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  1. Context. The detection of a branched alkyl molecule in the high-mass star forming protocluster Sagittarius (Sgr) B2(N) permitted by the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) revealed a new dimension of interstellar chemistry. Astrochemical simulations subsequently predicted that beyond a certain degree of molecular complexity, branched molecules could even dominate over their straight-chain isomers. Aims. More generally, we aim to probe further the presence in the interstellar medium of complex organic molecules with the capacity to exhibit both a normal and iso form, via the attachment of a functional group to either a primary or secondary carbon atom.more »Methods. We used the imaging spectral line survey ReMoCA performed with ALMA at high angular resolution and the results of a recent spectroscopic study of propanol to search for the iso and normal isomers of this molecule in the hot molecular core Sgr B2(N2). We analyzed the interferometric spectra under the assumption of local thermodynamical equilibrium. We expanded the network of the astrochemical model MAGICKAL to explore the formation routes of propanol and put the observational results in a broader astrochemical context. Results. We report the first interstellar detection of iso-propanol, ¿-C 3 H 7 OH, toward a position of Sgr B2(N2) that shows narrow linewidths. We also report the first secure detection of the normal isomer of propanol, n-C 3 H 7 OH, in a hot core. Iso-propanol is found to be nearly as abundant as normal-propanol, with an abundance ratio of 0.6 which is similar to the ratio of 0.4 that we obtained previously for iso- and normal-propyl cyanide in Sgr B2(N2) at lower angular resolution with our previous ALMA survey, EMoCA. The observational results are in good agreement with the outcomes of our astrochemical models, which indicate that the OH-radical addition to propylene in dust-grain ice mantles, driven by water photodissociation, can produce appropriate quantities of normal- and iso-propanol. The normal-to-iso ratio in Sgr B2(N2) may be a direct inheritance of the branching ratio of this reaction process. Conclusions. The detection of normal- and iso-propanol and their ratio indicate that the modest preference for the normal form of propyl cyanide determined previously may be a more general feature among similarly sized interstellar molecules. Detecting other pairs of interstellar organic molecules with a functional group attached either to a primary or secondary carbon may help in pinning down the processes that dominate in setting their normal-to-iso ratios. Butanol and its isomers would be the next obvious candidates in the alcohol family, but their detection in hot cores will be challenging.« less
    Free, publicly-accessible full text available June 1, 2023
  2. 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 thismore »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.« less
    Free, publicly-accessible full text available March 1, 2023
  3. Context. Glycinamide (NH 2 CH 2 C(O)NH 2 ) is considered to be one of the possible precursors of the simplest amino acid, glycine. Its only rotational spectrum reported so far has been in the centimetre-wave region on a laser-ablation generated supersonic expansion sample. Aims. The aim of this work is to extend the laboratory spectrum of glycinamide to the millimetre (mm) wave region to support searches for this molecule in the interstellar medium and to perform the first check for its presence in the high-mass star forming region Sagittarius B2(N). Methods. Glycinamide was synthesised chemically and was studied withmore »broadband rotational spectroscopy in the 90–329 GHz region with the sample in slow flow at 50°C. Tunnelling across a low-energy barrier between two symmetry equivalent configurations of the molecule resulted in splitting of each vibrational state and many perturbations in associated rotational energy levels, requiring careful coupled state fits for each vibrational doublet. We searched for emission of glycinamide in the imaging spectral line survey ReMoCA performed with the Atacama Large Millimetre/submillimetre Array towards Sgr B2(N). The astronomical spectra were analysed under the assumption of local thermodynamic equilibrium. Results. We report the first analysis of the mm-wave rotational spectrum of glycinamide, resulting in fitting – to experimental measurement accuracy – of over 1200 assigned and measured transition frequencies for the ground-state tunnelling doublet and of many lines for tunnelling doublets for two singly excited vibrational states. We also determine the precise vibrational separation in each doublet. We did not detect emission from glycinamide in the hot molecular core Sgr B2(N1S). We derived a column density upper limit of 1.5 × 10 16 cm −2 , which implies that glycinamide is at least seven times less abundant than aminoacetonitrile and 1.8 times less abundant than urea in this source. A comparison with results of astrochemical kinetics models for species related to glycinamide suggests that its abundance may be at least one order of magnitude below the upper limit obtained towards Sgr B2(N1S). This means that glycinamide emission in this source likely lies well below the spectral confusion limit in the frequency range covered by the ReMoCA survey. Conclusions. Thanks to the spectroscopic data provided by this study, the search for glycinamide in the interstellar medium can continue on a firm basis. Targetting sources with a lower level of spectral confusion, such as the Galactic Center shocked region G+0.693-0.027, may be a promising avenue.« less
