Andrés Megías

In recent years, several aromatic molecules (i.e., rings based on benzene, c- ) have been reported toward the cold molecular cloud TMC-1; and benzonitrile (the CN-derivative of benzene, C6H6 has been identified in other nearby cold clouds with similar physical properties. The derived molecular abundances differ significantly from those predicted by current chemical models, indicating that its chemistry is still poorly understood. This motivates the search for these compounds in other physically distinct sources beyond the solar vicinity. In this work, we present new detections of in two warmer molecular clouds of the Central Molecular Zone (CMZ) of our Galaxy, G+0.693-0.027 and G+0.633-0.0604. We used Yebes 40m ultra-deep surveys toward both CMZ clouds covering the 31-50 GHz range, and applied local thermodynamic equilibrium (LTE) and non-LTE analysis to identify and derive the physical parameters of the emission. We derived column densities of N=(7.4±0.5)times1012 and (2.60±0.13) of (6±1) cm^-2, which translate into very similar molecular abundances relative to H2 and (4.3±0.9) , respectively, consistent with those found toward cold clouds in the Galactic disk. We found that the ratio of the unsaturated carbon chain to -11 HC7N shows a dependence on the environmental conditions, being lower (2.15--2.4) in both CMZ clouds compared to colder sources (4.5-30). This points to local effects on the formation and destruction of these species, and a predominance of aromatic chemistry in the CMZ compared with linear carbon chains. This work confirms that is widespread in our Galaxy and can survive in harsher environments (high kinetic temperatures, strong shocks, and enhanced cosmic-ray ionization rates) than those in Galactic cold clouds, which suggests that aromatics are stable and abundant species that can significantly contribute to the total budget of interstellar carbon in molecular clouds in diverse environments. A top-down scenario, in which aromatic species are produced through the fragmentation and processing of carbonaceous solids or larger aromatics, agrees well with the strikingly nearly constant behavior of their molecular abundances with molecular size.

Context . In cores deuterium fractionation becomes highly efficient due to low temperatures and CO freeze-out. Cyclopropenylidene (c-C ₃ H ₂ ), a small cyclic molecule formed early in chemical evolution, and its deuterated forms serve as valuable tracers of gas-phase deuteration in these environments. Aims . In order to statistically explore the c-C ₃ H ₂ deuteration ratios towards starless and prestellar cores, we present observations of c-C ₃ H ₂ and its deuterated isotopologues on a sample of cores in the Perseus molecular cloud. Methods . Transitions of c-C ₃ H ₂ , c-C ₃ HD, and c-C ₃ D ₂ were observed with the Yebes 40m, the Arizona Radio Observatory (ARO) 12m and the Institut de Radioastronomie Millimétrique (IRAM) 30 metre telescopes towards a total of 16 starless and prestellar cores in the Perseus molecular cloud. The lines were fitted with Gaussian profiles and their column densities were computed using the non-local thermodynamic equilibrium (non-LTE) software RADEX. Results . The main isotopologue c-C ₃ H ₂ is detected in 93% (14/15) of the targeted cores (for one of the 16 cores, none of its transitions were covered), its singly deuterated form c-C ₃ HD is detected in 94% (15/16) of the targeted cores, and its doubly deuterated form c-C ₃ D ₂ is detected towards 56% (9/16) of the cores detected. A range of column densities towards the different cores was derived: for c-C ₃ H ₂ , (0.5–8.1)×10 ¹³ cm ⁻² ; for c-C ₃ HD, (0.2–2.1)×10 ¹² cm ⁻² ; and for c-C ₃ D ₂ , (0.6–1.6)×10 ¹¹ cm ⁻² . The ortho-to-para ratio of c-C ₃ H ₂ was obtained for all except one core with a median value of 3.5 ± 0.4. The D/H and D ₂ /D ratios were obtained for the cores with detections, yielding a statistically corrected D/H range of 0.5–9.2% with a median value of 1.5 ± 0.2% and a statistically corrected D ₂ /D range 9.0–55.2% with a median value of 25.9 ± 4.3%. Conclusions . No apparent trend is seen with the ortho-to-para ratio of c-C ₃ H ₂ and the evolutionary stage of the core, as traced by volume density n H ₂ . The median c-C ₃ H ₂ D/H ratio in Perseus’ starless cores appears lower than the value for the Taurus molecular cloud and the Chamaeleon molecular cloud. The D ₂ /D ratio is equivalent between the Perseus and Taurus molecular clouds within the uncertainties. There is a correlation with the D/H ratio and the n H ₂ of the cores in Perseus, strengthening the idea of D/H being a tracer of core evolution. The D ₂ /D ratio does not correlate with n H ₂ , but positively correlates with T _kin , suggesting that its formation is favoured by a slightly endothermic reaction.

