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Context. Protonated molecular species have been proven to be abundant in the interstellar gas. This class of molecules is also pivotal for the determination of important physical parameters for the evolution of the interstellar medium (e.g. gas ionisation fraction) or as tracers of non-polar species which are not directly observable. The identification of these molecular species through radioastronomical observations is directly linked to precise laboratory spectral characterisation. Aims. The goal of the present work is to extend the laboratory measurements of the pure rotational spectrum of the ground electronic state of protonated carbonyl sulfide (HSCO+) and its deuterium substituted isotopomer (DSCO+). At the same time, we show how implementing different laboratory techniques allows for the determination of different spectroscopical properties of asymmetric-top protonated species. Methods. Three different high-resolution experiments were used in conjunction to detect for the first time the b-type rotational spectrum of HSCO+, and to extend, well into the sub-millimetre region, the a-type spectrum of the same molecular species and DSCO+. The electronic ground-state of both ions was investigated in the 273–405 GHz frequency range, allowing for the detection of 60 and 50 new rotational transitions for HSCO+ and DSCO+, respectively. Results. The combination of our new measurements with the three rotational transitions previously observed in the microwave region permits the rest frequencies of the most astronomically relevant transitions to be predicted to better than 100 kHz for both HSCO+ and DSCO+ up to 500 GHz, equivalent to better than 60 m s−1 in terms of equivalent radial velocity. Conclusions. The present work illustrates the importance of using different laboratory techniques to spectroscopically characterise a protonated species at high frequency. Each instrument addressed a complementary part of the same spectroscopic challenge, demonstrating the potential of such an approach for future studies of similar reactive species. [ABSTRACT FROM AUTHOR]

Context. Turbulence is a key component of molecular cloud structure. It is usually described by a cascade of energy down to the dissipation scale. The power spectrum for subsonic incompressible turbulence is ∝k−5/3, while for supersonic turbulence it is ∝k−2. Aims. We determine the power spectrum in an actively star-forming molecular cloud, from parsec scales down to the expected magnetohydrodynamic (MHD) wave cutoff (dissipation scale). Methods. We analyzed observations of the nearby NGC 1333 star-forming region in three different tracers to cover the different scales from ∼10 pc down to 20 mpc. The largest scales are covered with the low-density gas tracer 13CO (1–0) obtained with a single dish, the intermediate scales are covered with single-dish observations of the C18O (3–2) line, while the smallest scales are covered in H13CO+ (1–0) and HNC (1–0) with a combination of NOEMA interferometer and IRAM 30m single-dish observations. The complementarity of these observations enables us to generate a combined power spectrum covering more than two orders of magnitude in spatial scale. Results. We derive the power spectrum in an active star-forming region spanning more than 2 decades of spatial scales. The power spectrum of the intensity maps shows a single power-law behavior, with an exponent of 2.9 ± 0.1 and no evidence of dissipation. Moreover, there is evidence that the power spectrum of the ions to have more power at smaller scales than the neutrals, which is opposite to the theoretical expectations. Conclusions. We show new possibilities for studying the dissipation of energy at small scales in star-forming regions provided by interferometric observations. [ABSTRACT FROM AUTHOR]

Context. The detections of narrow channels of accretion toward protostellar disks, known as streamers, have increased in number in the last few years. However, it is unclear whether streamers are a common feature around protostars that were previously missed, or if they are a rare phenomenon. Aims. Our goals are to obtain the incidence of streamers toward a region of clustered star formation and to trace the origins of their gas to determine whether they originate within the filamentary structure of molecular clouds or from beyond. Methods. We used combined observations of the nearby NGC 1333 star-forming region, carried out with the NOEMA interferometer and the IRAM 30m single dish. Our observations cover the area between the systems IRAS 4 and SVS 13. We traced the chemically fresh gas within NGC 1333 with HC3N molecular gas emission and the structure of the fibers in this region with N2H+ emission. We fit multiple velocity components in both maps and used clustering algorithms to recover velocity-coherent structures. Results. We find streamer candidates toward 7 out of 16 young stellar objects within our field of view. This represents an incidence of approximately 40% of young stellar objects with streamer candidates in a clustered star-forming region. The incidence increases to about 60% when we only considered embedded protostars. All streamers are found in HC3N emission. Conclusions. Given the different velocities between HC3N and N2H+ emission, and because by construction, N2H+ traces the fiber structure, we suggest that the gas that forms the streamers comes from outside the fibers. This implies that streamers can connect cloud material that falls onto the filaments with protostellar disk scales. [ABSTRACT FROM AUTHOR]

