arXiv subjects

A. Megías

Publications and source records attributed to A. Megías.

6 recordsLinked to original sources

arXiv2026

Aromatic rings in the Central Molecular Zone: Benzonitrile

In recent years, several aromatic molecules (benzene-based rings) have been detected in the cold molecular cloud TMC-1, with its CN-derivative, benzonitrile (c-C$_6$H$_5$CN), also identified in other nearby cold sources. However, observed abundances differ significantly from chemical model predictions, indicating an incomplete understanding of its chemistry and motivating searches in distinct environments. We report new detections of benzonitrile in two warmer molecular clouds of the Central Molecular Zone (CMZ): G+0.693-0.027 and G+0.633-0.0604. Using Yebes 40m ultra-deep surveys in the 31--50 GHz range, we performed LTE and non-LTE analyses to derive the physical parameters of the emission. We obtain column densities of $N$=(7.4$\pm$0.5)$\times10^{12}$ and (2.60$\pm$0.13)$\times10^{12}$ cm$^{-2}$, corresponding to abundances relative to H$_2$ of (6$\pm$1)$\times10^{-11}$ and (4.3$\pm$0.9)$\times10^{-11}$, consistent with values in cold Galactic clouds. The HC$_7$N/benzonitrile ratio is lower (2.15-2.4) than in colder sources (4.5-30), suggesting environmental effects and a relative enhancement of aromatic chemistry in the CMZ. These results confirm that benzonitrile is widespread and can survive in harsher environments (e.g., high temperatures, shocks, enhanced cosmic-ray ionization) than those in Galactic cold clouds. This suggests that aromatics are stable and abundant species that can significantly contribute to the total budget of interstellar carbon in molecular clouds. A top-down formation scenario, involving fragmentation of larger carbonaceous species, is consistent with the nearly constant abundances observed with molecular size.

astro-ph.GA↗

arXiv2026

c-C3H2 deuteration towards prestellar and starless cores in the Perseus Molecular Cloud

Deuterium fractionation becomes highly efficient in cold, dense cores where CO is frozen out. Cyclopropenylidene (c-C3H2), an early-formed carbon ring, and its deuterated isotopologues trace gas-phase deuteration in these environments. We present a statistical study of c-C3H2 deuteration in starless and prestellar cores of the Perseus Molecular Cloud using observations of c-C3H2, c-C3HD and c-C3D2 obtained with the Yebes 40 m, ARO 12 m and IRAM 30 m telescopes towards 16 cores. Gaussian fits and RADEX modeling yield column densities for the detected species. c-C3H2 is detected in 14/15 covered cores, c-C3HD in 15/16, and c-C3D2 in 9/16. Derived column densities range from 0.5-8.1 x 10^{13} cm^{-2} for c-C3H2, 0.2-2.1 x 10^{12} cm^{-2} for c-C3HD, and 0.6-1.6 x 10^{11} cm^{-2} for c-C3D2. The ortho-to-para ratio of c-C3H2 is obtained for all but one core, with a median value of 3.5\pm0.4. Statistically corrected D/H ratios span 0.5-9.2% (median 1.5\pm0.2%), and D2/D ratios 9-55% (median 25.9\pm4.3%). No trend is found between the c-C3H2 ortho-to-para ratio and core evolutionary stage traced by n(H2). The median D/H ratio in Perseus appears lower than values reported for Taurus and Chamaeleon, while the D2/D ratio agrees with Taurus within uncertainties. A positive correlation between D/H and n(H2) supports the use of D/H as an evolutionary tracer. D2/D does not correlate with n(H2), but shows a positive correlation with T_{kin}, suggesting that its formation is influenced by a mildly endothermic pathway.

astro-ph.GA↗

arXiv2026

Expanding the C$_3$H$_6$O$_2$ Isomeric Interstellar Inventory: Discovery of Lactaldehyde and Methoxyacetaldehyde in G+0.693-0.027

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. We use 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. 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 $gauche$- conformers of ethyl formate (CH$_3$CH$_2$OC(O)H), the latter tentatively. 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 $\leq$1.5 $\times$ 10$^{-10}$ and $\leq$3.7 $\times$ 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.

