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To date, 241 individual molecular species, composed of 19 different elements, have been detected in the interstellar and circumstellar medium by astronomical observations. These molecules range in size from two atoms to 70 and have been detected across the electromagnetic spectrum from centimeter wavelengths to the ultraviolet. This census presents a summary of the first detection of each molecular species, including the observational facility, wavelength range, transitions, and enabling laboratory spectroscopic work, as well as listing tentative and disputed detections. Tables of molecules detected in interstellar ices, external galaxies, protoplanetary disks, and exoplanetary atmospheres are provided. A number of visual representations of these aggregate data are presented and briefly discussed in context.

The detection of interstellar complex organic molecules (iCOMs), such as acetaldehyde (CH3CHO), in cold astrophysical environments challenges standard grain-surface chemistry models, which rely on radical diffusion. At prestellar core temperatures (∼10 K), radical mobility is severely limited, making prevailing formation pathways inefficient. We here present a computational investigation of a nondiffusive, three-body reaction (3BR) mechanism for acetaldehyde formation on interstellar water ices. The mechanism involves the hydrogenation of CO to form HCO near a CH3 radical, enabling immediate reaction without requiring diffusion. We characterize the potential energy surface of the 3BR on a crystalline water ice model by identifying key minima and transition states, as well as evaluating competing pathways such as hydrogen abstraction. To assess the efficiency of this mechanism, ab initio molecular dynamics simulations were performed, with results indicating that CH3CHO can form efficiently at temperatures as low as 10 K. However, the formation of alternative products, such as CH4 and CO, or nonreaction between the newly formed HCO and the nearby CH3, is also observed, leading to an outcome distribution. These findings support the viability of 3BRs as a potential route for iCOM formation in cold interstellar environments.

The typical amount of molecular hydrogen (H2) in interstellar ices is not known, but significant freeze-out of H2 on dust grains is not expected. However, chemical models ubiquitously predict large amounts of H2 freeze-out in dense cloud conditions, and specialized treatments are needed to control the H2 population on grains. Here we present a numerical desorption model where the effect of weak heating events induced by cosmic rays (CRs) that heat grains to temperatures of a few tens of kelvin at high frequencies is included, improving upon earlier desorption models that only consider strong heating events (maximum grain temperature close to 100 K) that occur at a low frequency. A temperature of a few tens of kelvin is high enough to induce efficient desorption of H2, but we find that even the weak heating events do not occur often enough to lead to significant H2 desorption. Taking the weak heating events into account does affect the predicted abundances of other lightly bound species, but the effect is restricted to low column densities. We make here the canonical assumption that the grains are spherical with a radius of 0.1 μm. It is conceivable that in the case of a grain size distribution, weak heating events could provide a boost to H2 desorption coming off small grains, which are the most numerous. Further studies are still required to better quantify the role of CRs in the desorption of H2 and other weakly bound species.

We present some of the first infrared spectral maps acquired by SPHEREx. These maps, which to our knowledge are the largest of their type ever compiled in the near-infrared, reveal multiple strong lines due to interstellar ices and polycyclic aromatic hydrocarbons (PAHs) throughout the Cygnus X and North American Nebula regions. The maps emphasize the strongest features arising from the 3 μm H2O, 4.27 μm CO2, and 4.67 μm CO lines and the 3.28 μm PAH feature, all of which are detected over large areas with complex and filamentary spatial distributions. The ice absorption maps of H2O and CO2 in particular broadly trace dense, cold, and well-shielded regions across Cygnus X, consistent with the established picture of efficient ice formation in dense molecular clouds. The interstellar ice features are also detected abundantly in diffuse absorption over wide areas. The relative strengths of the H2O and CO2 features vary among different lines of sight, indicating possible differences in local physical conditions or chemical variations. The 3.28 μm PAH emission correlates with the emission from the 7.7 and 11.2 μm features but shows small differences that may trace the grain-size distribution and variations in the ambient UV field. SPHEREx all-sky spectral imaging—only a small fraction of which is showcased in this work—will support numerous science investigations, including the structure of the Galaxy, the physics of the interstellar medium, and the chemistry of stars.

The successive addition of H atoms to CO in the solid phase has been hitherto regarded as the primary route to form methanol in dark molecular clouds. However, recent Monte Carlo simulations of interstellar ices alternatively suggested the radical-molecule H-atom abstraction reaction CH3O + H2CO → CH3OH + HCO, in addition to CH3O + H → CH3OH, as a very promising and possibly dominating (70%–90%) final step to form CH3OH in those environments. Here, we compare the contributions of these two steps leading to methanol by experimentally investigating hydrogenation reactions on H2CO and D2CO ices, which ensures comparable starting points between the two scenarios. The experiments are performed under ultrahigh vacuum conditions and astronomically relevant temperatures, with H:H2CO (or D2CO) flux ratios of 10:1 and 30:1. The radical-molecule route in the partially deuterated scenario, CHD2O + D2CO → CHD2OD + DCO, is significantly hampered by the isotope effect in the D-abstraction process, and can thus be used as an artifice to probe the efficiency of this step. We observe a significantly smaller yield of D2CO + H products in comparison to H2CO + H, implying that the CH3O-induced abstraction route must play an important role in the formation of methanol in interstellar ices. Reflection-absorption infrared spectroscopy and temperature-programmed desorption-quadrupole mass spectrometry analyses are used to quantify the species in the ice. Both analytical techniques indicate constant contributions of ∼80% for the abstraction route in the 10–16 K interval, which agrees well with the Monte Carlo calculations. Additional H2CO + D experiments confirm these conclusions.

