a low-mass protostar, supported by modeling of their ~7-9 μm spectra (Figure 3). Several COMs are detected and all of them are O-bearing COMs. The abundances of several COMs in ices are consistent with those in the gas-phase as well as the abundances of Comet 67P, suggesting an inheritance. Their analysis also highlights systematic uncertainties due to the challenge of determining a continuum without absorption features, which could substantially affect broad ice features, such as CH3OCHO.
N-bearing COMs are also searched using JWST spectroscopic data. Before JWST, there is no detection of N-bearing COMs. Using the same dataset of Brunken et al. (2024), Nazari et al. (2024) found tentative detection of CH3CN ice in three protostars and derived upper limits in another two protostars. Despite being heavily blended with the C-N stretching mode of other N-bearing COMs, CH3CN ice feature is likely to present in HOPS 153, HOPS 370, and IRAS 20126+4104, the three more luminous protostars in the five-sources sample. They also found tentative evidence of enhanced icy N-COM abundances in warmer ice. These studies of icy COMs portray a promising picture of identifying unique absorption features of COMs and connecting the ice chemistry to what we have learned from gas-phase measurements.
Concluding Remarks
We are only in the second year of JWST’s science operation. The selected JWST’s Cycle 1 results already show the scientific questions that we can address only because of JWST. These observations highlight the need for sensitivity; they also demonstrate the complexity in constructing a robust ice inventory, contributed by various mixing ratios and the limitations inherited from laboratory measurements. We are now standing at a vantage point where we can characterize the changes of mixing ratios of simple ice species and their processing. We also start to detect COMs in ice and quantify their abundances. These first results are the first step in revamping our understanding of ice chemistry and their evolution in the protostellar stage.
Acknowledgement
Y.-L. Yang acknowledges support from Grant-in-Aid from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (20H05845, 20H05844, 22K20389), and a pioneering project in RIKEN (Evolution of Matter in the Universe).
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