astro-ph.GA↗

arXiv2025

High deuteration of methanol in L1544

Isotopic fractionation is a very powerful tool to follow the evolution of material from one stage to the next in the star-formation process. Pre-stellar cores exhibit some of the highest levels of deuteration because their physical conditions greatly favor deuteration processes. Deuteration maps are a measure of the effectiveness of the deuteration across the core, and they are useful to study both the deuteration as well as the formation mechanism of the main species. Methanol is the simplest O-bearing complex organic molecule (COM) detected in the interstellar medium (ISM). It represents the beginning of molecular complexity in star-forming regions, thus a complete understanding of its formation and deuteration is a necessary step to understand the development of further chemical complexity. In this paper, we use single-dish observations with the IRAM 30 m telescope and state-of-the-art chemical models to investigate the deuteration of methanol towards the prototypical pre-stellar core L1544. We also compare the results of the chemical models with previous observations of deuterated methanol towards the pre-stellar cores HMM1 and L694-2. The spectra extracted from the CHD$_2$OH map show that the emission is concentrated in the center and towards the north-west of the core. Using deep observations towards the dust and the methanol peaks of the core, we derive a very large deuterium fraction for methanol ($\sim20\%$) towards both peaks. The comparison of our observational results with chemical models has highlighted the importance of H-abstraction processes in the formation and deuteration of methanol. Deep observations combined with state-of-the-art chemical models are of fundamental importance in understanding the development of molecular complexity in the ISM. Our analysis also shows the importance of non-LTE effects when measuring the D/H ratios in methanol.

astro-ph.GA↗

arXiv2025

High-energy interstellar isomers: cis-N-methylformamide in the G+0.693-0.027 molecular cloud

Isomerism in complex organic molecules provides key insights into the formation mechanisms and physical conditions of the interstellar medium (ISM). Among the C$_2$H$_5$NO isomers, only acetamide and trans-N-methylformamide (trans-NMF) have been detected in space. The recent detection of higher-energy isomers in other chemical families raises questions about the formation and abundance of less stable isomers. We used ultra-sensitive wide-band spectral surveys obtained with the Yebes 40 m and IRAM 30 m telescopes to search for cis-NMF towards the Galactic Centre molecular cloud G+0.693-0.027. We present the first detection of cis-NMF in the ISM, with 55 unblended or slightly blended transitions, 44 of which were new transitions identified based on extrapolated spectroscopic data. Due to the lack of collisional rate coefficients, a quasi-non-LTE analysis, which separated the transitions into different K$_a$ ladders, was used to determine the excitation conditions. The resulting trans/cis-NMF isomeric ratio of 2.9$\pm$0.6 deviates significantly from thermodynamic expectations, suggesting that kinetic non-equilibrium processes and stereospecific chemical pathways are responsible for the formation of cis-NMF in this environment. The detection of cis-NMF expands the known inventory of interstellar C$_2$H$_5$NO isomers and challenges the assumption that isomer abundances strictly correlate with thermodynamic stability. Laboratory and theoretical studies propose formation via CH$_3$NCO hydrogenation or spin-forbidden reactions involving CH$_2$ and NH$_2$CHO, though these may not reflect typical ISM conditions. This finding highlights the need for further investigation into isomerisation mechanisms and constrains astrochemical models of complex organic molecules.

astro-ph.GA↗

arXiv2022

Negative and Positive Feedback from a Supernova Remnant with SHREC: A detailed Study of the Shocked Gas in IC443

Supernova remnants (SNRs) contribute to regulate the star formation efficiency and evolution of galaxies. As they expand into the interstellar medium (ISM), they transfer vast amounts of energy and momentum that displace, compress and heat the surrounding material. Despite the extensive work in galaxy evolution models, it remains to be observationally validated to what extent the molecular ISM is affected by the interaction with SNRs. We use the first results of the ESO-ARO Public Spectroscopic Survey SHREC, to investigate the shock interaction between the SNR IC443 and the nearby molecular clump G. We use high sensitivity SiO(2-1) and H$^{13}$CO$^+$(1-0) maps obtained by SHREC together with SiO(1-0) observations obtained with the 40m telescope at the Yebes Observatory. We find that the bulk of the SiO emission is arising from the ongoing shock interaction between IC443 and clump G. The shocked gas shows a well ordered kinematic structure, with velocities blue-shifted with respect to the central velocity of the SNR, similar to what observed toward other SNR-cloud interaction sites. The shock compression enhances the molecular gas density, n(H$_2$), up to $>$10$^5$ cm$^{-3}$, a factor of >10 higher than the ambient gas density and similar to values required to ignite star formation. Finally, we estimate that up to 50\% of the momentum injected by IC443 is transferred to the interacting molecular material. Therefore the molecular ISM may represent an important momentum carrier in sites of SNR-cloud interactions.

astro-ph.GA↗