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1. Laboratory rotational spectroscopy of acrylamide and a search for acrylamide and propionamide toward Sgr B2(N) with ALMA

   https://doi.org/10.1051/0004-6361/202142448  

   Kolesniková, L.; Belloche, A.; Koucký, J.; Alonso, E. R.; Garrod, R. T.; Luková, K.; Menten, K. M.; Müller, H. S.; Kania, P.; Urban, Š. (March 2022, Astronomy & Astrophysics)

   Context. Numerous complex organic molecules have been detected in the universe and among them are amides, which are considered as prime models for species containing a peptide linkage. In its backbone, acrylamide (CH 2 CHC(O)NH 2 ) bears not only the peptide bond, but also the vinyl functional group that is a common structural feature in many interstellar compounds. This makes acrylamide an interesting candidate for searches in the interstellar medium. In addition, a tentative detection of the related molecule propionamide (C 2 H 5 C(O)NH 2 ) has been recently claimed toward Sgr B2(N). Aims. The aim of this work is to extend the knowledge of the laboratory rotational spectrum of acrylamide to higher frequencies, which would make it possible to conduct a rigorous search for interstellar signatures of this amide using millimeter wave astronomy. Methods. We measured and analyzed the rotational spectrum of acrylamide between 75 and 480 GHz. We searched for emission of acrylamide in the imaging spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array toward Sgr B2(N). We also searched for propionamide in the same source. The astronomical spectra were analyzed under the assumption of local thermodynamic equilibrium. Results. We report accurate laboratory measurements and analyses of thousands of rotational transitions in the ground state and two excited vibrational states of the most stable syn form of acrylamide. In addition, we report an extensive set of rotational transitions for the less stable skew conformer. Tunneling through a low energy barrier between two symmetrically equivalent configurations has been revealed for this higher-energy species. Neither acrylamide nor propionamide were detected toward the two main hot molecular cores of Sgr B2(N). We did not detect propionamide either toward a position located to the east of the main hot core, thereby undermining the recent claim of its interstellar detection toward this position. We find that acrylamide and propionamide are at least 26 and 14 times less abundant, respectively, than acetamide toward the main hot core Sgr B2(N1S), and at least 6 and 3 times less abundant, respectively, than acetamide toward the secondary hot core Sgr B2(N2). Conclusions. A comparison with results of astrochemical kinetics model for related species suggests that acrylamide may be a few hundred times less abundant than acetamide, corresponding to a value that is at least an order of magnitude lower than the observational upper limits. Propionamide may be as little as only a factor of two less abundant than the upper limit derived toward Sgr B2(N1S). Lastly, the spectroscopic data presented in this work will aid future searches of acrylamide in space. 

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2. Thermal Properties of the Hot Core Population in Sagittarius B2 Deep South

   https://doi.org/10.3847/1538-4357/ad1507  

   Jeff, Desmond; Ginsburg, Adam; Bulatek, Alyssa; Budaiev, Nazar; Sánchez-Monge, Álvaro; Bonfand, Mélisse; Battersby, Cara; Meng, Fanyi; Schilke, Peter; Schmiedeke, Anika (February 2024, The Astrophysical Journal)

   Abstract We report the discovery of nine new hot molecular cores in the Deep South (DS) region of Sagittarius B2 using Atacama Large Millimeter/submillimeter Array Band 6 observations. We measure the rotational temperature of CH₃OH and derive the physical conditions present within these cores and the hot core Sgr B2(S). The cores show heterogeneous temperature structure, with peak temperatures between 252 and 662 K. We find that the cores span a range of masses (203–4842M_⊙) and radii (3587–9436 au). CH₃OH abundances consistently increase with temperature across the sample. Our measurements show the DS hot cores are structurally similar to Galactic disk hot cores, with radii and temperature gradients that are comparable to sources in the disk. They also show shallower density gradients than disk hot cores, which may arise from the Central Molecular Zone’s higher density threshold for star formation. The hot cores have properties which are consistent with those of Sgr B2(N), with three associated with Class II CH₃OH masers and one associated with an ultra-compact Hiiregion. Our sample nearly doubles the high-mass star-forming gas mass near Sgr B2(S) and suggests the region may be a younger, comparably massive counterpart to Sgr B2(N) and (M). The relationship between peak CH₃OH abundance and rotational temperature traced by our sample and a selection of comparable hot cores is qualitatively consistent with predictions from chemical modeling. However, we observe constant peak abundances at higher temperatures (T≳ 250 K), which may indicate mechanisms for methanol survival that are not yet accounted for in models. 