    Free, publicly-accessible full text available January 1, 2023
  4. Young massive stars warm up the large amount of gas and dust that condenses in their vicinity, exciting a forest of lines from different molecular species. Their line brightness is a diagnostic tool of the gas’s physical conditions locally, which we use to set constraints on the environment where massive stars form. We made use of the Atacama Large Millimeter/submillimeter Array at frequencies near 349 GHz, with an angular resolution of 0′′.1, to observe the methyl cyanide (CH 3 CN) emission which arises from the accretion disk of a young massive star. We sample the disk midplane with twelve distinctmore »beams, where we get an independent measure of the gas’s (and dust’s) physical conditions. The accretion disk extends above the midplane, showing a double-armed spiral morphology projected onto the plane of the sky, which we sample with ten additional beams: Along these apparent spiral features, gas undergoes velocity gradients of about 1 km s −1 per 2000 au. The gas temperature ( T ) rises symmetrically along each side of the disk, from about 98 K at 3000 au to 289 K at 250 au, following a power law with radius R −0.43 . The CH 3 CN column density ( N ) increases from 9.2 × 10 15 cm −2 to 8.7 × 10 17 cm −2 at the same radii, following a power law with radius R −1.8 . In the framework of a circular gaseous disk observed approximately edge-on, we infer an H 2 volume density in excess of 4.8 ×10 9 cm −3 at a distance of 250 au from the star. We study the disk stability against fragmentation following the methodology by Kratter et al. (2010, ApJ, 708, 1585), which is appropriate under rapid accretion, and we show that the disk is marginally prone to fragmentation along its whole extent.« less
    Free, publicly-accessible full text available November 1, 2022
  5. ABSTRACT ATLASGAL is an 870-µm dust survey of 420 deg2 the inner Galactic plane and has been used to identify ∼10 000 dense molecular clumps. Dedicated follow-up observations and complementary surveys are used to characterize the physical properties of these clumps, map their Galactic distribution, and investigate the evolutionary sequence for high-mass star formation. The analysis of the ATLASGAL data is ongoing: We present an up-to-date version of the catalogue. We have classified 5007 clumps into four evolutionary stages (quiescent, protostellar, young stellar objects and H ii regions) and find similar numbers of clumps in each stage, suggesting a similar lifetime. Themore »luminosity-to-mass (Lbol/Mfwhm) ratio curve shows a smooth distribution with no significant kinks or discontinuities when compared to the mean values for evolutionary stages indicating that the star formation process is continuous and that the observational stages do not represent fundamentally different stages or changes in the physical mechanisms involved. We compare the evolutionary sample with other star formation tracers (methanol and water masers, extended green objects and molecular outflows) and find that the association rates with these increases as a function of evolutionary stage, confirming that our classification is reliable. This also reveals a high association rate between quiescent sources and molecular outflows, revealing that outflows are the earliest indication that star formation has begun and that star formation is already ongoing in many of the clumps that are dark even at 70 µm.« less
    Free, publicly-accessible full text available January 11, 2023
  6. The morphology of the Milky Way is still a matter of debate. In order to shed light on uncertainties surrounding the structure of the Galaxy, in this paper, we study the imprint of spiral arms on the distribution and properties of its molecular gas. To do so, we take full advantage of the SEDIGISM (Structure, Excitation, and Dynamics of the Inner Galactic Interstellar Medium) survey that observed a large area of the inner Galaxy in the 13 CO (2–1) line at an angular resolution of 28′′. We analyse the influences of the spiral arms by considering the features of themore »molecular gas emission as a whole across the longitude–velocity map built from the full survey. Additionally, we examine the properties of the molecular clouds in the spiral arms compared to the properties of their counterparts in the inter-arm regions. Through flux and luminosity probability distribution functions, we find that the molecular gas emission associated with the spiral arms does not differ significantly from the emission between the arms. On