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Molecular deuteration is a powerful diagnostic tool for probing the physical conditions and chemical processes in astrophysical environments. In this work, we focus on formaldehyde deuteration in the protobinary system NGC\,1333 IRAS\,4A, located in the Perseus molecular cloud. Using high-resolution ($\sim$\,100\,au) ALMA observations, we investigate the [D$_2$CO]/[HDCO] ratio along the cavity walls of the outflows emanating from IRAS\,4A1. Our analysis reveals a consistent decrease in the deuteration ratio (from $\sim$\,60-20\% to $\sim$\,10\%) with increasing distance from the protostar (from $\sim$\,2000\,au to $\sim$\,4000\,au). Given the large measured [D$_2$CO]/[HDCO], both HDCO and D$_2$CO are likely injected by the shocks along the cavity walls into the gas-phase from the dust mantles, formed in the previous prestellar phase. We propose that the observed [D$_2$CO]/[HDCO] decrease is due to the density profile of the prestellar core from which NGC\,1333 IRAS\,4A was born. When considering the chemical processes at the base of formaldehyde deuteration, the IRAS\,4A's prestellar precursor had a predominantly flat density profile within 3000\,au and a decrease of density beyond this radius.

The exploration of outflows in protobinary systems presents a challenging yet crucial endeavour, offering valuable insights into the dynamic interplay between protostars and their evolution. In this study, we examine the morphology and dynamics of jets and outflows within the IRAS\,4A protobinary system. This analysis is based on ALMA observations of SiO(5--4), H$_2$CO(3$_{0,3}$--2$_{0,3}$), and HDCO(4$_{1,4}$--3$_{1,3}$) with a spatial resolution of $\sim$150\,au. Leveraging an astrochemical approach involving the use of diverse tracers beyond traditional ones has enabled the identification of novel features and a comprehensive understanding of the broader outflow dynamics. Our analysis reveals the presence of two jets in the redshifted emission, emanating from IRAS\,4A1 and IRAS\,4A2, respectively. Furthermore, we identify four distinct outflows in the region for the first time, with each protostar, 4A1 and 4A2, contributing to two of them. We characterise the morphology and orientation of each outflow, challenging previous suggestions of bends in their trajectories. The outflow cavities of IRAS\,4A1 exhibit extensions of 10$''$ and 13$''$ with position angles (PA) of 0$^{\circ}$ and -12$^{\circ}$, respectively, while those of IRAS\,4A2 are more extended, spanning 18$''$ and 25$''$ with PAs of 29$^{\circ}$ and 26$^{\circ}$. We propose that the misalignment of the cavities is due to a jet precession in each protostar, a notion supported by the observation that the more extended cavities of the same source exhibit lower velocities, indicating they may stem from older ejection events.

Methyl cyanide (CH$_3$CN) is one of the most abundant and widely spread interstellar complex organic molecules (iCOMs). Several studies found that, in hot corinos, methyl cyanide and methanol abundances are correlated suggesting a chemical link, often interpreted as a synthesis of them on the interstellar grain surfaces. In this article, we present a revised network of the reactions forming methyl cyanide in the gas-phase. We carried out an exhaustive review of the gas-phase CH$_3$CN formation routes, propose two new reactions and performed new quantum mechanics computations of several reactions. We found that 13 of the 15 reactions reported in the databases KIDA and UDfA have incorrect products and/or rate constants. The new corrected reaction network contains 10 reactions leading to methyl cyanide. We tested the relative importance of those reactions in forming CH$_3$CN using our astrochemical model. We confirm that the radiative association of CH${_3}{^+}$ and HCN, forming CH$_{3}$CNH$^{+}$, followed by the electron recombination of CH$_{3}$CNH$^{+}$, is the most important CH$_3$CN formation route in both cold and warm environments, notwithstanding that we significantly corrected the rate constants and products of both reactions. The two newly proposed reactions play an important role in warm environments. Finally, we found a very good agreement between the CH$_3$CN predicted abundances with those measured in cold ($\sim$10 K) and warm ($\sim$90 K) objects. Unexpectedly, we also found a chemical link between methanol and methyl cyanide via the CH$_{3}^{+}$ ion, which can explain the observed correlation between the CH$_3$OH and CH$_3$CN abundances measured in hot corinos., Comment: 24 pages, 19 figures, accepted in MNRAS

