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Abstract Despite the organic molecule inventory detected in the Orion Kleinmann–Low Nebula (Orion KL), acetaldehyde (CH₃CHO)—one of the most ubiquitous interstellar aldehydes—has not been firmly identified with millimeter-wave interferometry. We analyze extensive Atacama Large Millimeter/submillimeter Array archival data sets (142–355 GHz) to search for acetaldehyde, revealing two distinct acetaldehyde emission peaks and one component with more complex kinematic structures. One peak aligns with MF10/IRc2, where emissions of other O-bearing complex organic molecules are rarely reported, while the other is coincident with the ethanol peak in the southwest region of the hot core. The MF10/IRc2 detection suggests unique chemistry, possibly influenced by repeated heating events. In contrast, codetection with ethanol indicates an ice origin and suggests a potential chemical relationship between the two species. We determine acetaldehyde column densities and kinetic temperatures toward these two peaks under local thermodynamic equilibrium assumptions and compare its distribution with ethanol and other molecules that have an aldehyde (HCO) group, such as methyl formate, glycolaldehyde, and formic acid. Toward the ethanol peak, the observed abundance ratios between HCO-containing species are analyzed using a chemical model. The model suggests two key points: (1) the destruction of ethanol to form acetaldehyde in the ice may contribute to the observed correlation between the two species; and (2) a long cold-collapse timescale and a methyl formate binding energy similar to or lower than water are needed to explain the observations. The relative abundance ratios obtained from the model are highly sensitive to the assumed kinetic temperature, which accounts for the high spatial variability of the aldehyde ratios observed toward Orion KL. 

ABSTRACT Sodium-bearing species such as NaCl in the gas phase have been observed in an assortment of carbon-rich and oxygen-rich stellar atmospheres and interstellar environments such as the high-mass protostellar disc surrounding Orion Src1 and the proto-binary system, IRAS 16547−4247. Their detection in relatively low-temperature regions is yet to be made. In this paper, we consider the synthesis of sodium-bearing species with an emphasis on NaCl, via both gas-phase and grain-surface chemistry under assorted interstellar conditions. We also consider the chemistry leading to the gas-phase species NaH and NaOH. Two classes of numerical simulations were run: models under isothermal conditions at temperatures from 10 to 800 K with varied intervals, and three-phase warm-up models that consist of an initial isothermal collapse at 10 K, followed by a warm-up phase in which temperature rises linearly to 200 K, and finally a hot core phase. We have included reactive desorption for both models to produce gaseous NaCl, NaH, and NaOH. We found that for isothermal models over a broad parameter space, the fractional abundances of gaseous NaCl and NaOH can reach above 2 × 10−10 and approx. 1 × 10−10, respectively, are in the detection range of observational facilities such as Atacama Large Millimeter/Submillimeter Array and JWST. For warm-up models, we found that if we consider molecules to be co-desorbed with water, gaseous NaCl can have a sufficiently large abundance for detection. We then conclude that both gaseous NaCl and NaOH can be detected; however, more experiments and quantum mechanical calculations are needed to constrain the relevant reaction rates better. 

Abstract At centimeter wavelengths, single-dish observations have suggested that the Sagittarius (Sgr) B2 molecular cloud at the Galactic Center hosts weak maser emission from several organic molecules, including CH₂NH, HNCNH, and HCOOCH₃. However, the lack of spatial distribution information on these new maser species has prevented us from assessing the excitation conditions of the maser emission as well as their pumping mechanisms. Here, we present a mapping study toward Sgr B2 north (N) to locate the region where the complex maser emission originates. We report the first detection of the Class I methanol (CH₃OH) maser at 84 GHz and the first interferometric map of the methanimine (CH₂NH) maser at 5.29 GHz toward this region. In addition, we present a tool for modeling and fitting the unsaturated molecular maser signals with non-LTE radiative transfer models and Bayesian analysis using the Markov Chain Monte Carlo approach. These enable us to quantitatively assess the observed spectral profiles. The results suggest a two-chain-clump model for explaining the intense CH₃OH Class I maser emission toward a region with low continuum background radiation. By comparing the spatial origin and extent of maser emission from several molecular species, we find that the 5.29 GHz CH₂NH maser has a close spatial relationship with the 84 GHz CH₃OH Class I masers. This relationship serves as observational evidence to suggest a similar collisional pumping mechanism for these maser transitions. 