average, spiral arms show masses per unit length of ~10 5 –10 6 M ⊙ kpc −1 . This is similar to values inferred from data sets in which emission distributions were segmented into molecular clouds. By examining the cloud distribution across the Galactic plane, we infer that the molecular mass in the spiral arms is a factor of 1.5 higher than that of the inter-arm medium, similar to what is found for other spiral galaxies in the local Universe. We observe that only the distributions of cloud mass surface densities and aspect ratio in the spiral arms show significant differences compared to those of the inter-arm medium; other observed differences appear instead to be driven by a distance bias. By comparing our results with simulations and observations of nearby galaxies, we conclude that the measured quantities would classify the Milky Way as a flocculent spiral galaxy, rather than as a grand-design one.« less
    Free, publicly-accessible full text available February 1, 2023
  7. We present the first data release of the ALMA-IMF Large Program, which covers the 12m-array continuum calibration and imaging. The ALMA-IMF Large Program is a survey of fifteen dense molecular cloud regions spanning a range of evolutionary stages that aims to measure the core mass function. We describe the data acquisition and calibration done by the Atacama Large Millimeter/submillimeter Array (ALMA) observatory and the subsequent calibration and imaging we performed. The image products are combinations of multiple 12 m array configurations created from a selection of the observed bandwidth using multi-term, multi-frequency synthesis imaging and deconvolution. The data products aremore »self-calibrated and exhibit substantial noise improvements over the images produced from the delivered data. We compare different choices of continuum selection, calibration parameters, and image weighting parameters, demonstrating the utility and necessity of our additional processing work. Two variants of continuum selection are used and will be distributed: the “best-sensitivity” ( bsens ) data, which include the full bandwidth, including bright emission lines that contaminate the continuum, and “cleanest” ( cleanest ), which select portions of the spectrum that are unaffected by line emission. We present a preliminary analysis of the spectral indices of the continuum data, showing that the ALMA products are able to clearly distinguish free-free emission from dust emission, and that in some cases we are able to identify optically thick emission sources. The data products are made public with this release.« less
    Free, publicly-accessible full text available June 1, 2023
  8. Aims. Thanks to the high angular resolution, sensitivity, image fidelity, and frequency coverage of ALMA, we aim to improve our understanding of star formation. One of the breakthroughs expected from ALMA, which is the basis of our Cycle 5 ALMA-IMF Large Program, is the question of the origin of the initial mass function (IMF) of stars. Here we present the ALMA-IMF protocluster selection, first results, and scientific prospects. Methods. ALMA-IMF imaged a total noncontiguous area of ~53 pc 2 , covering extreme, nearby protoclusters of the Milky Way. We observed 15 massive (2.5 −33 × 10 3 M ⊙ ),more »nearby (2−5.5 kpc) protoclusters that were selected to span relevant early protocluster evolutionary stages. Our 1.3 and 3 mm observations provide continuum images that are homogeneously sensitive to point-like cores with masses of ~0.2 M ⊙ and ~0.6 M ⊙ , respectively, with a matched spatial resolution of ~2000 au across the sample at both wavelengths. Moreover, with the broad spectral coverage provided by ALMA, we detect lines that probe the ionized and molecular gas, as well as complex molecules. Taken together, these data probe the protocluster structure, kinematics, chemistry, and feedback over scales from clouds to filaments to cores. Results. We classify ALMA-IMF protoclusters as Young (six protoclusters), Intermediate (five protoclusters), or Evolved (four proto-clusters) based on the amount of dense gas in the cloud that has potentially been impacted by H  II region(s). The ALMA-IMF catalog contains ~700 cores that span a mass range of ~0.15 M ⊙ to ~250 M ⊙ at a typical size of ~2100 au. We show that this core sample has no significant distance bias and can be used to build core mass functions (CMFs) at similar physical scales. Significant gas motions, which we highlight here in the G353.41 region, are traced down to core scales and can be used to look for inflowing gas streamers and to quantify the impact of the possible associated core mass growth on the shape of the CMF with time. Our first analysis does not reveal any significant evolution of the matter concentration from clouds to cores (i.e., from 1 pc to 0.01 pc scales) or from the youngest to more evolved protoclusters, indicating that cloud dynamical evolution and stellar feedback have for the moment only had a slight effect on the structure of high-density gas in our sample. Furthermore, the first-look analysis of the line richness toward bright cores indicates that the survey encompasses several tens of hot cores, of which we highlight the most massive in the G351.77 cloud. Their homogeneous characterization can be used to constrain the emerging molecular complexity in protostars of high to intermediate masses. Conclusions. The ALMA-IMF Large Program is uniquely designed to transform our understanding of the IMF origin, taking the effects of cloud characteristics and evolution into account. It will provide the community with an unprecedented database with a high legacy value for protocluster clouds, filaments, cores, hot cores, outflows, inflows, and stellar clusters studies.« less