The gas-phase reaction networks are the backbone of astrochemical models. However, due to their complexity and non-linear impact on the astrochemical modeling, they can be the first source of error in the simulations if incorrect reactions are present. Over time, following the increasing number of species detected, astrochemists have added new reactions, based on laboratory experiments and quantum mechanics (QM) computations as well as reactions inferred by chemical intuition and similarity principle. However, sometimes no verification of their feasibility in the interstellar conditions, namely their exothermicity, was performed. In this work, we present a new gas-phase reaction network, GRETOBAPE, based on the KIDA2014 network and updated with several reactions, cleaned from endothermic reactions not explicitly recognized as such. To this end, we characterized all the species in the GRETOBAPE network with accurate QM calculations. We found that 5% of the reactions in the original network are endothermic although most of them are reported as barrierless. The reaction network of Si-bearing species is the most impacted by the endothermicity cleaning process. We also produced a cleaned reduced network, GRETOBAPE-red, to be used to simulate astrochemical situations where only C-, O-, N- and S- bearing species with less than 6 atoms are needed. Finally, the new GRETOBAPE network, its reduced version, as well as the database with all the molecular properties are made publicly available. The species properties database can be used in the future to test the feasibility of possibly new reactions., Comment: ApJS submitted

We report a comprehensive study of the cyanopolyyne chemistry in the prototypical prestellar core L1544. Using the 100m Robert C. Byrd Green Bank Telescope (GBT) we observe 3 emission lines of HC$_3$N, 9 lines of HC$_5$N, 5 lines of HC$_7$N, and 9 lines of HC$_9$N. HC$_9$N is detected for the first time towards the source. The high spectral resolution ($\sim$ 0.05 km s$^{-1}$) reveals double-peak spectral line profiles with the redshifted peak a factor 3-5 brighter. Resolved maps of the core in other molecular tracers indicates that the southern region is redshifted. Therefore, the bulk of the cyanopolyyne emission is likely associated with the southern region of the core, where free carbon atoms are available to form long chains, thanks to the more efficient illumination of the interstellar field radiation. We perform a simultaneous modelling of the HC$_5$N, HC$_7$N, and HC$_9$N lines, to investigate the origin of the emission. To enable this analysis, we performed new calculation of the collisional coefficients. The simultaneous fitting indicates a gas kinetic temperature of 5--12 K, a source size of 80$\arcsec$, and a gas density larger than 100 cm$^{-3}$. The HC$_5$N:HC$_7$N:HC$_9$N abundance ratios measured in L1544 are about 1:6:4. We compare our observations with those towards the the well-studied starless core TMC-1 and with the available measurements in different star-forming regions. The comparison suggests that a complex carbon chain chemistry is active in other sources and it is related to the presence of free gaseous carbon. Finally, we discuss the possible formation and destruction routes in the light of the new observations., Comment: 16 pages, 7 figure, 3 tables

We have performed a theoretical investigation of the S$^+$($^4$S) + SiH$_{2}$($^1$A$_1$) reaction, a possible formation route of the HSiS$^+$ and SiSH$^+$ cations that are alleged to be precursors of interstellar silicon sulfide, SiS. Electronic structure calculations allowed us to identify the main reaction pathways for the systems. The reaction has two exothermic channels leading to the isomeric species $^3$HSiS$^{+}$ and $^3$SiSH$^{+}$ formed in conjunction with H atoms. The reaction is not characterized by an entrance barrier and, therefore, it is believed to be fast also under the very low temperature conditions of interstellar clouds. The two ions are formed in their first electronically excited state because of the spin multiplicity of the overall potential energy surface. In addition, following the suggestion that neutral species are formed by proton transfer of protonated cations to ammonia, we have derived the potential energy surface for the reactions $^3$HSiS$^{+}$/$^3$SiSH$^{+}$ + NH$_{3}$($^{1}$A$_1$)., Comment: 15 pages, 7 figures. Preprint version submitted to LNCS (Springer) ICCSA2022

In this work we characterize an initial van der Waals adduct in the potential energy surface of reaction between cyanoacetylene HC3N and the cyano radical. The geometry of the CN-HC3N adduct has been optimized through calculations employing ab initio methods. Results show that the energy of the adduct lays below the reactants. Additionally, a saddle point that connects the adduct to an important intermediate of the PES has been localized, with energy below the reactants. Calculations of the intermolecular potential have been performed and results show that the energy of the van der Waals adduct is higher than estimated with the ab initio methods., Comment: 13 pages, 5 figures. Preprint version submitted to LNCS (Springer) ICCSA2021

Ethanol (CH$_3$CH$_2$OH) is a relatively common molecule, often found in star forming regions. Recent studies suggest that it could be a parent molecule of several so-called interstellar complex organic molecules (iCOMs). Yet, the formation route of this species remains debated. In the present work, we study the formation of ethanol through the reaction of CCH with one H$_2$O molecule belonging to the ice, as a test case to investigate the viability of chemical reactions based on a "radical + ice component" scheme as an alternative mechanism for the synthesis of iCOMs, beyond the usual radical-radical coupling. This has been done by means of DFT calculations adopting two clusters of 18 and 33 water molecules as ice models. Results indicate that CH$_3$CH$_2$OH can potentially be formed by this proposed reaction mechanism. The reaction of CCH with H$_2$O on the water ice clusters can be barrierless (thanks to the help of boundary icy water molecules acting as proton transfer assistants) leading to the formation of vinyl alcohol precursors (H$_2$CCOH and CHCHOH). Subsequent hydrogenation of vinyl alcohol yielding ethanol is the only step presenting a low activation energy barrier. We finally discuss the astrophysical implications of these findings., Comment: 30 pages, 6 figures Accepted for publication ACS Earth and Space Chemistry