Observations carried out toward starless and prestellar cores have revealed that complex organic molecules are prevalent in these objects, but it is unclear what chemical processes are involved in their formation. Recently, it has been shown that complex organics are preferentially produced at an intermediate-density shell within the L1544 prestellar core at radial distances of ∼4000 au with respect to the core center. However, the spatial distribution of complex organics has only been inferred toward this core, and it remains unknown whether these species present a similar behavior in other cores. We report high-sensitivity observations carried out toward two positions in the L1498 starless core, the dust peak and a position located at a distance of ∼11,000 au from the center of the core where the emission of CH3OH peaks. Similarly to L1544, our observations reveal that small O-bearing molecules and N-bearing species are enhanced by factors of ∼4–14 toward the outer shell of L1498. However, unlike L1544, large O-bearing organics such as CH3CHO, CH3OCH3, or CH3OCHO are not detected within our sensitivity limits. For N-bearing organics, these species are more abundant toward the outer shell of the L1498 starless core than toward the one in L1544. We propose that the differences observed between O-bearing and N-bearing species in L1498 and L1544 are due to the different physical structure of these cores, which in turn is a consequence of their evolutionary stage, with L1498 being younger than L1544. [ABSTRACT FROM AUTHOR]

Prestellar cores represent the initial conditions in the process of star and planet formation. Their low temperatures (<10 K) allow the formation of thick icy dust mantles, which will be partially preserved in future protoplanetary disks, ultimately affecting the chemical composition of planetary systems. Previous observations have shown that carbon- and oxygen-bearing species, in particular CO, are heavily depleted in prestellar cores due to the efficient molecular freeze-out onto the surface of cold dust grains. However, N-bearing species such as NH3 and, in particular, its deuterated isotopologues appear to maintain high abundances where CO molecules are mainly in the solid phase. Thanks to ALMA, we present here the first clear observational evidence of NH2D freeze-out toward the L1544 prestellar core, suggestive of the presence of a “complete depletion zone” within a ≃1800 au radius, in agreement with astrochemical prestellar core model predictions. Our state-of-the-art chemical model coupled with a non-LTE radiative transfer code demonstrates that NH2D becomes mainly incorporated in icy mantles in the central 2000 au and starts freezing out already at ≃7000 au. Radiative transfer effects within the prestellar core cause the NH2D(111 âˆ' 101) emission to appear centrally concentrated, with a flattened distribution within the central ≃3000 au, unlike the 1.3 mm dust continuum emission, which shows a clear peak within the central ≃1800 au. This prevented NH2D freeze-out from being detected in previous observations, where the central 1000 au cannot be spatially resolved. [ABSTRACT FROM AUTHOR]

The infrared spectrum of 15NH3 has been investigated by high-resolution Fourier transform infrared spectroscopy in the region of the stretching fundamentals. A large number of ro-vibration transitions in the 3050–3650 cm−1 spectral range has been recorded and assigned to the fundamentals ν 1 and ν 3, and to the 2ν 4 overtone bands. In total, 1606 transitions involving the (s) and (a) inversion–rotation–vibration levels have been identified and assigned. They include 256 perturbation-allowed transitions with selection rules ΔK = ±2, Δl = −1 in ν 3 and Δl = +2 in , and ΔK = ±3, Δl = 0 in ν 1 and . All assigned transitions were fitted simultaneously to a model Hamiltonian that includes all symmetry-allowed interactions between and within the excited state levels in order to obtain accurate sets of spectroscopic parameters for both inversion states. The standard deviation of the fit, 0.034 cm−1, is about 70 times larger than the estimated measurement precision. This result is similar to that reported for the same band system in 14NH3 by Kleiner et al. [J. Mol. Spectrosc. 193, 46 (1999)] and is a consequence of the neglect of vibration and ro-vibration interactions between the analysed states and vibrationally excited states with close energies. [ABSTRACT FROM PUBLISHER]

