Rotational spectroscopy of the HCCO and DCCO radicals in the millimeter and submillimeter range

The ketenyl radical, HCCO, has recently been detected in the ISM for the first time. Further astronomical detections of HCCO will help us understand its gas-grain chemistry, and subsequently revise the oxygen-bearing chemistry towards dark clouds. Moreover, its deuterated counterpart, DCCO, has never been observed in the ISM. HCCO and DCCO still lack a broad spectroscopic investigation, although they exhibit a significant astrophysical relevance. In this work we aim to measure the pure rotational spectra of the ground state of HCCO and DCCO in the millimeter and submillimeter region, considerably extending the frequency range covered by previous studies. The spectral acquisition was performed using a frequency-modulation absorption spectrometer between 170 and 650 GHz. The radicals were produced in a low-density plasma generated from a select mixture of gaseous precursors. For each isotopologue we were able to detect and assign more than 100 rotational lines. The new lines have significantly enhanced the previous data set allowing the determination of highly precise rotational and centrifugal distortion parameters. In our analysis we have taken into account the interaction between the ground electronic state and a low-lying excited state (Renner-Teller pair) which enables the prediction and assignment of rotational transitions with $K_a$ up to 4. The present set of spectroscopic parameters provides highly accurate, millimeter and submillimeter rest-frequencies of HCCO and DCCO for future astronomical observations. We also show that towards the pre-stellar core L1544, ketenyl peaks in the region where $c$-$\mathrm{C_3H_2}$ peaks, suggesting that HCCO follows a predominant hydrocarbon chemistry, as already proposed by recent gas-grain chemical models

HSCO+^+ and DSCO+^+: a multi-technique approach in the laboratory for the spectroscopy of interstellar ions

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 ISM evolution (e.g. gas ionisation fraction) or as tracers of non-polar, hence not directly observable, species. The identification of these molecular species through radioastronomical observations is directly linked to a precise laboratory spectral characterisation. 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 the determination of different spectroscopical properties of asymmetric-top protonated species. Three different high-resolution experiments were involved to detected for the first time the $b-$type rotational spectrum of HSCO$^+$, and to extend, well into the sub-millimeter region, the $a-$type spectrum of the same molecular species and DSCO$^+$. The electronic ground-state of both ions have been investigated in the 273-405 GHz frequency range, allowing the detection of 60 and 50 new rotational transitions for HSCO$^+$ and DSCO$^+$, respectively. The combination of our new measurements with the three rotational transitions previously observed in the microwave region permits the rest frequencies of the astronomically most 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 in terms of equivalent radial velocity. The present work illustrates the importance of using different laboratory techniques to spectroscopically characterise a protonated species at high frequency, and how a similar approach can be adopted when dealing with reactive species.Comment: 7 pages, 4 figures. Accepted for publication in Astronomy and Astrophysic

A 1.3 cm line survey toward IRC +10216

IRC +10216 is the prototypical carbon star exhibiting an extended molecular circumstellar envelope. Its spectral properties are therefore the template for an entire class of objects. The main goal is to systematically study the $\lambda$ $\sim$1.3 cm spectral line characteristics of IRC +10216. We carried out a spectral line survey with the Effelsberg-100 m telescope toward IRC +10216. It covers the frequency range between 17.8 GHz and 26.3 GHz (K-band). In the circumstellar shell of IRC +10216, we find 78 spectral lines, among which 12 remain unidentified. The identified lines are assigned to 18 different molecules and radicals. A total of 23 lines from species known to exist in this envelope are detected for the first time outside the Solar System and there are additional 20 lines first detected in IRC +10216. The potential orgin of "U" lines is also discussed. Assuming local thermodynamic equilibrium (LTE), we then determine rotational temperatures and column densities of 17 detected molecules. Molecular abundances relative to H$_{2}$ are also estimated. A non-LTE analysis of NH$_{3}$ shows that the bulk of its emission arises from the inner envelope with a kinetic temperature of 70$\pm$20 K. Evidence for NH$_{3}$ emitting gas with higher kinetic temperature is also obtained, and potential abundance differences between various $^{13}$C-bearing isotopologues of HC$_{5}$N are evaluated. Overall, the isotopic $^{12}$C/$^{13}$C ratio is estimated to be 49$\pm$9. Finally, a comparison of detected molecules in the $\lambda$ $\sim$1.3 cm range with the dark cloud TMC-1 indicates that silicate-bearing molecules are more predominant in IRC +10216.Comment: 32 pages, 9 figures, Accepted by A&