Aims . The tentative detection of 3-hydroxypropanal (HO(CH ₂ ) ₂ C(O)H) toward the Galactic center molecular cloud G+0.693-0.027 prompts a systematic survey in this source aimed at detecting all C ₃ H ₆ O ₂ isomers with available spectroscopy. Methods . We used an ultra-deep broadband spectral survey of G+0.693-0.027, carried out with the Yebes 40 m and IRAM 30 m telescopes, to conduct the astronomical search. Results . We report the first interstellar detection of lactaldehyde (CH ₃ CH(OH)C(O)H) and methoxyacetaldehyde (CH ₃ OCH ₂ C(O)H), together with the second detections (i.e., confirmation) of methyl acetate (CH ₃ C(O)OCH ₃ ) and hydroxyacetone (CH ₃ C(O)CH ₂ OH), and new detections in this source of both anti - and g auche - conformers of ethyl formate (CH ₃ CH ₂ OC(O)H; the latter being tentative). For these species, we derived a fractional abundance relative to H ₂ of ~(0.81, 0.24, 16, 1.6, 1.3, 1.4) × 10 ⁻¹⁰ , respectively. In contrast, neither propionic acid, CH ₃ CH ₂ C(O)OH, nor glycidol, c-CH ₂ OCHCH ₂ OH (i.e., the most and the least stable species within the C ₃ H ₆ O ₂ family, respectively) were detected, and we provide upper limits on their fractional abundances of ≤1.5 × 10 ⁻¹⁰ and ≤3.7 × 10 ⁻¹¹ . Interestingly, all C ₃ H ₆ O ₂ isomers can be synthesized through radical-radical reactions on the surface of dust grains, ultimately tracing back to CO as the parent molecule. We suggest that formation of the detected isomers is mainly driven by successive hydrogenation of CO, producing CH ₃ OH and CH ₃ CH ₂ OH as the primary parent species. Conversely, propionic acid is thought to originate from the oxygenation of CO via the HOCO intermediate, which help us rationalize its non-detection. Overall, our findings notably expand the known chemical inventory of the interstellar medium and provide direct observational evidence that increasingly complex chemistry involving O-bearing species occurs in space.

Molecules harbouring sulfur are thought to have played a key role in the biological processes of life on Earth, and thus, they are of much interest when found in space. Here we report on the astronomical detection of a six-membered sulfur-bearing cyclic hydrocarbon in the interstellar medium. Observations of the Galactic Centre molecular cloud G+0.693-0.027 reveal the presence of 2,5-cyclohexadien-1-thione, which is a structural isomer of thiophenol ( c -C ₆ H ₆ S). For the astronomical identification, we first performed precise laboratory measurements of the thiophenol discharge products system. These measurements, conducted in the radio band using a chirped-pulse Fourier transform microwave spectrometer, enabled us to characterize this highly polar molecular species and provided unambiguous fingerprints needed to identify this organosulfur compound in space, which now ranks as the largest interstellar sulfur-bearing molecule. These results herald the discovery of a family of prebiotically relevant sulfur-bearing species, which potentially act as a bridge between the chemical inventory of the interstellar medium and the composition of the minor bodies of the Solar System.