ABSTRACT Two closely related isomeric pairs of cyanides, CH3[CN/NC] and H2C[CN/NC], are studied in cold, dark interstellar cloud conditions. In contrast to the diverse detections of methyl cyanide (CH3CN) in space, methyl isocyanide (CH3NC) has previously only been observed in warm and hot star-forming regions. We report the detection of CH3NC in the cold pre-stellar core Taurus Molecular Cloud (TMC-1) using the Green Bank Telescope with a detection significance of 13.4σ. Hyperfine transitions in H2CCN and quadrupole interactions in CH3CN and CH3NC were matched to a spectral line survey from the Green Bank Telescope Observations of TMC-1: Hunting for Aromatic Molecules large project on the Green Bank Telescope, resulting in abundances with respect to hydrogen of $$1.92^{+0.13}_{-0.07} \times 10^{-9}$$ for the cyanomethyl radical (H2CCN), $$5.02^{+3.08}_{-2.06} \times 10^{-10}$$ for CH3CN, and $$2.97^{+2.10}_{-1.37} \times 10^{-11}$$ for CH3NC. Efforts to model these molecules with the three-phase gas-grain code nautilus in TMC-1 conditions overproduce both CH3CN and CH3NC, though the ratio of ∼5.9 per cent is consistent across observations and models of these species. This may point to missing destruction routes in the model. The models capture the larger abundance of H2CCN well. Dissociative recombination is found to be the primary production route for these molecules, and reactions with abundant ions are found to be the primary destruction routes. H + CH3NC is investigated with transition state theory as a potential destruction route, but found to be too slow in cold cloud conditions to account for the discrepancy in modelled and observed abundances of CH3NC. 

ABSTRACT The formation of complex organic molecules (COMs) in interstellar conditions is influenced by several different processes occurring both in the gas and solid phases. Here we perform an extension of previous work to understand the influence of electronically excited metastable species on condensed phase COM formation via insertion-type reactions. These reactions involve the insertion of a chemical entity on a previously existing chemical bond. Such insertion processes involving a metastable species allow for rapid reactions with the surrounding grain ice in the absence of activation energy or diffusion barriers even under cold, dark cloud conditions. In this paper, the production of a number of interstellar species including COMs in cold dark clouds is treated both via the metastable process as well as existing suggested pathways such as radical recombination and hydrogenation of unsaturated species in order to gain insight about the relative importance of the newly added process. 

Abstract We present Atacama Large Millimeter/submillimeter Array Band 3 data toward five massive young stellar objects (MYSOs), and investigate relationships between unsaturated carbon-chain species and saturated complex organic molecules (COMs). An HC 5 N ( J = 35–34) line has been detected from three MYSOs, where nitrogen (N)-bearing COMs (CH 2 CHCN and CH 3 CH 2 CN) have been detected. The HC 5 N spatial distributions show compact features and match with a methanol (CH 3 OH) line with an upper-state energy around 300 K, which should trace hot cores. The hot regions are more extended around the MYSOs where N-bearing COMs and HC 5 N have been detected compared to two MYSOs without these molecular lines, while there are no clear differences in the bolometric luminosity and temperature. We run chemical simulations of hot-core models with a warm-up stage, and compare with the observational results. The observed abundances of HC 5 N and COMs show good agreements with the model at the hot-core stage with temperatures above 160 K. These results indicate that carbon-chain chemistry around the MYSOs cannot be reproduced by warm carbon-chain chemistry, and a new type of carbon-chain chemistry occurs in hot regions around MYSOs. 

ABSTRACT In the interstellar medium (ISM), the formation of complex organic molecules (COMs) is largely facilitated by surface reactions. However, in cold dark clouds, thermal desorption of COMs is inefficient because of the lack of thermal energy to overcome binding energies to the grain surface. Non-thermal desorption methods are therefore important explanations for the gas-phase detection of many COMs that are primarily formed on grains. Here, we present a new non-thermal desorption process: cosmic ray sputtering of grain ice surfaces based on water, carbon dioxide, and a simple mixed ice. Our model applies estimated rates of sputtering to the three-phase rate equation model nautilus-1.1, where this inclusion results in enhanced gas-phase abundances for molecules produced by grain reactions such as methanol (CH3OH) and methyl formate (HCOOCH3). Notably, species with efficient gas-phase destruction pathways exhibit less of an increase in models with sputtering compared to other molecules. These model results suggest that sputtering is an efficient, non-specific method of non-thermal desorption that should be considered as an important factor in future chemical models. 

Abstract We report a comprehensive study of the cyanopolyyne chemistry in the prototypical prestellar core L1544. Using the 100 m Robert C. Byrd Green Bank Telescope, we observe three emission lines of HC₃N, nine lines of HC₅N, five lines of HC₇N, and nine lines of HC₉N. HC₉N is detected for the first time toward the source. The high spectral resolution (∼0.05 km s⁻¹) reveals double-peak spectral line profiles with the redshifted peak a factor 3–5 brighter. Resolved maps of the core in other molecular tracers indicate that the southern region is redshifted. Therefore, the bulk of the cyanopolyyne emission is likely associated with the southern region of the core, where free carbon atoms are available to form long chains, thanks to the more efficient illumination of the interstellar field radiation. We perform a simultaneous modeling of the HC₅N, HC₇N, and HC₉N lines to investigate the origin of the emission. To enable this analysis, we performed new calculation of the collisional coefficients. The simultaneous fitting indicates a gas kinetic temperature of 5–12 K, a source size of 80″, and a gas density larger than 100 cm⁻³. The HC₅N:HC₇N:HC₉N abundance ratios measured in L1544 are about 1:6:4. We compare our observations with those toward the well-studied starless core TMC-1 and with the available measurements in different star-forming regions. The comparison suggests that a complex carbon chain chemistry is active in other sources and is related to the presence of free gaseous carbon. Finally, we discuss the possible formation and destruction routes in light of the new observations.