We studied the line emission from CH3CN and its deuterated isotopologue CH$_2$DCN towards the prototypical Class I object SVS13-A, where the deuteration of a large number of species has already been reported. Our goal is to measure the CH$_3$CN deuteration in a Class I protostar, for the first time, in order to constrain the CH$_3$CN formation pathways and the chemical evolution from the early prestellar core and Class 0 to the evolved Class I stages. We imaged CH2DCN towards SVS13-A using the IRAM NOEMA interferometer at 3mm in the context of the Large Program SOLIS (with a spatial resolution of 1.8"x1.2"). The NOEMA images have been complemented by the CH$_3$CN and CH$_2$DCN spectra collected by the IRAM-30m Large Program ASAI, that provided an unbiased spectral survey at 3mm, 2mm, and 1.3mm. The observed line emission has been analysed using LTE and non-LTE LVG approaches. The NOEMA/SOLIS images of CH2DCN show that this species emits in an unresolved area centered towards the SVS13-A continuum emission peak, suggesting that methyl cyanide and its isotopologues are associated with the hot corino of SVS13-A, previously imaged via other iCOMs. In addition, we detected 41 and 11 ASAI transitions of CH$_3$CN and CH2DCN, respectively, which cover upper level energies (Eup) from 13 to 442 K and from 18 K to 200 K, respectively. The derived [CH2DCN]/[CH3CN] ratio is $\sim$9\%. This value is consistent with those measured towards prestellar cores and a factor 2-3 higher than those measured in Class 0 protostars. Contrarily to what expected for other molecular species, the CH3CN deuteration does not show a decrease in SVS13-A with respect to measurements in younger prestellar cores and Class 0 protostars. Finally, we discuss why our new results suggest that CH3CN was likely synthesised via gas-phase reactions and frozen onto the dust grain mantles during the cold prestellar phase., Comment: 20 pages, 2 tables, 12 figures

The formation of the interstellar complex organic molecules (iCOMs) is a hot topic in astrochemistry. One of the main paradigms trying to reproduce the observations postulates that iCOMs are formed on the ice mantles covering the interstellar dust grains as a result of radical--radical coupling reactions. We investigate iCOMs formation on the icy surfaces by means of computational quantum mechanical methods. In particular, we study the coupling and direct hydrogen abstraction reactions involving the CH$_3$ + X systems (X = NH$_2$, CH$_3$, HCO, CH$_3$O, CH$_2$OH) and HCO + Y (Y = HCO, CH$_3$O, CH$_2$OH), plus the CH$_2$OH + CH$_2$OH and CH$_3$O + CH$_3$O systems. We computed the activation energy barriers of these reactions as well as the binding energies of all the studied radicals, by means of density functional theory (DFT) calculations on two ice water models, made of 33 and 18 water molecules. Then, we estimated the efficiency of each reaction using the reaction activation, desorption and diffusion energies and derived kinetics with the Eyring equations. We find that radical--radical chemistry on surfaces is not as straightforward as usually assumed. In some cases, direct H abstraction reactions can compete with radical--radical couplings, while in others they may contain large activation energies. Specifically, we found that (i) ethane, methylamine and ethylene glycol are the only possible products of the relevant radical--radical reactions; (ii) glyoxal, methyl formate, glycolaldehyde, formamide, dimethyl ether and ethanol formation is likely in competition with the respective H-abstraction products, and (iii) acetaldehyde and dimethyl peroxide do not seem a likely grain surface products., Comment: 46 pages, 15 figures. Accepted for publication in ApJs

We report a study of the low-mass Class-0 multiple system VLA 1623AB in the Ophiuchus star-forming region, using H$^{13}$CO$^+$ ($J=3-2$), CS ($J=5-4$), and CCH ($N=3-2$) lines as part of the ALMA Large Program FAUST. The analysis of the velocity fields revealed the rotation motion in the envelope and the velocity gradients in the outflows (about 2000 au down to 50 au). We further investigated the rotation of the circum-binary VLA 1623A disk as well as the VLA 1623B disk. We found that the minor axis of the circum-binary disk of VLA 1623A is misaligned by about 12 degrees with respect to the large-scale outflow and the rotation axis of the envelope. In contrast, the minor axis of the circum-binary disk is parallel to the large-scale magnetic field according to previous dust polarization observations, suggesting that the misalignment may be caused by the different directions of the envelope rotation and the magnetic field. If the velocity gradient of the outflow is caused by rotation, the outflow has a constant angular momentum and the launching radius is estimated to be $5-16$ au, although it cannot be ruled out that the velocity gradient is driven by entrainments of the two high-velocity outflows. Furthermore, we detected for the first time a velocity gradient associated with rotation toward the VLA 16293B disk. The velocity gradient is opposite to the one from the large-scale envelope, outflow, and circum-binary disk. The origin of its opposite gradient is also discussed., Comment: 27 pages, 21 figures, 2 Tables, Accepted for publication in ApJ

