Carbon disulfide (CS_2) is one of the sulfur-bearing species expected to be present in the interstellar medium (ISM). In this study, we investigated the surface reactions of solid CS_2 with hydrogen (H) atoms on amorphous solid water (ASW) using laboratory experiments supported by computational calculations. Our results show that CS_2 reacts with H atoms through quantum tunneling in the initial step, followed by successive H addition reactions, with or without activation barriers, on icy surfaces. These processes lead to the formation of several sulfur-bearing species, including hydrogen sulfide (H_2S), methyl mercaptan (CH_3SH), and small amounts of dithioformic acid (HC(S)SH) and methanedithiol (CH_2(SH)_2). The observed reactivity of CS_2 with H atoms provides a plausible explanation for the non-detection of CS_2 in interstellar ices. Furthermore, the efficient hydrogenation of the complex molecules derived from CS_2, namely HC(S)SH and CH_2(SH)_2, suggests that these species could be easily undergone with H atoms to produce other S-bearing species under ISM conditions.

Sulfur-bearing molecules are key constituents of the interstellar medium (ISM). Particularly, hydrogen sulfide (H2S) and cyano (CN) radicals are key precursors of prebiotic molecules in the ISM. However, the ultralow-temperature gas-phase reactivity remains poorly characterized yet. We report the first experimental and theoretical investigation of the CN + H2S reaction under conditions relevant to cold molecular clouds. Rate coefficients were determined between 11.7 and 45.5 K using the Cin & eacute;tique de R & eacute;action en Ecoulement Supersonique Uniforme technique coupled with pulsed laser photolysis-laser-induced fluorescence, yielding negligible temperature dependence values around 4.0 & times; 10-10 cm3 s-1 in excellent agreement with complementary rate coefficients calculations. AutoMeKin and coupled-cluster theory reveal that the dominant channel involves CN addition to H2S, followed by H elimination, forming HSCN. This pathway is energetically more favorable than the previously assumed HCN + SH channel and exhibits submerged transition states, suggesting efficient reactivity at ultracold temperatures. Astrochemical modeling indicates that inclusion of this reaction in chemical networks enhances HSCN abundances in dark clouds, with contributions comparable to those from dissociative recombination routes. Although the CN + H2S reaction is absent from current astrochemical databases, our results demonstrate its potential role in sulfur-nitrogen coupling and the formation of prebiotic molecules in the ISM. These findings underscore the need to update chemical models to account for this process and improve predictions of sulfur chemistry in star-forming regions.

Cosmic dust is mainly formed in the atmospheres of evolved stars. In carbon rich stars, amorphous carbon along with silicon carbide are the main constituents of dust grains yet the mechanisms involved in the formation of these grains are still poorly understood. Several molecular precursors have been proposed to form silicon carbide grains. Here, we have simulated in the laboratory the formation of silicon carbide dust starting from atomic C, atomic Si and H_2 and we have clearly identified SiC_2 as a key molecular precursor of nanodust analogues. We show that the interaction of molecular hydrogen with atomic carbon initiates the formation of hydrocarbons, which then react with atomic silicon to produce gas-phase SiC_2. In our experiments, the silicon carbide nanodust analogues are partially hydrogenated. Chemical routes for the formation of SiC_2 and organosilicon species are discussed on the basis of thermochemical calculations and chemical kinetics modelling. Our findings reveal the central role of molecular hydrogen in the formation of SiC_2 and contribute to a deeper understanding of silicon carbide dust formation processes in evolved stars, from atoms to molecules, clusters, and ultimately dust grains.

Cosmic dust mainly forms in the atmospheres of evolved stars. In carbon-rich stars, amorphous carbon along with silicon carbide are the main constituents of dust grains, yet the mechanisms involved in the formation of these grains are poorly understood. Several molecular precursors have been proposed to form silicon carbide grains. Here we have simulated in the laboratory the formation of silicon carbide dust starting from atomic C, atomic Si and H2, and we have clearly identified SiC2 as a key molecular precursor of nanodust analogues. We show that the interaction of molecular hydrogen with atomic carbon initiates the formation of hydrocarbons, which then react with atomic silicon to produce gas-phase SiC2. In our experiments, the silicon carbide nanodust analogues are partially hydrogenated. Chemical routes for the formation of SiC2 and organo-silicon species are discussed on the basis of thermochemical calculations and chemical kinetics modelling. Our findings reveal the central role of molecular hydrogen in the formation of SiC2, and they contribute to a deeper understanding of silicon carbide dust formation processes in evolved stars, from atoms to molecules, clusters and ultimately dust grains.

