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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 Spectral line surveys of the Taurus Molecular Cloud-1 (TMC-1) have led to the detection of more than 100 new molecular species, making it the most prolific source of interstellar molecular discoveries. These wide-band, high-sensitivity line surveys have been enabled by advances in telescope and receiver technology, particularly at centimeter and millimeter wavelengths. In this work, we present a statistical analysis of the molecular inventory of TMC-1 as probed by the GOTHAM large program survey from 3.9 to 36.4 GHz. To fully unlock the potential of the ∼29 GHz spectral bandwidth, we developed an automated pipeline for data reduction and calibration. We applied a Bayesian approach with Markov Chain Monte Carlo fitting to the calibrated spectra and constrained column densities for 102 molecular species detected in TMC-1, including 75 main isotopic species, 20 carbon-13 substituted species, and seven deuterium-substituted species. This list of the detected gas-phase molecules is populated by unsaturated hydrocarbons, in stark contrast to the oxygen-rich organics found in sublimated ices around protostars. Of note, 10 individual aromatic molecules were identified in the GOTHAM observations, contributing 0.011% of the gas-phase carbon budget probed by detected molecules when including CO and 6% when excluding CO. This work provides a reference set of observed gas-phase molecular abundances for interstellar clouds, offering a new benchmark for astrochemical theoretical models. 

Abstract We used new high spectral resolution observations of propynal (HCCCHO) toward TMC-1 and in the laboratory to update the spectral line catalog available for transitions of HCCCHO—specifically at frequencies lower than 30 GHz, which were previously discrepant in a publicly available catalog. The observed astronomical frequencies provided a high enough spectral resolution that, when combined with high-resolution (∼2 kHz) measurements taken in the laboratory, a new, consistent fit to both the laboratory and astronomical data was achieved. Now with a nearly exact (<1 kHz) frequency match to theJ= 2–1 and 3–2 transitions in the astronomical data, using a Markov Chain Monte Carlo analysis, a best fit to the total HCCCHO column density of ${7.28}_{-1.94}^{+4.08}\times {10}^{12}$cm⁻²was found with a surprisingly low excitation temperature of just over 3 K. This column density is around a factor of 5 times larger than reported in previous studies. Finally, this work highlights that care is needed when using publicly available spectral catalogs to characterize astronomical spectra. The availability of these catalogs is essential to the success of modern astronomical facilities and will only become more important as the next generation of facilities comes online. 

Abstract We present the spectroscopic characterization of cyclopropenethione in the laboratory and detect it in space using the Green Bank Telescope Observations of TMC-1: Hunting Aromatic Molecules survey. The detection of this molecule—the missing link in understanding the C₃H₂S isomeric family in TMC-1—completes the detection of all three low-energy isomers of C₃H₂S, as both CH₂CCS and HCCCHS have been previously detected in this source. The total column density of this molecule (N_Tof $5.72_{-1.61}^{+2.65}\times 10^{10}$cm⁻²at an excitation temperature of $4.7_{-1.1}^{+1.3}$K) is smaller than both CH₂CCS and HCCCHS and follows nicely the relative dipole principle (RDP), a kinetic rule of thumb for predicting isomer abundances that suggests that, all other chemistry among a family of isomers being the same, the member with the smallest dipole (μ) should be the most abundant. The RDP now holds for the astronomical abundance ratios of both the S-bearing and O-bearing counterparts observed in TMC-1; however, CH₂CCO continues to elude detection in any astronomical source. 

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 We present the synthesis and laboratory rotational spectroscopy of the seven-ring polycyclic aromatic hydrocarbon (PAH) cyanocoronene (C₂₄H₁₁CN) using a laser-ablation-assisted cavity-enhanced Fourier transform microwave spectrometer. A total of 71 transitions were measured and assigned between 6.8 and 10.6 GHz. Using these assignments, we searched for emission from cyanocoronene in the Green Bank Telescope (GBT) Observations of TMC-1: Hunting Aromatic Molecules project observations of the cold dark molecular cloud TMC-1 using the 100 m GBT. We detect a number of individually resolved transitions in ultrasensitiveX-band observations and perform a Markov Chain Monte Carlo analysis to derive best-fit parameters, including a total column density of $N\left({\mathrm{C}}_{24}{\mathrm{H}}_{11}\mathrm{CN}\right)=2.69_{-0.23}^{+0.26}\times 10^{12}{\mathrm{cm}}^{-2}$at a temperature of $6.05_{-0.37}^{+0.38}$K. A spectral stacking and matched filtering analysis provides a robust 17.3σsignificance to the overall detection. The derived column density is comparable to that of cyano-substituted naphthalene, acenaphthylene, and pyrene, defying the trend of decreasing abundance with increasing molecular size and complexity found for carbon chains. We discuss the implications of the detection for our understanding of interstellar PAH chemistry and highlight major open questions and next steps. 

Polycyclic aromatic hydrocarbons (PAHs) are organic molecules containing adjacent aromatic rings. Infrared emission bands show that PAHs are abundant in space, but only a few specific PAHs have been detected in the interstellar medium. We detected 1-cyanopyrene, a cyano-substituted derivative of the related four-ring PAH pyrene, in radio observations of the dense cloud TMC-1, using the Green Bank Telescope. The measured column density of 1-cyanopyrene is $\sim \text{1}{\text{.52×10}}^{\text{12}}$cm⁻², from which we estimate that pyrene contains up to 0.1% of the carbon in TMC-1. This abundance indicates that interstellar PAH chemistry favors the production of pyrene. We suggest that some of the carbon supplied to young planetary systems is carried by PAHs that originate in cold molecular clouds. 

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. 

Abstract The extraordinary 2021 September–October outburst of Centaur 29P/Schwassmann–Wachmann 1 afforded an opportunity to test the composition of primitive Kuiper disk material at high sensitivity. We conducted nearly simultaneous multiwavelength spectroscopic observations of 29P/Schwassmann–Wachmann 1 using iSHELL at the NASA Infrared Telescope Facility (IRTF) and nFLASH at the Atacama Pathfinder EXperiment (APEX) on 2021 October 6, with follow-up APEX/nFLASH observations on 2021 October 7 and 2022 April 3. This coordinated campaign between near-infrared and radio wavelengths enabled us to sample molecular emission from a wealth of coma molecules and to perform measurements that cannot be accomplished at either wavelength alone. We securely detected CO emission on all dates with both facilities, including velocity-resolved spectra of the CO (J= 2–1) transition with APEX/nFLASH and multiple CO (v= 1–0) rovibrational transitions with IRTF/iSHELL. We report rotational temperatures, coma kinematics, and production rates for CO and stringent (3σ) upper limits on abundance ratios relative to CO for CH₄, C₂H₆, CH₃OH, H₂CO, CS, and OCS. Our upper limits for CS/CO and OCS/CO represent their first values in the literature for this Centaur. Upper limits for CH₄, C₂H₆, CH₃OH, and H₂CO are the most stringent reported to date, and are most similar to values found in ultra CO-rich Oort cloud comet C/2016 R2 (PanSTARRS), which may have implications for how ices are preserved in cometary nuclei. We demonstrate the superb synergy of coordinated radio and near-infrared measurements, and advocate for future small-body studies that jointly leverage the capabilities of each wavelength.