Interstellar grains are known to be important actors in the formation of interstellar molecules such as H$_2$, water, ammonia, and methanol. It has been suggested that the so-called interstellar complex organic molecules (iCOMs) are also formed on the interstellar grain icy surfaces by the combination of radicals via reactions assumed to have an efficiency equal to unity. In this work, we aim to investigate the robustness or weakness of this assumption by considering the case of acetaldehyde (CH$_3$CHO) as a starting study case. In the literature, it has been postulated that acetaldehyde is formed on the icy surfaces via the combination of HCO and CH$_3$. Here we report new theoretical computations on the efficiency of its formation. To this end, we coupled quantum chemical calculations of the energetics and kinetics of the reaction CH$_3$ + HCO, which can lead to the formation of CH$_3$CHO or CO + CH$_4$. Specifically, we combined reaction kinetics computed with the Rice-Ramsperger-Kassel-Marcus (RRKM) theory (tunneling included) method with diffusion and desorption competitive channels. We provide the results of our computations in the format used by astrochemical models to facilitate their exploitation. Our new computations indicate that the efficiency of acetaldehyde formation on the icy surfaces is a complex function of the temperature and, more importantly, of the assumed diffusion over binding energy ratio $f$ of the CH$_3$ radical. If the ratio $f$ is $\geq$0.4, the efficiency is equal to unity in the range where the reaction can occur, namely between 12 and 30 K. However, if $f$ is smaller, the efficiency dramatically crashes: with $f$=0.3, it is at most 0.01. In addition, the formation of acetaldehyde is always in competition with that of CO + CH$_4$., Comment: 14 pages, 12 figures, 11 tables, accepted in A&A

Positive ions play a fundamental role in the interstellar chemistry, especially in cold environments where chemistry is believed to be mainly ion-driven. However, in contrast with neutral species, most of the cations present in the astrochemical reaction networks are not fully characterized in the astrochemical literature. To fill up this gap, we have carried out new accurate quantum chemical calculations to identify the structures and energies of 262 cations with up to 14 atoms that are postulated to have a role in the interstellar chemistry. Optimised structures and rotational constants were obtained at M06-2X/cc-pVTZ level, while electric dipoles and total electronic energies were computed with CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ single point energy calculations. The present work complements the study by Woon & Herbst (2009), who characterised the structure and energies of 200 neutral species involved as well in the interstellar chemistry. Taken together, the two datasets can be used to estimate whether a reaction, postulated in present astrochemical reaction networks, is feasible from a thermochemistry point of view and, consequently, to improve the reliability of the present networks used to simulate the interstellar chemistry. We provide an actual example of the potential use of the cations plus neutral datasets. It shows that two reactions, involving Si-bearing ions and present in the widely used reaction networks KIDA and UMIST, cannot occur in cold ISM because endothermic.

The present work focuses on the characterization of the reaction between cyanoacetylene and cyano radical by electronic structure calculations of the stationary points along the minimum energy path. One channel, leading to C4N2 (2-Butynedinitrile) + H, was selected due to the importance of its products. Using different ab initio methods, a number of stationary points of the potential energy surface were characterized. The energy values of these minima were compared in order to weight the computational costs in relation to chemical accuracy. The results of this works suggests that B2PLYP (and B2PLYPD3) gave a better description of the saddle point geometry, while B3LYP works better for minima., Comment: 11 pages, 1 figure. Preprint version submitted to LNCS (Springer) ICCSA2020