    Free, publicly-accessible full text available June 1, 2023
  9. Context. For all the amides detected in the interstellar medium (ISM), the corresponding nitriles or isonitriles have also been detected in the ISM, some of which have relatively high abundances. Among the abundant nitriles for which the corresponding amide has not yet been detected is cyanoacetylene (HCCCN), whose amide counterpart is propiolamide (HCCC(O)NH 2 ). Aims. With the aim of supporting searches for this amide in the ISM, we provide a complete rotational study of propiolamide from 6 to 440 GHz. Methods. Time-domain Fourier transform microwave spectroscopy under supersonic expansion conditions between 6 and 18 GHz was used to accuratelymore »measure and analyze ground-state rotational transitions with resolved hyperfine structure arising from nuclear quadrupole coupling interactions of the 14 N nucleus. We combined this technique with the frequency-domain room-temperature millimeter wave and submillimeter wave spectroscopies from 75 to 440 GHz in order to record and assign the rotational spectra in the ground state and in the low-lying excited vibrational states. We used the ReMoCA spectral line survey performed with the Atacama Large Millimeter/submillimeter Array toward the star-forming region Sgr B2(N) to search for propiolamide. Results. We identified and measured more than 5500 distinct frequency lines of propiolamide in the laboratory. These lines were fitted using an effective semi-rigid rotor Hamiltonian with nuclear quadrupole coupling interactions taken into consideration. We obtained accurate sets of spectroscopic parameters for the ground state and the three low-lying excited vibrational states. We report the nondetection of propiolamide toward the hot cores Sgr B2(N1S) and Sgr B2(N2). We find that propiolamide is at least 50 and 13 times less abundant than acetamide in Sgr B2(N1S) and Sgr B2(N2), respectively, indicating that the abundance difference between both amides is more pronounced by at least a factor of 8 and 2, respectively, than for their corresponding nitriles. Conclusions. Although propiolamide has yet to be included in astrochemical modeling networks, the observed upper limit to the ratio of propiolamide to acetamide seems consistent with the ratios of related species as determined from past simulations. The comprehensive spectroscopic data presented in this paper will aid future astronomical searches.« less
  10. Context. Glycolamide is a glycine isomer and also one of the simplest derivatives of acetamide (e.g., one hydrogen atom is replaced with a hydroxyl group), which is a known interstellar molecule. Aims. In this context, the aim of our work is to provide direct experimental frequencies of the ground vibrational state of glycolamide in the centimeter-, millimeter- and submillimeter-wavelength regions in order to enable its identification in the interstellar medium. Methods. We employed a battery of state-of-the-art rotational spectroscopic techniques in the frequency and time domain to measure the frequencies of glycolamide. We used the spectral line survey named Exploringmore »Molecular Complexity with ALMA (EMoCA), which was performed toward the star forming region Sgr B2(N) with ALMA to search for glycolamide in space. We also searched for glycolamide toward Sgr B2(N) with the Effelsberg radio telescope. The astronomical spectra were analyzed under the local thermodynamic equilibrium approximation. We used the gas-grain chemical kinetics model MAGICKAL to interpret the results of the astronomical observations. Results. About 1500 transitions have been newly assigned up to 460 GHz to the most stable conformer, and a precise set of spectroscopic constants was determined. Spectral features of glycolamide were then searched for in the prominent hot molecular core Sgr B2(N2). We report the nondetection of glycolamide toward this source with an abundance at least six and five times lower than that of acetamide and glycolaldehyde, respectively. Our astrochemical model suggests that glycolamide may be present in this source at a level just below the upper limit, which was derived from the EMoCA survey. We could also not detect the molecule in the region’s extended molecular envelope, which was probed with the Effelsberg telescope. We find an upper limit to its column density that is similar to the column densities obtained earlier for acetamide and glycolaldehyde with the Green Bank Telescope.« less