Context. Core-collapse Supernovae (CCSNe) classed as Type II contribute to the chemical enrichment of galaxies through explosion. Their role as dust producers in the high-redshift Universe may be of paramount importance. However, the type and amount of dust they synthesise following the outburst are still a matter of debate and their formation processes also remain unclear. Aims. We aim to identify and understand the chemical processes at play in the dust formation scenario. We also derive mass yields for molecules and dust clusters at late post-explosion time. Methods. We revisited existing models by improving on the physics and chemistry of the supernova ejecta. We identified and evaluated new chemical species and pathways underpinning the formation of dust clusters. We applied a unique exhaustive chemical network to the entire ejecta of a SN with a 15 M⊙ progenitor. We tested this new chemistry for various gas conditions in the ejecta, and derived mass yields for molecules and dust clusters. Results. We obtained the molecular component of the ejecta up to 11 years after explosion. The most abundant species are, in order of decreasing masses, O2, CO, SiS, SiO, CO2, SO2, CaS, N2, and CS. Atomic oxygen is quickly depleted after 300 days post-explosion in a large part of the oxygen core owing to the efficient synthesis of O2. Caution should then be exercised in the use of atomic oxygen masses as a supernova diagnostic. We identified molecules that are tracers of high-density clumps. As for dust clusters, we find the composition is dominated by silicates and silica, along with carbon dust, but with modest amounts of alumina. Pure metal clusters and metal sulphide and oxide clusters have negligible masses. High-density gas favours the formation of carbon clusters in the outer ejecta region whereas low temperatures hamper the formation of silicates in the oxygen core. These results are in good agreement with existing astronomical data and recent observations with the James Webb Space Telescope (JWST). They highlight the importance of chemistry in the derivation of dust budgets from supernovae.

The combined use of laboratory rotational spectroscopy and radio astronomical observations remains the most effective approach for identifying molecules in the interstellar medium (ISM). Following the recent detections of several polycyclic aromatic hydrocarbons (PAHs) and their cyano derivatives in the dense Taurus Molecular Cloud (TMC-1), it is reasonable to extend such searches to other PAHs within the same source. In this work, we report a rotational spectroscopy study of commercially available fluoranthene (FA) and its synthesized cyano derivative, 3-cyanofluoranthene (3-CNFA), using chirped-pulse Fourier-transform microwave spectroscopy. The analysis of the rotational spectra, supported by quantum chemical calculations, yielded molecular parameters for the parent species of both FA and 3-CNFA molecules. The experimental data of 3-CNFA were later used for its astronomical search in TMC-1 with the QUIJOTE line survey but proved unsuccessful. Despite the nondetection of 3-CNFA in this source, the upper limit to its abundance was established. The experimental data will support future astronomical searches in the ISM.

Sulfur is one of the most abundant elements in the interstellar medium (ISM) and a key component for life, yet little is understood about its chemistry in the ISM. While increasingly larger molecules containing sulfur and oxygen are being observed in the ISM, the largest organic molecule containing oxygen and sulfur, monothioformic acid (HC(O)SH), was only recently detected. There is still no identification of a complex organic molecule (COM, carbon-bearing molecule with six or more atoms) containing both oxygen- and sulfur-bearing functional groups. We extended the laboratory rotational spectrum of 2-mercaptoethanol (HSCH2CH2OH), one of the simplest saturated COMs containing both oxygen and sulfur, into millimeter/submillimeter wavelengths, providing an improved spectral catalog at frequencies required for its interstellar identification. Millimeter/submillimeter transitions were measured for 2-mercaptoethanol from similar to 82 to 450 GHz. Using the resulting spectral catalog, we searched for its rotational emission toward the Galactic Center molecular cloud G+0.693-0.027, the high-mass star-forming region Sgr B2(N), the cold dark core TMC-1, the hot core in Orion, and toward the hot corino surrounding the low-mass protostar IRAS 16293-2422B. An extensive analysis of 8584 transitions of 2-mercaptoethanol with Jmax '' = 104, and Kamax '' = 64 is provided. The resulting fit includes a full set of quartic, sextic, and octic distortion constants. We report the nondetection of 2-mercaptoethanol and provide column density upper limits toward each source. While our interstellar search for 2-mercaptoethanol did not result in a detection, the upper limits on its column density provide important constraints for chemical models on the formation of oxygen- and sulfur-bearing COMs.