Molecular hydrogen is the most abundant molecular species in the Universe. While no doubts exist that it is mainly formed on the interstellar dust grain surfaces, many details of this process remain poorly known. In this work, we focus on the fate of the energy released by the H$_2$ formation on the dust icy mantles, how it is partitioned between the substrate and the newly formed H$_2$, a process that has a profound impact on the interstellar medium. We carried out state-of-art \textit{ab-initio} molecular dynamics simulations of H$_2$ formation on periodic crystalline and amorphous ice surface models. Our calculations show that up to two thirds of the energy liberated in the reaction ($\sim$300 kJ/mol $\sim$3.1 eV) is absorbed by the ice in less than 1 ps. The remaining energy ($\sim$140 kJ/mol $\sim$1.5 eV) is kept by the newly born H$_2$. Since it is ten times larger than the H$_2$ binding energy on the ice, the new H$_2$ molecule will eventually be released into the gas-phase. The ice water molecules within $\sim$4 \AA ~from the reaction site acquire enough energy, between 3 and 14 kJ/mol (360--1560 K), to potentially liberate other frozen H$_2$ and, perhaps, frozen CO molecules. If confirmed, the latter process would solve the long standing conundrum of the presence of gaseous CO in molecular clouds. Finally, the vibrational state of the newly formed H$_2$ drops from highly excited states ($\nu = 6$) to low ($\nu \leq 2$) vibrational levels in a timescale of the order of ps., Comment: 13 pages, 7 figures Submitted to ApJ (in press)

We have observed the very low-mass Class 0 protostar IRAS 15398-3359 at scales ranging from 50 au to 1800 au, as part of the ALMA Large Program FAUST. We uncover a linear feature, visible in H2CO, SO, and C18O line emission, which extends from the source along a direction almost perpendicular to the known active outflow. Molecular line emission from H2CO, SO, SiO, and CH3OH further reveals an arc-like structure connected to the outer end of the linear feature and separated from the protostar, IRAS 15398-3359, by 1200 au. The arc-like structure is blue-shifted with respect to the systemic velocity. A velocity gradient of 1.2 km/s over 1200 au along the linear feature seen in the H2CO emission connects the protostar and the arc-like structure kinematically. SO, SiO, and CH3OH are known to trace shocks, and we interpret the arc-like structure as a relic shock region produced by an outflow previously launched by IRAS 15398-3359. The velocity gradient along the linear structure can be explained as relic outflow motion. The origins of the newly observed arc-like structure and extended linear feature are discussed in relation to turbulent motions within the protostellar core and episodic accretion events during the earliest stage of protostellar evolution.

Among all the interstellar complex organic molecules (iCOMs), acetaldehyde is one of the most widely detected species. The question of its formation route(s) is, therefore, of a major interest regarding astrochemical models. In this paper, we provide an extensive review of the gas-phase formation paths that were, or are, reported in the literature and the major astrochemical databases. Four different gas-phase formation routes stand out : (1) CH$_3$OCH$_3$ + H$^+$ / CH$_3$CHOH$^+$ + e$^-$, (2) C$_2$H$_5$ + O($^3$P), (3) CH$_3$OH + CH and (4) CH$_3$CH$_2$OH + OH / CH$_3$CHOH + O($^3$P). Paths (2) and (3) were not studied neither via laboratory or theoretical works in the low temperature and density regime valid for the ISM. Thus, we carried out new accurate quantum chemistry computations. A theoretical kinetics study at low temperatures (7-300 K), adopting the RRKM scheme, was also performed. We confirm that reaction (2) is efficient in forming acetaldehyde in the 7-300 temperature range (alpha = 1.21 x 10$^{-10}$ cm$^3$ s$^{-1}$ and beta = 0.16). On the contrary, our new computations disprove the formation of acetaldehyde through reaction (3) (alpha = 1.84, 0.67 x 10$^{-13}$ cm$^3$ s$^{-1}$ and beta = -0.07, -0.95). Path (1) was showed to be inefficient too by recent computations, while path (4) was formerly considered for glycolaldehyde formation, having acetaldehyde as a by-product. In conclusions, of the four above paths only two, the (2) and (4), are potentially efficient gas-phase reaction routes for the formation of acetaldehyde and we encourage astrochemical modellers to only consider them. Comparison with astronomical observations suggest that path (4) may actually play the major role.

Molecular clouds are the cold regions of the Milky Way where stars form. They are enriched by rather complex molecules. Many of these molecules are believed to be synthesized on the icy surfaces of the interstellar submicron-sized dust grains that permeate the Galaxy. At 10 K thermal desorption is ineffcient and, therefore, why these molecules are found in the cold gas has tantalized astronomers for years. The assumption of the current models, called chemical desorption, is that the molecule formation energy released by the chemical reaction at the grain surface is partially absorbed by the grain and the remaining one causes the ejection of the newly formed molecule into the gas. Here we report an accurate ab-initio molecular dynamics simulations aimed to study the fate of the energy released by the first reaction of the H addition chain on CO, CO + H $\rightarrow$ HCO, occurring on a crystalline ice surface model. We show that about 90% of the HCO formation energy is injected towards the ice in the first picosecond, leaving HCO with an energy content (10-15 kJ mol$^{-1}$) more than a factor two lower than its adsorption energy (30 kJ mol$^{-1}$). As a result, in agreement with laboratory experiments, we conclude that chemical desorption is ineffcient for this specific system, namely H + CO on crystalline ice. We suspect this behavior to be quite general when dealing with hydrogen bonds, which are responsible of both the cohesive energy of the ice mantle and the interaction with adsorbates, as the HCO radical, even though ad hoc simulations are needed to draw specific conclusions on other systems., Comment: Appendix not included. Submitted to ApJ