Context. Current observations taken by James Webb Space Telescope (JWST) allow us to observe the absorption features of icy mantles that cover interstellar dust grains, which are mainly composed of H₂O, CO, and CO₂, along with other minor species. Thanks to its sensitivity and spectral resolution, JWST has the potential to observe ice features towards hundreds of sources at different stages along the process of star formation. However, identifying the spectral features of the different species and quantifying the ice composition is not trivial and requires complex spectroscopic analysis. Aims. We reduce the difficulty and the time employed in doing this task by developing a new software analysis tool based on machine learning. Methods. We present Automatic Ice Composition Estimator (AICE), a new tool based on artificial neural networks. Based on the infrared (IR) ice absorption spectrum between 2.5 and 10 μm, AICE predicts the ice fractional composition in terms of H₂O, CO, CO₂, CH₃OH, NH₃, and CH₄. To train the model, we used hundreds of laboratory experiments of ice mixtures from different databases, which were reprocessed with baseline subtraction and normalisation. Results. Once trained, AICE takes less than one second on a conventional computer to predict the ice composition associated with the observed IR absorption spectrum, with typical errors of ~3% in the species fraction. We tested its performance on two spectra reported towards the NIR38 and J110621 background stars observed within the JWST Ice Age program, demonstrating a good agreement with previous estimations of the ice composition. Conclusions. The fast and accurate performance of AICE enables the systematic analysis of hundreds of different ice spectra with a modest time investment. In addition, this model can be enhanced and re-trained with more laboratory data, improving the precision of the predictions and expanding the list of predicted species.

Phosphorus is a key element that plays an essential role in biological processes important for living organisms on Earth. The origin and connection of phosphorus-bearing molecules to early solar system objects and star-forming molecular clouds is therefore of great interest, yet there are limited observations throughout different stages of low-mass (M < a few solar masses) star formation. Observations from the Yebes 40 m and IRAM 30 m telescopes detect for the first time in the 7 mm, 3 mm, and 2 mm bands multiple transitions of PN and PO, as well as a single transition of PO⁺, toward a low-mass starless core. The presence of PN, PO, and PO⁺ is kinematically correlated with bright SiO(1–0) emission. Our results reveal not only that shocks are the main driver of releasing phosphorus from dust grains and into the gas phase but that the emission originates from gas not affiliated with the shock itself but quiescent gas that has been shocked in the recent past. From radiative transfer calculations, the PO/PN abundance ratio is found to be 3 . 1 − 0.6 + 0.4 , consistent with other high-mass and low-mass star-forming regions. This first detection of PO⁺ toward any low-mass star-forming region reveals a PO⁺/PO ratio of 0.011 5 − 0.0009 + 0.0008 , a factor of 10 lower than previously determined from observations of a Galactic center molecular cloud, suggesting its formation can occur under more standard Galactic cosmic-ray ionization rates. These results motivate the need for additional observations that can better disentangle the physical mechanisms and chemical drivers of this precursor of prebiotic chemistry.

Context. Recent JWST observations have measured the ice chemical composition towards two highly extinguished background stars, NIR38 and J110621, in the Chamaeleon I molecular cloud. The observed excess of extinction on the long-wavelength side of the H₂O ice band at 3 μm has been attributed to a mixture of CH₃OH with ammonia hydrates NH₃·H₂O), which suggests that CH₃OH ice in this cloud could have formed in a water-rich environment with little CO depletion. Laboratory experiments and quantum chemical calculations suggest that CH₃OH could form via the grain surface reactions CH₃ + OH and/or C + H₂O in water-rich ices. However, no dedicated chemical modelling has been carried out thus far to test their efficiency. In addition, it remains unexplored how the efficiencies of the proposed mechanisms depend on the astrochemical code employed. Aims. We modelled the ice chemistry in the Chamaeleon I cloud to establish the dominant formation processes of CH₃OH, CO, CO₂, and of the hydrides CH₄ and NH₃ (in addition to H₂O). By using a set of state-of-the-art astrochemical codes (MAGICKAL, MONACO, Nautilus, UCLCHEM, and KMC simulations), we can test the effects of the different code architectures (rate equation vs. stochastic codes) and of the assumed ice chemistry (diffusive vs. non-diffusive). Methods. We consider a grid of models with different gas densities, dust temperatures, visual extinctions, and cloud-collapse length scales. In addition to the successive hydrogenation of CO, the codes’ chemical networks have been augmented to include the alternative processes for CH₃OH ice formation in water-rich environments (i.e. the reactions CH₃ + OH → CH₃OH and C + H₂O → H₂CO). Results. Our models show that the JWST ice observations are better reproduced for gas densities ≥10⁵ cm⁻³ and collapse timescales ≥10⁵ yr. CH₃OH ice formation occurs predominantly (>99%) via CO hydrogenation. The contribution of reactions CH₃ + OH and C + H₂O is negligible. The CO₂ ice may form either via CO + OH or CO + O depending on the code. However, KMC simulations reveal that both mechanisms are efficient despite the low rate of the CO + O surface reaction. CH₄ is largely underproduced for all codes except for UCLCHEM, for which a higher amount of atomic C is available during the translucent cloud phase of the models. Large differences in the predicted abundances are found at very low dust temperatures (T_dustdust. This could explain the rather constant ice chemical composition found in Chamaeleon I and other dense cores despite the different visual extinctions probed.