Nitrogen-bearing molecules, like ammonia (NH3) or methylamine (CH3NH2) can be precursors of prebiotic molecules in the interstellar medium. In this work, the gas-phase kinetics of the CN reaction with NH3 (reaction 1) and CH3NH2 (reaction 2) was experimentally investigated between 11.7 and 177.5 K. A pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) apparatus was employed to determine the temperature dependence of the rate coefficients (k1(T) for NH3 and k2(T) for CH3NH2). It was observed that both rate coefficients increase when temperature decreases. The modified Arrhenius expressions obtained by combining our results with those from the literature for reactions 1 and 2 are: k1(11.7-295 K) = (2.74 +/- 0.09) & times; 10-11 (T/300 K)-1.36 +/- 0.04 exp[-(16.6 +/- 2.2) K/T] cm3 s-1 and k2(49.1-297 K) = (3.10 +/- 0.25) & times; 10-10 (T/300 K)-0.27 +/- 0.07 cm3 s-1. Additionally, k2 was found to be independent on temperature between 11.7 and 49.1 K, (5.23 +/- 0.65) & times; 10-10 cm3 s-1. No pressure dependence of k1(T) and k2(T) was observed in the (3.37-16.7) & times; 1016 cm-3 total gas density range at 22 K. The impact of the determined rate coefficients k1(11.7 K) and k2(11.7 K) in the modeled abundances of the reactants NH3 and CH3NH2 is only moderate, although k2(11.7 K) drives a significant enhancement of the reaction products, especially for CH3NH radical, the abundance of which increases by about 1 order of magnitude.

Iminomethylium, HCNH+, is a ubiquitous molecular cation in the interstellar medium (ISM). Although isotopologues of HCNH+ precursors are known to exist, no isotopically substituted derivative of HCNH+ has been identified in space, primarily due to the absence of hyperfine-resolved spectral data. In this work, we present the first hyperfine-resolved spectroscopic data for DCNH (+), (HCNH)-C-13 (+), (HCNH)-N-15 (+), and HCND (+). In addition, we provide hyperfine-resolved collisional rate coefficients with H 2 and Einstein coefficients that enable non-LTE predictions of line intensities in cold molecular clouds. Radiative-transfer calculations tailored to TMC-1 indicate that HCND+ is the most detectable isotopologue, with the j = 2-1 rotational line expected at similar to 7-40mK (while the j = 1-0 line is blocked by atmospheric O (2) ), whereas (HCNH)-C-13+ and (HCNH+)-N-15 yield mK-level emission for the j = 1-0 and 2-1 lines. On the other hand, DCNH+ lines are intrinsically weak owing to its nearly-zero dipole moment. The frequencies, hyperfine patterns, and radiative and collisional coefficients reported here remove the main barrier to targeted searches of HCNH+ isotopologues and enable robust non-LTE determinations of isotopic ratios in the cold ISM.

In recent years, obsessive interest in the observation of TMC-1 has brought a boost in our knowledge of the chemistry of cold dark clouds. The number of molecules detected in this particular cloud has been more than doubled. Two observational programs, GOTHAM and QUIJOTE, are responsible for this spectacular achievement. Here, we provide an overall view of QUIJOTE, which is a line survey carried out in the Q-band (31-50 GHz) with the Yebes 40m radiotelescope, summarize the actual observational status of TMC-1, and discuss the chemistry of this remarkable source. We highlight the successes and failures of state-of-the-art chemical models to describe their chemical composition, with a particular emphasis on the origin of polycyclic aromatic hydrocarbons, which is yet far from being understood.