Infrared spectroscopic observations have established the presence of solid methanol (CH3OH) in the interstellar medium and in solar system ices, but the abundance of frozen CH3OH cannot be deduced without accurate band strengths, optical constants, and reference spectra. In this paper we identify disagreements, omissions, and gaps in the literature on infrared (IR) intensities of methanol ices, including unaddressed concerns that reach back several decades. New spectra are presented with intensity measurements aided by new data on the index of refraction and density of solid CH3OH. The result is that the large discordant results from different laboratory groups can now be reconciled. Multiple ices have been used to determine, apparently for the first time, IR intensities of H2O + CH3OH mixtures of accurately known composition for use with observations of interstellar ices. Also for the first time, measurements on CH3OH ices with different thicknesses have allowed us to report both near-IR band strengths and optical constants for two near-IR features used by planetary scientists. We have used our new IR results to determine vapor pressures of solid CH3OH and have compared them to measurements made with a quartz-crystal microbalance. Thermal annealings of methanol ices have been carried out and phase changes in the solid state examined. Comparisons of our results to earlier work are presented where possible, and electronic versions of our new results are made available.

Comets have similar compositions to interstellar medium ices, suggesting at least some of their molecules may be inherited from an earlier stage of evolution. To investigate the degree to which this might have occurred, we compare the composition of individual comets to that of the well-studied protostellar region IRAS 16293–2422B. We show that the observed molecular abundance ratios in several comets correlate well with those observed in the protostellar source. However, this does not necessarily mean that the cometary abundances are identical to protostellar. We find the abundance ratios of many molecules present in comets are enhanced compared to their protostellar counterparts. For COH molecules, the data suggest higher abundances relative to methanol of more complex species, e.g., HCOOH, CH3CHO, and HCOOCH3, are found in comets. For N-bearing molecules, the ratios of nitriles relative to CH3CN—HC3N/CH3CN and HCN/CH3CN—tend to be enhanced. The abundances of cometary SO and SO2 relative to H2S are enhanced, whereas OCS/H2S is reduced. Using a subset of comets with a common set of observed molecules, we suggest a possible means of determining the relative degree to which they retain interstellar ices. This analysis suggests that over 84% of COH-bearing molecules can be explained by the protostellar composition. The possible fraction inherited from the protostellar region is lower for nitrogen molecules, at only 26%–74%. While this is still speculative, especially since few comets have large numbers of observed molecules, it provides a possible route for determining the relative degree to which comets contain disk-processed material.

Although oxygenated benzene derivatives are key precursors in the abiotic synthesis of biorelevant molecules and fundamental building blocks of functionalized polycyclic aromatic hydrocarbons, their formation mechanisms under interstellar conditions have remained largely unexplored. Here, we report the first bottom-up formation of phenol (C6H5OH) in low-temperature interstellar ice analogs composed of acetylene and water (C2H2–H2O). Utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry and resonance-enhanced multiphoton ionization, phenol, along with aromatic hydrocarbons including benzene (C6H6), phenylacetylene (C6H5CCH), styrene (C6H5CHCH2), naphthalene (C10H8), and phenanthrene (C14H10), were identified in the gas phase during temperature-programmed desorption. Among these species, styrene, naphthalene, and phenanthrene have not yet been detected in the interstellar medium, suggesting that they are suitable targets for future astronomical searches. These findings reveal viable low-temperature formation pathways for phenol through nonequilibrium chemistry in acetylene-containing interstellar ices, thereby advancing our understanding of the abiotic formation of oxygenated benzene derivatives in extraterrestrial environments.

The interaction of carbon atoms with solid carbon monoxide (CO) is a fundamental process in astrochemistry, influencing the formation of complex organic molecules in interstellar environments. This study investigates the adsorption and reaction mechanisms of carbon atoms on solid CO under cryogenic conditions, employing a combination of experimental techniques, including the combination of photostimulated desorption and resonance-enhanced multiphoton ionization and infrared spectroscopy, alongside quantum chemical calculations. The results reveal the formation of oxygenated carbon chains, such as CCO, C3O2, and C5O2, as well as CO2. The findings highlight the role of chemisorption and subsequent reactions in driving molecular complexity on solid CO, with implications for the chemical evolution of interstellar ices and the potential formation of prebiotic molecules.

Enols—tautomers of ketones or aldehydes—are considered key intermediates in the formation of prebiotic sugars and sugar acids. Although laboratory simulation experiments suggest that enols should be ubiquitous in the interstellar medium, the underlying formation mechanisms of enols in interstellar environments are largely elusive. Here, we present the laboratory experiments on the formation of glyoxal (HCOCHO) along with its ynol tautomer acetylenediol (HOCCOH) in interstellar ice analogs composed of carbon monoxide (CO) and water (H2O) upon exposure to energetic electrons as a proxy for secondary electrons generated from Galactic cosmic rays. Utilizing tunable vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry, glyoxal and acetylenediol were detected in the gas phase during temperature-programmed desorption. Our results reveal the formation pathways of glyoxal via radical–radical recombination of two formyl (HĊO) radicals, and that of acetylenediol via keto-enol-ynol tautomerization. Due to the abundance of carbon monoxide and water in interstellar ices, glyoxal and acetylenediol are suitable candidates for future astronomical searches. Furthermore, the detection of acetylenediol in astrophysically relevant ices advances our understanding for the formation pathways of high-energy tautomers such as enols in deep space.