A 1.3 cm Line Survey toward Orion KL

Orion KL has served as a benchmark for spectral line searches throughout the (sub)millimeter regime. The main goal is to systematically study spectral characteristics of Orion KL in the 1.3 cm band. We carried out a spectral line survey (17.9 GHz to 26.2 GHz) with the Effelsberg-100 m telescope towards Orion KL. We find 261 spectral lines, yielding an average line density of about 32 spectral features per GHz above 3$\sigma$. The identified lines include 164 radio recombination lines (RRLs) and 97 molecular lines. A total of 23 molecular transitions from species known to exist in Orion KL are detected for the first time in the interstellar medium. Non-metastable 15NH3 transitions are detected in Orion KL for the first time. Based on the velocity information of detected lines and the ALMA images, the spatial origins of molecular emission are constrained and discussed. A narrow feature is found in SO2 ($8_{1,7}-7_{2,6}$), possibly suggesting the presence of a maser line. Column densities and fractional abundances relative to H2 are estimated for 12 molecules with LTE methods. Rotational diagrams of non-metastable 14NH3 transitions with J=K+1 to J=K+4 yield different results; metastable 15NH3 is found to have a higher excitation temperature than non-metastable 15NH3, indicating that they may trace different regions. Elemental and isotopic abundance ratios are estimated: 12C/13C=63+-17, 14N/15N=100+-51, D/H=0.0083+-0.0045. The dispersion of the He/H ratios derived from H$\alpha$/He$\alpha$ pairs to H$\delta$/He$\delta$ pairs is very small, which is consistent with theoretical predictions that the departure coefficients bn factors for hydrogen and helium are nearly identical. Based on a non-LTE code neglecting excitation by the infrared radiation field and a likelihood analysis, we find that the denser regions have lower kinetic temperature, which favors an external heating of the Hot Core.Comment: 70 pages, 26 figures, 12 tables, accepted for publication in A&A. Figs. 1, 2, 8, 9 have been downsize

Similar levels of deuteration in the pre-stellar core L1544 and the protostellar core HH211

In the centre of pre-stellar cores, deuterium fractionation is enhanced due to the low temperatures and high densities. Therefore, the chemistry of deuterated molecules can be used to study the earliest stages of star formation. We analyse the deuterium fractionation of simple molecules, comparing the level of deuteration in the envelopes of the pre-stellar core L1544 in Taurus and the protostellar core HH211 in Perseus. We used single-dish observations of CCH, HCN, HNC, HCO$^+$, and their $^{13}$C-, $^{18}$O- and D-bearing isotopologues, detected with the Onsala 20m telescope. We derived the column densities and the deuterium fractions of the molecules. Additionally, we used radiative transfer simulations and results from chemical modelling to reproduce the observed molecular lines. We used new collisional rate coefficients for HNC, HN$^{13}$C, DNC, and DCN that consider the hyperfine structure of these molecules. We find high levels of deuteration for CCH (10%) in both sources, consistent with other carbon chains, and moderate levels for HCN (5-7%) and HNC (8%). The deuterium fraction of HCO$^+$ is enhanced towards HH211, most likely caused by isotope-selective photodissociation of C$^{18}$O. Similar levels of deuteration show that the process is likely equally efficient towards both cores, suggesting that the protostellar envelope still retains the chemical composition of the original pre-stellar core. The fact that the two cores are embedded in different molecular clouds also suggests that environmental conditions do not have a significant effect on the deuteration within dense cores. Radiative transfer modelling shows that it is necessary to include the outer layers of the cores to consider the effects of extended structures. Besides HCO$^+$ observations, HCN observations towards L1544 also require the presence of an outer diffuse layer where the molecules are relatively abundant.Comment: 27 pages, 17 figures, accepted for publication in A&

Seeds of Life in Space (SOLIS): XI. First measurement of nitrogen fractionation in shocked clumps of the L1157 protostellar outflow