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3. Toward the limits of complexity of interstellar chemistry: Rotational spectroscopy and astronomical search for n - and i -butanal

   https://doi.org/10.1051/0004-6361/202142848  

   Sanz-Novo, M.; Belloche, A.; Rivilla, V. M.; Garrod, R. T.; Alonso, J. L.; Redondo, P.; Barrientos, C.; Kolesniková, L.; Valle, J. C.; Rodríguez-Almeida, L.; et al (October 2022, Astronomy & Astrophysics)

   Context.In recent times, large organic molecules of exceptional complexity have been found in diverse regions of the interstellar medium. Aims.In this context, we aim to provide accurate frequencies of the ground vibrational state of two key aliphatic aldehydes,n-butanal and its branched-chain isomer, i-butanal, to enable their eventual detection in the interstellar medium. We also want to test the level of complexity that interstellar chemistry can reach in regions of star formation. Methods.We employ a frequency modulation millimeter-wave absorption spectrometer to measure the rotational features ofn- andi-butanal. We analyze the assigned rotational transitions of each rotamer separately using theA-reduced semirigid-rotor Hamiltonian. We use the spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array to search forn- andi-butanal toward the star-forming region Sgr B2(N). We also search for both aldehydes toward the molecular cloud G+0.693−0.027 with IRAM 30 m and Yebes 40 m observations. The observational results are compared with computational results from a recent gas-grain astrochemical model. Results.Several thousand rotational transitions belonging to the lowest-energy conformers of two distinct linear and branched isomers have been assigned in the laboratory spectra up to 325 GHz. A precise set of the relevant rotational spectroscopic constants has been determined for each structure as a first step toward identifying both molecules in the interstellar medium. We report non-detections ofn-and i-butanal toward both sources, Sgr B2(N1S) and G+0.693-0.027. We find thatn- andi-butanal are at least 2-6 and 6-18 times less abundant than acetaldehyde toward Sgr B2(N1S), respectively, and thatn-butanal is at least 63 times less abundant than acetaldehyde toward G+0.693−0.027. While propanal is not detected toward Sgr B2(N1S) either, with an abundance at least 5–11 lower than that of acetaldehyde, propanal is found to be 7 times less abundant than acetaldehyde in G+0.693−0.027. Comparison with astrochemical models indicates good agreement between observed and simulated abundances (where available). Grain-surface chemistry appears sufficient to reproduce aldehyde ratios in G+0.693−0.027; gas-phase production may play a more active role in Sgr B2(N1S). Model estimates for the larger aldehydes indicate that the observed upper limits may be close to the underlying values. Conclusions.Our astronomical results indicate that the family of interstellar aldehydes in the Galactic center region is characterized by a drop of one order of magnitude in abundance at each incrementation in the level of molecular complexity. 

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4. Radiolysis versus photolysis of nitrile ices: Laboratory constraints for interstellar chemistry

   https://doi.org/10.1051/0004-6361/202658969  

   Borengasser, Q D; Moore, B M; Hager, T J; Johnson, R; Renshaw, K T; Broderick, B M (May 2026, Astronomy & Astrophysics)

   Aims.Interstellar ice chemistry is a fundamental driver of the molecular complexity observed within the interstellar medium. Moreover, the chemical evolution of icy mantles is predominantly influenced by the energetic processes to which they are exposed. Differences between radiolytic and photolytic pathways for the formation of complex organic molecules (COMs) can lead to significant variations in the resulting molecular abundances. Methods.Application of Chirped Pulse ICE enables the energetic processing of astrophysical ices through both vacuum ultraviolet photolysis and high-energy electron bombardment, facilitating direct comparison of the chemical outcomes produced by each radiation source. Within a single apparatus, ices are characterized in the condensed phase using reflection-absorption infrared spectroscopy, and in the gas phase directly following temperature-programmed desorption via chirped-pulse millimeter-wave rotational spectroscopy coupled with buffer gas cooling. Comparative energetic processing of two parent ices, methyl and ethyl cyanide (CH₃CN and CH₃CH₂CN), reveals several key findings. Results.Following radiolysis, the formation of ketenimine (CH₂CNH) from methyl cyanide increases by a factor of 3.9–5.9 relative to hydrogen cyanide (HCN) production, compared to photolysis, with detection limited to the condensed phase. This enhancement increases to 7.0–8.3 for methyl ketenimine (CH₃CHCNH) formation from ethyl cyanide. In contrast, the abundance of the corresponding isonitrile relative to HCN remains consistent across both radiation sources, as evidenced by its detection in both the condensed and gas phase. Our results suggest that observed imine-to-HCN ratios may be diagnostic tracers of the prevailing energetic processing and chemical pathways to COM formation in specific interstellar environments. 