Methyl formate presents a challenge for the conventional chemical mechanisms assumed to guide interstellar organic chemistry. Previous studies of potential formation pathways for methyl formate in interstellar clouds ruled out gas-phase chemistry as a major production route, and more recent chemical kinetics models indicate that it may form efficiently from radical--radical chemistry on ice surfaces. Yet, recent chemical imaging studies of methyl formate and molecules potentially related to its formation suggest that it may form through previously unexplored gas-phase chemistry. Motivated by these findings, two new gas-phase ion-molecule formation routes are proposed and characterized using electronic structure theory with conformational specificity. The proposed reactions, add-cetalyzed Fisher esterification and methyl cation transfer, both produce the less stable trans-conformational isomer of protonated methyl formate in relatively high abundance under the kinetically controlled conditions relevant to interstellar chemistry. Gas-phase neutral methyl formate can be produced from its protonated counterpart through either a dissociative electron recombination reaction or a proton transfer reaction to a molecule with larger proton affinity. Retention (or partial retention) of the conformation in these neutralization reactions would yield trans-methyl formate in an abundance that exceeds predictions under thermodynamic equilibrium at typical interstellar temperatures of ≤ 100 K. For this reason, this conformer may prove to be an excellent probe of gas-phase chemistry in interstellar clouds. Motivated by new theoretical predictions, the rotational spectrum of trans-methyl formate has been measured for the first time in the laboratory, and seven lines have now been detected in the interstellar medium using the publicly available PRIMOS survey from the NRAO Green Bank Telescope. [ABSTRACT FROM AUTHOR]

[Display omitted] • NH 2 D and ND 2 H spectra revised from the submillimeter to the far-infrared region. • Quantum-chemical calculation of hyperfine interaction constants. • Nitrogen and deuterium hyperfine structures resolved for some low J transitions. • New line catalog used to analyze astrophysical NH 2 D emission in L1544. Quantifying molecular abundances of astrochemical species is a key step towards the understanding of the chemistry occurring in the interstellar medium. This process requires a profound knowledge of the molecular energy levels, including their structure resulting from weak interactions between nuclear spins and the molecular rotation. With the aim of increasing the quality of spectral line catalogs for the singly- and doubly-deuterated ammonia (NH 2 D and ND 2 H), we have revised their rotational spectra by observing many hyperfine-resolved lines and more accurate high-frequency transitions. The measurements have been performed in the submillimeter-wave region (265–1565 GHz) using a frequency modulation submillimeter spectrometer and in the far-infrared domain (45–220 cm−1) with a synchrotron-based Fourier-transform interferometer. The analysis of the new data, with the interpretation of the hyperfine structure supported by state-of-the-art quantum-chemical calculations, led to an overall improvement of all spectroscopic parameters. Moreover, the effect of the inclusion of deuterium splittings in the analysis of astrophysical NH 2 D emissions at millimeter wavelengths has been tested using recent observations of the starless core L1544, an ideal astrophysical laboratory for the study of deuterated species. Our results show that accounting for hyperfine interactions leads to a small but significant change in the physical parameters used to model NH 2 D line emissions. [ABSTRACT FROM AUTHOR]

The rotational spectrum of the higher-energy trans-conformational isomer of methyl formate has been assigned for the first time using several pulsed-jet Fourier transform microwave spectrometers in the 6-60 GHz frequency range. This species has also been sought toward the Sagittarius B2(N) molecular cloud using the publicly available PRIMOS survey from the Green Bank Telescope. We detect seven absorption features in the survey that coincide with laboratory transitions of trans-methyl formate, from which we derive a column density of 3.1 (+2.6, –1.2) × 1013 cm–2 and a rotational temperature of 7.6 ± 1.5 K. This excitation temperature is significantly lower than that of the more stable cis conformer in the same source but is consistent with that of other complex molecular species recently detected in Sgr B2(N). The difference in the rotational temperatures of the two conformers suggests that they have different spatial distributions in this source. As the abundance of trans-methyl formate is far higher than would be expected if the cis and trans conformers are in thermodynamic equilibrium, processes that could preferentially form trans-methyl formate in this region are discussed. We also discuss measurements that could be performed to make this detection more certain. This manuscript demonstrates how publicly available broadband radio astronomical surveys of chemically rich molecular clouds can be used in conjunction with laboratory rotational spectroscopy to search for new molecules in the interstellar medium. [ABSTRACT FROM AUTHOR]