We report the first detection in interstellar medium of the 1-cyano propargyl radical, HC3HCN. This species is an isomer of the 3-cyano propargyl radical (CH2C3N), which was recently discovered in TMC-1. The 1-cyano propargyl radical was observed in the cold dark cloud TMC-1 using data from the ongoing QUIJOTE line survey, which is being carried out with the Yebes 40m telescope. A total of seven rotational transitions with multiple hyperfine components were detected in the 31.0-50.4 GHz range. We derived a column density of (2.2 +/- 0.2) x 10(11) cm(-2) and a rotational temperature of 7 +/- 1 K. The abundance ratio between HC3HCN and CH2C3N is 1.4. The almost equal abundance of these isomers indicates that the two species may be produced in the same reaction with a similar efficiency, probably in the reaction C + CH2CHCN and perhaps also in the reaction C-2 + CH3CN and the dissociative recombination with electrons of CH2C3NH+.

Context. The chemical composition of diffuse interstellar clouds is not fully established. They host an active chemistry despite their relatively low density and the ubiquitous presence of far-UV radiation. Aims. To further explore the chemical composition of diffuse clouds, we performed a spectral scan toward the bright radio source BL Lac in the Q band (from 32 to 50 GHz) using the Yebes 40 m telescope. Methods. Yebes observations were performed interleaving frequency switching and position switching integrations toward BL Lac, using a spectral resolution of 38 kHz. The data were reduced with the CLASS software. Results. We achieved an unprecedented sensitivity on the continuum of 0.02-0.07%, allowing for the detection of very faint absorption features. We confirm previous detections of HCS+, C3H, C3H+, CH3CN, and HC3N in diffuse clouds and report new detections of CCS, C4H, CH3CHO, H2CCO, HNCO, and H2CS along the line of sight to BL Lac, with abundances relative to H-2 from a few 10(-11) to a few 10(-10). We compiled molecular detections toward diffuse clouds to obtain the chemical inventory of a typical diffuse interstellar cloud. Conclusions. The chemical inventory of diffuse interstellar clouds includes complex organic species with up to four heavy atoms. These species are efficiently formed in the diffuse interstellar gas and reach abundances similar to those measured in dense photodissociation regions, pointing to similar gas-phase chemical processes.

Cyclopentadiene (c-C5H6) is considered a key molecule in the formation of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). The synthesis of PAHs from simpler precursors is known as the "bottom-up" theory, which, so far, has been dominated by reactions between organic radicals. However, this mechanism struggles to account for the origin of the smallest cycles themselves. However, it struggles to account for the origin of the smallest cycles themselves. Ion-molecule reactions emerge as promising alternative pathways to explain the formation of these molecules. We investigated the reaction network of the main ionic precursor of cyclopentadiene, c-C5H7+. To this end, we established an integrated protocol that combines astrochemical modelling to identify viable formation routes under cold ISM conditions, automated reaction path searches, and kinetic simulations to obtain accurate descriptions of the reaction pathways and reliable rate constants. In particular, we examined the reaction between ethylene (C2H4) and the linear propargyl cation (CH2CCH+). Our results reveal that the formation of c-C5H7+ by radiative association is inefficient, contrary to our initial expectations. Instead, the system predominantly evolves through bimolecular channels yielding c-C5H5+ and CH3CCH2+, with the formation of c-C5H5+; this offers new insights into the reactivity that supports molecular growth in the ISM.

In recent years, the chemistry of sulfur in the interstellar medium has experienced renewed interest due to the detection of a large variety of molecules containing sulfur. We report the first identification in space of a new S-bearing molecule, thioacetaldehyde (CH3CHS), which is the sulfur counterpart of acetaldehyde (CH3CHO). The astronomical observations are part of QUIJOTE, a Yebes 40 m Q-band line survey of the cold dense cloud TMC-1. We detected seven individual lines corresponding to A and E components of the four most favorable rotational transitions of CH3CHS covered in the Q band (31.0-50.3 GHz). Assuming a rotational temperature of 9 K, we derive a column density of 9.8 x 10(10) cm(-2) for CH3CHS, which implies that it is 36 times less abundant than its oxygen counterpart CH3CHO. By comparing the column densities of the O- and S-bearing molecules detected in TMC-1, we find that as molecules increase their degree of hydrogenation, sulfur-bearing molecules become less abundant than their oxygen analog. That is, hydrogenation seems to be less favored for S-bearing molecules than for O-bearing ones in cold sources such as TMC-1. We explored potential formation pathways to CH3CHS and implemented them into a chemical model, which underestimates the observed abundance of thioacetaldehyde by several orders of magnitude, however. Quantum chemical calculations carried out for one of the potential formation pathways, the S + C2H5 reaction, indicate that formation of CH3CHS is only a minor channel in this reaction.