Following the discovery of dimethyl sulfide (DMS; CH₃SCH₃) signatures in comet 67P/Churyumov–Gerasimenko, we report the first detection of this organosulfur species in the interstellar medium during the exploration of an ultradeep molecular line survey performed toward the Galactic center molecular cloud G+0.693-0.027 with the Yebes 40 m and IRAM 30 m telescopes. We derive a molecular column density of N = (2.6 ± 0.3) × 10¹³ cm⁻², yielding a fractional abundance relative to H₂ of ∼1.9 × 10⁻¹⁰. This implies that DMS is a factor of ∼1.6 times less abundant than its structural isomer CH₃CH₂SH and ∼30 times less abundant than its O-analog dimethyl ether (CH₃OCH₃) toward this cloud, in excellent agreement with previous results on various O/S pairs. Furthermore, we find a remarkable resemblance between the relative abundance of DMS/CH₃OH in G+0.693-0.027 (∼1.7 × 10⁻³) and in the comet (∼1.3 × 10⁻³). Although the chemistry of DMS beyond Earth has yet to be fully disclosed, this discovery provides conclusive observational evidence on its efficient abiotic production in the interstellar medium, casting doubt on using DMS as a reliable biomarker in exoplanet science.

Cold (⁠|$\sim$|10 K) and dense (⁠|$\sim 10^{5}$| cm|$^{-3}$|⁠) cores of gas and dust within molecular clouds, known as starless and dynamically evolved pre-stellar cores, are the birthplaces of low-mass (M|$\le$| few M|$_\odot$|⁠) stars. As detections of interstellar complex organic molecules, or COMs, in starless cores has increased, abundance comparisons suggest that some COMs might be seeded early in the star formation process and inherited to later stages (i.e. protostellar discs and eventually comets). To date observations of COMs in starless cores have been limited, with most detections reported solely in the Taurus molecular cloud. It is therefore still a question whether different environments affect abundances. We have surveyed 35 starless and pre-stellar cores in the Perseus molecular cloud with the Arizona Radio Observatory (ARO) 12 m telescope detecting both methanol, CH|$_3$|OH, and acetaldehyde, CH|$_3$|CHO, in 100 per cent and 49 per cent of the sample, respectively. In the sub-sample of 15 cores where CH|$_3$|CHO was detected at |$\gt 3\sigma$| (⁠|$\sim$|18 mK) with the ARO 12 m, follow-up observations with the Yebes 40 m telescope were carried out. Detections of formic acid, t-HCOOH, ketene, H|$_2$|CCO, methyl cyanide, CH|$_3$|CN, vinyl cyanide, CH|$_2$|CHCN, methyl formate, HCOOCH|$_3$|⁠, and dimethyl ether, CH|$_3$|OCH|$_3$|⁠, are seen in at least 20 per cent of the cores. We discuss detection statistics, calculate column densities, and compare abundances across various stages of low-mass star formation. Our findings have more than doubled COM detection statistics in cold cores and show COMs are prevalent in the gas before star and planet formation in the Perseus molecular cloud.