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5. Revisiting rotationally excited CH at radio wavelengths: A case study towards W51

   https://doi.org/10.1051/0004-6361/202449603  

   Jacob, Arshia M; Nandakumar, Meera; Roy, Nirupam; Menten, Karl M; Neufeld, David A; Faure, Alexandre; Tiwari, Maitraiyee; Pillai, Thushara_G S; Robishaw, Timothy; Durán, Carlos A (December 2024, Astronomy & Astrophysics)

   Context.Ever since they were first detected in the interstellar medium, the radio wavelength (3.3 GHz) hyperfine-structure splitting transitions in the rotational ground state of CH were observed to show anomalous excitation. Astonishingly, this behaviour was uniformly observed towards a variety of different sources probing a wide range of physical conditions. While the observed level inversion could be explained globally by a pumping scheme involving collisions, a description of the extent of ‘over-excitation’ observed in individual sources required the inclusion of radiative processes, involving transitions at higher rotational levels. Therefore, a complete description of the excitation mechanism in the CH ground state, observed towards individual sources entails observational constraints from the rotationally excited levels of CH and in particular that of its first rotationally excited state (²Π_3/2,N= 1,J= 3/2). Aims.Given the limited detections of these lines, the objective of this work is to characterise the physical and excitation properties of the rotationally excited lines of CH between the Λ-doublet levels of its²Π_3/2,N= 1,J= 3/2 state near 700 MHz, and investigate their influence on the pumping mechanisms of the ground-state lines of CH. Methods.This work presents the first interferometric search for the rotationally excited lines of CH between the Λ-doublet levels of its²Π_3/2,N= 1,J= 3/2 state near 700 MHz carried out using the upgraded Giant Metrewave Radio Telescope (uGMRT) array towards six star-forming regions, W51 E, Sgr B2 (M), M8, M17, W43, and DR21 Main. Results.We detected the two main hyperfine structure lines within the first rotationally excited state of CH, in absorption towards W51 E. To jointly model the physical and excitation conditions traced by lines from both the ground and first rotationally excited states of CH, we performed non-local thermodynamic equilibrium (LTE) radiative transfer calculations using the code MOLPOP-CEP. These models account for the effects of line overlap and are aided by column density constraints from the far-infrared (FIR) wavelength rotational transitions of CH that connect to the ground state and use collisional rate coefficients for collisions of CH with H, H₂and electrons (the latter was computed in this work using cross-sections estimated within the Born approximation). Conclusions.The non-LTE analysis revealed that physical properties typical of diffuse and translucent clouds best reproduced the higher rates of level inversion seen in the ground-state lines at 3.3 GHz, observed at velocities near 66 km s⁻¹along the sightline towards W51 E. In contrast, the excited lines near 700 MHz were only excited in much denser environments withn_H~ 10⁵cm⁻³towards which the anomalous excitation in two of the three ground state lines is quenched, but not in the 3.264 GHz line. This is in alignment with our observations and suggests that while FIR pumping and line overlap effects are essential for exciting and producing line inversion in the ground state, excitation to the first rotational level is dominated by collisional excitation from the ground state. For the rotationally excited state of CH, the models indicated low excitation temperatures and column densities of 2 × 10¹⁴cm⁻². Furthermore, modelling these lines helps us understand the complexities of the spectral features observed in the 532/536 GHz rotational transitions of CH. These transitions, connecting sub-levels of the first rotationally excited state to the ground state, play a crucial role in trapping FIR radiation and enhancing the degree of inversion seen in the ground state lines. Based on the physical conditions constrained, we predict the potential of detecting hyperfine-splitting transitions arising from higher rotationally excited transitions of CH in the context of their current non-detections. 

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