Due to the importance of a reference atmospheric radiative transfer model for both planning and calibrating ground-based observations at millimetre and sub-millimetre wavelengths, we have undertaken a validation campaign consisting of acquiring atmospheric spectra under different weather conditions, in different diurnal moments and seasons, with the Atacama Pathfinder EXperiment (APEX), due to the excellent stability of its receivers and the very high frequency resolution of its back-ends. As a result, a dataset consisting of 56 spectra within the 157.3-742.1 GHz frequency range, at kilohertz resolution (smoothed to similar to 2-10 MHz for analysis), and spanning one order of magnitude (similar to 0.35-3.5 mm) in precipitable water vapour columns, has been gathered from October 2020 to September 2022. These data are unique for their quality and completeness and, due to the proximity of APEX to the Atacama Large Millimeter/Submillimeter Array (ALMA), they provide an excellent opportunity to validate the atmospheric radiative transfer model currently installed in the ALMA software. The main issues addressed in the study are possible missing lines in the model, line shapes, vertical profiles of atmospheric physical parameters and molecular abundances, seasonal and diurnal variations, and collision-induced absorption (CIA), to which this paper is devoted, in its N-2-N-2 + N-2-O-2 + O-2-O-2 (dry) and N-2-H2O + O-2 -H2O ('foreign' wet) mechanisms. All these CIA terms should remain unchanged in the above-mentioned ALMA atmospheric model as a result of this work.

We present the discovery of the unsubstituted polycyclic aromatic hydrocarbon (PAH) phenalene (C13H10) in TMC-1 as part of the QUIJOTE line survey. In spite of the low dipole moment of this three-ring PAH, we managed to identify a total of 267 rotational transitions with quantum numbers J and K-a up to 34 and 14, respectively, corresponding to 71 independent frequencies. The identification of this new PAH from our survey was based on the agreement between the rotational parameters derived from the analysis of the lines and those obtained by quantum chemical calculations. Our subsequent chemical synthesis of this PAH and investigation of its laboratory microwave spectrum unequivocally support our identification. We report the column density of phenalene in TMC-1 as (2.8 +/- 1.6) x 10(13) cm(-2).

We investigated the reaction of propene (C3H6) with the cyano radical (CN) in light of the recent detection of five cyanopropene isomers in TMC-1. To provide reliable branching ratios, we characterized the stationary points on the potential energy surface using the high-accuracy jun-ChS-F12 method. The resulting energetics were then employed to derive temperature-dependent rate constants. Our calculations show that the formation of all observed cyano derivatives is feasible through this reaction, although it is secondary compared to the dominant formation channel of vinyl cyanide (C2H3CN). The predicted branching ratios are in good agreement with the observations, with the discrepancies prompting further investigation on the destruction mechanisms of the different isomers. Overall, this work supports a scenario in which these cyano derivatives in TMC-1 arise primarily from pure gas-phase chemistry.

We detected the linear ^3Σ^- radicals SiC_3 and SiC_5 toward IRC+10216 using an ultrasensitive line survey gathered with the Yebes 40 m radio telescope. The derived column densities of l-SiC_3 and l-SiC_5 are (3.6±0.4)×10^12 cm^-2 and (1.8±0.2)×10^12 cm^-2, respectively. The linear SiC_3 radical is ∼2 times less abundant that its singlet rhomboidal prolate isomer, for which we provide a new analysis based on recent sensitive observations in the Q band (7 mm), and at 3 and 2 mm with the IRAM 30m telescope. The emission detected from these species arises from the cool external layers of the circumstellar envelope. We speculate whether ion-neutral routes involving SiC_nH_m^+ cations or neutral-neutral reactions involving Si and SiC_2 could efficiently synthesize these species.