The investigation of star forming regions have enormously benefited from the recent advent of the ALMA interferometer. More specifically, the unprecedented combination of high-sensitivity and high-angular resolution provided by ALMA allows one to shed light on the jet/disk systems associated with a Sun-like mass protostar. Also astrochemistry enjoyed the possibility to analyze complex spectra obtained using large bandwidths: several interstellar Complex Organic Molecules (iCOMs; C-bearing species with at least 6 atoms) have been imaged around protostars. This in turn boosted the study of the astrochemistry at work during the earliest phases of star formation paving the way to the chemical complexity in planetary systems where Life could emerge. There is mounting evidence that the observations of iCOMs can be used as unique tool to shed light, on Solar System scales (< 50 au), on the molecular content of protostellar disk. The increase of iCOMs abundances occur only under very selective physical conditions, such as those associated low-velocity shocks found where the infalling envelope is impacting the rotating accretion disk. The imaging of these regions with simpler molecules such as CO or CS is indeed paradoxically hampered by their high abundances and consequently high line opacities which do not allow the observers to disentangle all the emitting components at these small scales. In this respect, we review the state-of-the art of the ALMA analysis about the standard Sun-like star forming region in Orion named HH 212. We show (i) how all the physical components involved in the formation of a Sun-like star can be revealed only by observing different molecular tracers, and (ii) how the observation of iCOMs emission, observed to infer the chemical composition of star forming regions, can be used also as unique tracer to image protostellar disks on Solar System scales., Comment: To be published for ACS Earth and Space Chemistry

Interstellar complex organic molecules (iCOMs) can be loosely defined as chemical compounds with at least six atoms in which at least one is carbon. The observations of iCOMs in star-forming regions have shown that they contain an important fraction of carbon in a molecular form, which can be used to synthesize more complex, even biotic molecules. Hence, iCOMs are major actors in the increasing molecular complexity in space and they might have played a role in the origin of terrestrial life. Understand-ing how iCOMs are formed is relevant for predicting the ultimate organic chemistry reached in the interstellar medium. One possibility is that they are synthesized on the interstellar grain icy surfaces, via recombination of previously formed radicals. The present work focuses on the reactivity of HCO with CH3/NH2 on the grain icy sur-faces, investigated by means of quantum chemical simulations. The goal is to carry outa systematic study using different computational approaches and models for the icy surfaces. Specifically, DFT computations have been bench-marked with CASPT2 and CCSD(T) methods, and the ice mantles have been mimicked with cluster models of 1, 2, 18 and 33 H2O molecules, in which different reaction sites have been considered. Our results indicate that the HCO + CH3/NH2 reactions, if they actually occur, have two major competitive channels: the formation of iCOMs CH3CHO/NH2CHO, or the formation of CO + CH4/NH3. These two channels are either barrierless or presentrelatively low ($\leq$ 10 kJ/mol equal to about 1200 K) energy barriers. Finally, we briefly discuss the astrophysical implications of these findings., Comment: 38 pages, 6 figures

Formamide (NH$_2$CHO) has been identified as a potential precursor of a wide variety of organic compounds essential to life, and many biochemical studies propose it likely played a crucial role in the context of the origin of life on our planet. The detection of formamide in comets, which are believed to have --at least partially-- inherited their current chemical composition during the birth of the Solar System, raises the question whether a non-negligible amount of formamide may have been exogenously delivered onto a very young Earth about four billion years ago. A crucial part of the effort to answer this question involves searching for formamide in regions where stars and planets are forming today in our Galaxy, as this can shed light on its formation, survival, and chemical re-processing along the different evolutionary phases leading to a star and planetary system like our own. The present review primarily addresses the chemistry of formamide in the interstellar medium, from the point of view of (i) astronomical observations, (ii) experiments, and (iii) theoretical calculations. While focusing on just one molecule, this review also more generally reflects the importance of joining efforts across multiple scientific disciplines in order to make progress in the highly interdisciplinary science of astrochemistry., Comment: Review, 5 figures

Many species of complex organic molecules (COMs) have been observed in several astrophysical environments but it is not clear how they are produced, particularly in cold, quiescent regions. One process that has been proposed as a means to enhance the chemical complexity of the gas phase in such regions is the explosion of the ice mantles of dust grains. In this process, a build up of chemical energy in the ice is released, sublimating the ices and producing a short lived phase of high density, high temperature gas. The gas-grain chemical code UCLCHEM has been modified to treat these explosions in order to model the observed abundances of COMs towards the TMC-1 region. It is found that, based on our current understanding of the explosion mechanism and chemical pathways, the inclusion of explosions in chemical models is not warranted at this time. Explosions are not shown to improve the model's match to the observed abundances of simple species in TMC-1. Further, neither the inclusion of surface diffusion chemistry, nor explosions, results in the production of COMs with observationally inferred abundances., Comment: Accepted for publication in ApJ

