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Context. In the centre of pre-stellar cores, deuterium fractionation is enhanced due to low temperatures and high densities. Therefore, the chemistry of deuterated molecules can be used to probe the evolution and the kinematics in the earliest stages of star formation. Aims. We analyse the deuterium fractionation of simple molecules, comparing the level of deuteration in the envelopes of the prototypical pre-stellar core L1544 in Taurus and the young protostellar core HH211 in Perseus. Methods. We used single-dish observations of CCH, HCN, HNC, and HCO+ and their 13C-, 18O-, and D-bearing isotopologues, detected with the 20 m telescope at the Onsala Space Observatory. We derived the column densities, and subsequently the carbon isotopic ratios and 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, HN13C DNC, and DCN that consider the hyperfine structure of these molecules. Results. For CCH, we find high levels of deuteration (10%) in both sources, consistent with other carbon chains. We find moderate deuteration of HCN (5–7%), with a slight enhancement towards the protostellar core. Equal levels of deuteration for HNC towards both cores (~8%) indicate that HNC is tracing slightly different layers compared to HCN. We find that the deuterium fraction of HCO+ is enhanced towards HH211, most likely caused by isotope-selective photodissociation of C18O. With radiative transfer, we were able to reproduce the observed lines of CCH, HCN, H13CN HNC, HN13C and DNC towards L1544 as well as CCH, H13CN HN13C DNC, H13CO+ HC18O+ and DCO+ towards HH211. Conclusions. Similar levels of deuteration show that the deuterium fractionation is most probably 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 deuterium fractionation within dense cores. Our results highlight the uncertainties when dealing with 13C isotopologues and the influence of the applied carbon isotopic ratio. Radiative transfer modelling shows that it is crucial to include the effects of the hyperfine structure to reproduce the observed line shapes. In addition, to correctly model emission lines from pre-stellar cores, it is necessary to include the outer layers of the core to consider the effects of extended structures. In addition to HCO+ observations, HCN observations towards L1544 also require the presence of an outer diffuse layer where the molecules are relatively abundant.
Context. In the centre of pre-stellar cores, deuterium fractionation is enhanced due to low temperatures and high densities. Therefore, the chemistry of deuterated molecules can be used to probe the evolution and the kinematics in the earliest stages of star formation. Aims. We analyse the deuterium fractionation of simple molecules, comparing the level of deuteration in the envelopes of the prototypical pre-stellar core L1544 in Taurus and the young protostellar core HH211 in Perseus. Methods. We used single-dish observations of CCH, HCN, HNC, and HCO+ and their 13C-, 18O-, and D-bearing isotopologues, detected with the 20 m telescope at the Onsala Space Observatory. We derived the column densities, and subsequently the carbon isotopic ratios and 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, HN13C DNC, and DCN that consider the hyperfine structure of these molecules. Results. For CCH, we find high levels of deuteration (10%) in both sources, consistent with other carbon chains. We find moderate deuteration of HCN (5–7%), with a slight enhancement towards the protostellar core. Equal levels of deuteration for HNC towards both cores (~8%) indicate that HNC is tracing slightly different layers compared to HCN. We find that the deuterium fraction of HCO+ is enhanced towards HH211, most likely caused by isotope-selective photodissociation of C18O. With radiative transfer, we were able to reproduce the observed lines of CCH, HCN, H13CN HNC, HN13C and DNC towards L1544 as well as CCH, H13CN HN13C DNC, H13CO+ HC18O+ and DCO+ towards HH211. Conclusions. Similar levels of deuteration show that the deuterium fractionation is most probably 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 deuterium fractionation within dense cores. Our results highlight the uncertainties when dealing with 13C isotopologues and the influence of the applied carbon isotopic ratio. Radiative transfer modelling shows that it is crucial to include the effects of the hyperfine structure to reproduce the observed line shapes. In addition, to correctly model emission lines from pre-stellar cores, it is necessary to include the outer layers of the core to consider the effects of extended structures. In addition to HCO+ observations, HCN observations towards L1544 also require the presence of an outer diffuse layer where the molecules are relatively abundant.
The H2NC radical is the high-energy metastable isomer of H2CN radical, which has been recently detected for the first time in the interstellar medium towards a handful of cold galactic sources, besides a warm galaxy in front of the PKS 1830-211 quasar. These detections have shown that the H2CN/H2NC isomeric ratio, likewise the HCN/HNC ratio, might increase with the kinetic temperature (Tkin), but the shortage of them in warm sources still prevents us to confirm this hypothesis and shed light about their chemistry. In this work, we present the first detection of H2CN and H2NC towards a warm galactic source, the G+0.693-0.027 molecular cloud (with Tkin > 70K), using IRAM 30m observations. We have detected multiple hyperfine components of the $N_{K_\text{a}K_\text{c}} = 1_{01} - 0_{00}$ and 202 − 101 transitions. We derived molecular abundances with respect to H2 of (6.8±1.3) × 10−11 for H2CN and of (3.1±0.7) × 10−11 for H2CN, and a H2CN/H2NC abundance ratio of 2.2 ± 0.5. These detections confirm that the H2CN/H2NC ratio is ≳2 for sources with Tkin > 70K, larger than the ∼1 ratios previously found in colder cores (Tkin ∼ 10K). This isomeric ratio dependence with temperature cannot be fully explained with the currently proposed gas-phase formation and destruction pathways. Grain surface reactions, including the H2NC → H2CN isomerization, deserve consideration to explain the higher isomeric ratios and H2CN abundances observed in warm sources, where the molecules can be desorbed into the gas phase through thermal and/or shock-induced mechanisms.
The characterization of the molecular inventory of solar-type protostars is of crucial importance for a deep understanding of the chemical complexity underlying our cosmic origins. In this context, we present here the full millimetre line survey of the Class 0 protostellar object NGC 1333 IRAS 4A in the spectral bands at 3, 2, and 1.3 mm. In recognition of the powerful tool that unbiased spectral studies provide for investigating the chemistry and physics of star-forming regions, we provide a detailed description of the survey and the results of the analysis. We describe the identification of 1474 spectral lines belonging to 97 different molecular species, including complex organic molecules, which together cover the most ubiquitous chemical elements of life on Earth, namely carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur. The abundances obtained herein are compared with the Class 0 protostellar objects L483 and L1527, and selected molecular ratios are used as tracers of physicochemical properties of the sources. Particularly, the dominance of oxygen-bearing species and the presence of distinct excitation temperature regimes support the attribution of NGC 1333 IRAS 4A as a hot corino featuring three physical components with distinguished and diverse chemical composition.
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