Context. The isotopic ratio of nitrogen presents a wide range of values in the Solar System: from 140 in meteorites and comets to 441 in the solar wind. In star-forming systems, we observe evena higher spread of ~150-1000. The origin of these differences is still unclear. Aims. Chemical reactions in the gas phase are one of the possible processes that could modify the 14N/15N ratio. We aim to investigate if and how the passage of a shock wave in the interstellar medium, which activates a rich chemistry, can affect the relative fraction of nitrogen isotopes. Theideal place for such a study is the chemically rich outflow powered by the L1157-mm protostar, where several shocked clumps are present. Methods. We present the first measurement of the 14N/15N ratio in the two shocked clumps, B1 and B0, of the protostellar outflow L1157. The measurement is derived from the interferometeric maps of the H13CN (1-0) and the HC15N (1-0) lines obtained with the NOrthern Extended Millimeter Array (NOEMA) interferometeras part of the Seeds of Life in Space (SOLIS) programme. Results. In B1, we find that the H13CN (1-0) and HC15N (1-0) emission traces the front of the clump, that is the apex of the shocked region, where the fast jet impacts the lower velocity medium with an averaged column density of N(H13CN) ~ 7 × 1012 cm-2 and N(HC15N) 2 × 1012 cm-2. In this region, the ratio H13CN (1-0)/HC15N (1-0) is almost uniform with an average value of ~5 ± 1. The same average value isalso measured in the smaller clump B0e. Assuming the standard 12C/13C = 68, we obtain 14N/15N = 340 ± 70. This ratio is similar to those usually found with the same species in prestellar cores and protostars. We analysed the prediction of a chemical shock model for several shock conditions and we found that the nitrogen and carbon fractionations do not vary much for the first period after the shock. The observed H13CN/HC15N can be reproduced by a non-dissociative, C-type shock with pre-shock density n(H) = 105 cm-3, shock velocity Vs between 20 and 40 km s-1, and cosmic-ray ionization rate of 3 × 10-16 s-1; this agrees with previous modelling of other chemical species in L1157-B1. Conclusions. Both observations and chemical models indicate that the rich chemistry activated by the shock propagation does not affect the nitrogen isotopic ratio, which remains similar to that measured in lower temperature gas in prestellar cores and protostellar envelopes

Laboratory and tentative interstellar detection of trans-methyl formate using the publicly available Green Bank Telescope PRIMOS survey

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) \times 10^13 cm-2 and a rotational temperature of 7.6 \pm 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.Comment: 40 pages, 7 figures, 4 tables; accepted for publication in Ap

Gas Phase Elemental Abundances in Molecular CloudS (GEMS) V. Methanol in Taurus

Context. Methanol, one of the simplest complex organic molecules in the interstellar medium, has been shown to be present and extended in cold environments such as starless cores. Studying the physical conditions at which CH3OH starts its efficient formation is important to understand the development of molecular complexity in star-forming regions. Aims. We aim to study methanol emission across several starless cores and investigate the physical conditions at which methanol starts to be efficiently formed, as well as how the physical structure of the cores and their surrounding environment affect its distribution. Methods. Methanol and C18O emission lines at 3 mm have been observed with the IRAM 30 m telescope within the large programme Gas phase Elemental abundances in Molecular CloudS towards 66 positions across 12 starless cores in the Taurus Molecular Cloud. A non-LTE (local thermodynamic equilibrium) radiative transfer code was used to compute the column densities in all positions. We then used state-of-the-art chemical models to reproduce our observations. Results. We have computed N(CH3OH)/N(C18O) column density ratios for all the observed offsets, and the following two different behaviours can be recognised: the cores where the ratio peaks at the dust peak and the cores where the ratio peaks with a slight offset with respect to the dust peak (∼10 000 AU). We suggest that the cause of this behaviour is the irradiation on the cores due to protostars nearby which accelerate energetic particles along their outflows. The chemical models, which do not take irradiation variations into account, can reproduce the overall observed column density of methanol fairly well, but they cannot reproduce the two different radial profiles observed. Conclusions. We confirm the substantial effect of the environment on the distribution of methanol in starless cores. We suggest that the clumpy medium generated by protostellar outflows might cause a more efficient penetration of the interstellar radiation field in the molecular cloud and have an impact on the distribution of methanol in starless cores. Additional experimental and theoretical work is needed to reproduce the distribution of methanol across starless cores. © S. Spezzano et al. 2021.Acknowledgements. The authors are grateful to the anonymous referee for insightful comments. A large part of the data analysis described in this paper was performed during the spring of 2020, in the beginning of the COVID pandemic and during a hard lockdown. S.S. wishes to thank the Max Planck Society for the flexibility that was allowed during the pandemic, because it contributed to maintaining a clear and focus mind during the hours that she could dedicate to her work, and overall to keep calm, while waiting for the ‘storm’ to pass. Based on analysis carried out with the CASSIS software (http://cassis.irap. omp.eu) and CDMS and JPL spectroscopic databases and LAMDA molecular databases. CASSIS has been developed by IRAP-UPS/CNRS. S.S. wishes to thank the Max Planck Society for the Independent Max Planck Research Group funding. A.F., D.N.A. and M.R.B. are funded by Spanish MICINN through PID2010-106235GB-I00 national research project. V.W. acknowledges the CNRS program Physique et Chimie du Milieu Interstellaire (PCMI) co-funded by the Centre National d’Etudes Spatiales (CNES). A.V. and A.P. are the members of the Max Planck Partner Group at the Ural Federal University. A.V. and A.P. acknowledge the support of the Russian Ministry of Science and Education via the State Assignment Contract no. FEUZ-2020-0038