To correctly model the abundances of interstellar complex organic molecules (iCOMS) in different environments, both formation and destruction routes should be appropriately accounted for. While several scenarios have been explored for the formation of iCOMs via grain and gas-phase processes, much less work has been devoted to understanding the relevant destruction pathways, with special reference to (dissociative) charge exchange or proton transfer reactions with abundant atomic and molecular ions such as He$^+$, H$_3^+$ and HCO$^+$. By using a combined experimental and theoretical methodology we provide new values for the rate coefficients and branching ratios (BRs) of the reactions of He$^+$ ions with two important iCOMs, namely dimethyl ether (DME) and methyl formate (MF). We also review the destruction routes of DME and MF by other two abundant ions, namely H$_3^+$ and HCO$^+$. Based on our recent laboratory measurements of cross sections and BRs for the DME/MF + He$^+$ reactions over a wide collision energy range, we extend our theoretical insights on the selectivity of the microscopic dynamics to calculate the rate coefficients $k(T)$ in the temperature range from 10 to 298 K. We implement these new and revised kinetic data in a general model of cold and warm gas, simulating environments where DME and MF have been detected. Due to stereodynamical effects present at low collision energies, the rate coefficients, BRs and temperature dependences here proposed differ substantially from those reported in KIDA and UDfA, two of the most widely used astrochemical databases. These revised rates impact the predicted abundances of DME and MF, with variations up to 40% in cold gases and physical conditions similar to those present in prestellar cores, Comment: accepted for publication in Astronomy and Astrophysics (manuscript no. AA/2018/34585), 10 pages, 3 figures

The reaction between the amidogen, NH, radical and the ethyl, C2H5, radical has been investigated by performing electronic structure calculations of the underlying doublet potential energy surface. Rate coefficients and product branching ratios have also been estimated by combining capture and RRKM calculations. According to our results, the reaction is very fast, close to the gas-kinetics limit. However, the main product channel, with a yield of ca. 86-88% in the range of temperatures investigated, is the one leading to methanimine and the methyl radical. The channels leading to the two E-, Z- stereoisomers of ethanimine account only for ca. 5-7% each. The resulting ratio [E-CH3CHNH]/[Z-CH3CHNH] is ca. 1.2, that is a value rather lower than that determined in the Green Bank Telescope PRIMOS radio astronomy survey spectra of Sagittarius B2 North (ca. 3). Considering that ice chemistry would produce essentially only the most stable isomer, a possible conclusion is that the observed [E-CH3CHNH]/[Z-CH3CHNH] ratio is compatible with a combination of gas-phase and grain chemistry. More observational and laboratory data are needed to definitely address this issue.

Dimethyl ether is one of the most abundant interstellar complex organic molecules. Yet its formation route remains elusive. In this work, we have performed electronic structure and kinetics calculations to derive the rate coefficients for two ion-molecule reactions recently proposed as a gas-phase formation route of dimethyl ether in interstellar objects, namely CH$_3$OH + CH$_3$OH$_2^+$ $\rightarrow$ (CH$_3$)$_2$OH$^+$ + H$_2$O followed by (CH$_3$)$_2$OH$^+$ + NH$_3$ $\rightarrow$ CH$_3$OCH$_3$ + NH$_4^+$. A comparison with previous experimental rate coefficients for the reaction CH$_3$OH + CH$_3$OH$_2^+$ sustains the accuracy of the present calculations and allow a more reliable extrapolation at the low temperatures of interest in interstellar objects (10-100 K). The rate coefficient for the reaction (CH$_3$)$_2$OH$^+$ + NH$_3$ is, instead, provided for the first time ever. The rate coefficients derived in this work essentially confirm the prediction by Taquet et al. (2016) concerning dimethyl ether formation in hot cores/corinos. Nevertheless, this formation route cannot be efficient in cold objects (like prestellar cores) where dimethyl ether is also detected, because ammonia has a very low abundance in those environments.