The astronomical detection of CN-functionalized ring molecules in TMC-1 has opened a new and unexplored area in space-based aromatic organic chemistry. The rotational spectrum of 1-cyano-cyclopentene (1-CNCPE) has been investigated using a combination of low- and high-resolution microwave spectroscopy techniques under jet-cooled and room temperature conditions. A Stark-modulated low-resolution spectrum revealed a clear a- type R- branch band structure, indicative of a single dominant rotamer with prolate character. High-resolution spectra recorded with broadband (CP-FTMW) and narrowband (MB-FTMW) techniques exhibited a doubling of transitions, attributed to tunneling between two ring-puckering vibrational substates, denoted 0 ^+ and 0 ^− . In addition, the millimeter-wave spectrum in the 329–394 GHz region has also been analyzed. Due to strong coupling between these levels, the spectral analysis required a Hamiltonian including Coriolis interaction terms within the reduced axis system formalism. This approach enabled the successful assignment and fitting of over 1244 rotational transitions, including hyperfine structure from the ^14 N nucleus and weak b- and c -type lines. The final spectroscopic parameters provide a reliable basis for astrophysical searches, and partition functions are reported at standard temperatures. We have searched for this species toward TMC-1 using the data of the QUIJOTE line survey. Only a 3 σ upper limit to the column density of 3 × 10 ^10 cm ^−2 has been obtained. Using the available spectroscopic data, we also searched for the two axial and equatorial conformers of 3-cyanocyclopentene, establishing an upper limit similar to that of 1-CNCPE.

Context.Mid-infrared emission features are important probes of the properties of ionized gas and hot or warm molecular gas, which are difficult to probe at other wavelengths. The Orion Bar photodissociation region (PDR) is a bright, nearby, and frequently studied target containing large amounts of gas under these conditions. Under the “PDRs4All” Early Release Science Program for JWST, a part of the Orion Bar was observed with MIRI integral field unit (IFU) spectroscopy, and these high-sensitivity IR spectroscopic images of very high angular resolution (0.2″) provide a rich observational inventory of the mid-infrared (MIR) emission lines, while resolving the HIIregion, the ionization front, and multiple dissociation fronts.Aims.We list, identify, and measure the most prominent gas emission lines in the Orion Bar using the new MIRI IFU data. An initial analysis summarizes the physical conditions of the gas and demonstrates the potential of these new data and future IFU observations with JWST.Methods.The MIRI IFU mosaic spatially resolves the substructure of the PDR, its footprint cutting perpendicularly across the ionization front and three dissociation fronts. We performed an up-to-date data reduction, and extracted five spectra that represent the ionized, atomic, and molecular gas layers. We identified the observed lines through a comparison with theoretical line lists derived from atomic data and simulated PDR models. The identified species and transitions are summarized in the main table of this work, with measurements of the line intensities and central wavelengths.Results.We identified around 100 lines and report an additional 18 lines that remain unidentified. The majority consists of HIrecombination lines arising from the ionized gas layer bordering the PDR. The HIline ratios are well matched by emissivity coefficients from H recombination theory, but deviate by up to 10% because of contamination by HeIlines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni. We show how the NeIII/NeII, SIV/SIII, and ArIII/ArIIratios trace the conditions in the ionized layer bordering the PDR, while FeIII/FeIIand NiIII/NiIIexhibit a different behavior, as there are significant contributions to FeIIand NiIIfrom the neutral PDR gas. We observe the pure-rotational H2lines in the vibrational ground state from 0–0S(1) to 0–0S(8), and in the first vibrationally excited state from 1–1S(5) to 1–1 S(9). We derive H2excitation diagrams, and for the three observed dissociation fronts, the rotational excitation can be approximated with one thermal (~700 K) component representative of an average gas temperature, and one nonthermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model of the Orion Bar PDR, and find that the predicted excitation matches the data qualitatively, while adjustments to the parameters of the PDR model are required to reproduce the intensity of the 0–0 S (6) to S (8) lines.