Interstellar formamide (NH2CHO) has recently attracted significant attention due to its potential role as a molecular building block in the formation of precursor biomolecules relevant for the origin of life. Its formation, whether on the surfaces of the interstellar grains or in the gas phase, is currently debated. The present article presents new theoretical quantum chemical computations on possible NH2CHO formation routes in water-rich amorphous ices, simulated by a 33-H2O-molecule cluster. We have considered three possible routes. The first one refers to a scenario used in several current astrochemical models, that is, the radical-radical association reaction between NH2 and HCO. Our calculations show that formamide can indeed be formed, but in competition with formation of NH3 and CO through a direct H transfer process. The final outcome of the NH2 + HCO reactivity depends on the relative orientation of the two radicals on the ice surface. We then analyzed two other possibilities, suggested here for the first time: reaction of either HCN or CN with water molecules of the ice mantle. The reaction with HCN has been found to be characterized by large energy barriers and, therefore, cannot occur under the interstellar ice conditions. On the contrary, the reaction with the CN radical can occur, possibly leading through multiple steps to the formation of NH2CHO. For this reaction, water molecules of the ice act as catalytic active sites since they help the H transfers involved in the process, thus reducing the energy barriers (compared to the gas-phase analogous reaction). Additionally, we apply a statistical model to estimate the reaction rate coefficient when considering the cluster of 33-H2O-molecules as an isolated moiety with respect to the surrounding environment, i.e., the rest of the ice.

In dense molecular clouds, interstellar grains are covered by mantles of iced molecules. The formation of the grain mantles has two important consequences: it removes species from the gas phase and promotes the synthesis of new molecules on the grain surfaces. The composition of the mantle is a strong function of the environment which the cloud belongs to. Therefore, clouds in high-zeta galaxies, where conditions -like temperature, metallicity and cosmic rays flux- are different from those in the Milky Way, will have different grain mantles. In the last years, several authors have suggested that silicate grains might grow by accretion of silicon bearing species on smaller seeds. This would occur simultaneously to the formation of the iced mantles and be greatly affected by its composition as a function of time. In this work, we present a numerical study of the grain mantle formation in high-zeta galaxies and we quantitatively address the possibility of silicate growth. We find that the mantle thickness decreases with increasing redshift, from about 120 to 20 layers for z varying from 0 to 8. Furthermore, the mantle composition is also a strong function of the cloud redshift, with the relative importance of CO, CO2, ammonia, methane and methanol highly varying with z. Finally, being Si-bearing species always a very minor component of the mantle, the formation of silicates in molecular clouds is practically impossible., Comment: MNRAS in press

Towards the pre-stellar core L1544, the methanol (CH$_3$OH) emission forms an asymmetric ring around the core centre, where CH$_3$OH is mostly in solid form, with a clear peak 4000~au to the north-east of the dust continuum peak. As part of the NOEMA Large Project SOLIS (Seeds of Life in Space), the CH$_3$OH peak has been spatially resolved to study its kinematics and physical structure and to investigate the cause behind the local enhancement. We find that methanol emission is distributed in a ridge parallel to the main axis of the dense core. The centroid velocity increases by about 0.2~km~s$^{-1}$ and the velocity dispersion increases from subsonic to transonic towards the central zone of the core, where the velocity field also shows complex structure. This could be indication of gentle accretion of material onto the core or interaction of two filaments, producing a slow shock. We measure the rotational temperature and show that methanol is in local thermodynamic equilibrium (LTE) only close to the dust peak, where it is significantly depleted. The CH$_3$OH column density, $N_{tot}({\rm CH_3OH})$, profile has been derived with non-LTE radiative transfer modelling and compared with chemical models of a static core. The measured $N_{tot}({\rm CH_3OH})$ profile is consistent with model predictions, but the total column densities are one order of magnitude lower than those predicted by models, suggesting that the efficiency of reactive desorption or atomic hydrogen tunnelling adopted in the model may be overestimated; or that an evolutionary model is needed to better reproduce methanol abundance., Comment: 19 pages, 18 figures. Accepted by ApJ

Despite the harsh conditions of the interstellar medium, chemistry thrives in it, especially in star forming regions where several interstellar complex organic molecules (iCOMs) have been detected. Yet, how these species are synthesised is a mystery. The majority of current models claim that this happens on interstellar grain surfaces. Nevertheless, evidence is mounting that neutral gas-phase chemistry plays an important role. In this article, we propose a new scheme for the gas-phase synthesis of glycolaldehyde, a species with a prebiotic potential and for which no gas-phase formation route was previously known. In the proposed scheme, the ancestor is ethanol and the glycolaldehyde sister species are acetic acid (another iCOM with unknown gas-phase formation routes) and formic acid. For the reactions of the new scheme with no available data, we have performed electronic structure and kinetics calculations deriving rate coefficients and branching ratios. Furthermore, after a careful review of the chemistry literature, we revised the available chemical networks, adding and correcting several reactions related to glycolaldehyde, acetic acid and formic acid. The new chemical network has been used in an astrochemical model to predict the abundance of glycolaldehyde, acetic acid and formic acid. The predicted abundance of glycolaldehyde depends on the ethanol abundance in the gas phase and is in excellent agreement with the measured one in hot corinos and shock sites. Our new model overpredicts the abundance of acetic acid and formic acid by about a factor of ten, which might imply a yet incomplete reaction network.