Interstellar Complex Organic Molecules Research Articles

Unveiling the gas phase H2NCO radical: Laboratory rotational spectroscopy and interstellar searches in the direction of IRAS16293-2422

The carbamoyl radical ( H2NCO ) is believed to play a central role in the ice-grain chemistry of crucial interstellar complex organic molecules such as formamide and acetamide. Yet, little is known about this radical, which remains elusive in laboratory gas-phase experiments. In order to enable interstellar searches of H2NCO we have undertaken a mandatory laboratory characterisation of its pure rotational spectrum. We report the gas-phase laboratory detection of H2NCO produced by H-atom abstraction from formamide, using pure rotational spectroscopy at millimetre and submillimetre wavelengths. Millimetre-wave data were acquired using chirped-pulse Fourier-transform spectroscopy, while submillimetre-wave ones were obtained using Zeeman-modulated spectroscopy. Experimental measurements were guided by quantum-chemical calculations at the omega B97X-D/cc-pVQZ level of theory. Interstellar searches for the radical have been undertaken in the Protostellar Interferometric Line Survey (PILS) towards the solar-type protostar IRAS 16293–2422. From the assignment and fit of experimental transitions up to 660\,GHz, reliable spectroscopic parameters for H2NCO in its ground vibrational state have been derived, enabling accurate spectral predictions. No transitions of the radical were detected in the PILS survey. The inferred upper limit shows that the H2NCO abundance is at least times below that of formamide and 160 times below that of HNCO in this source; a value that is in agreement with predictions from a physico-chemical model of this young protostar.

Comprehensive laboratory constraints on thermal desorption of interstellar ice analogues

Gas accretion and sublimation in various astrophysical conditions are crucial aspects of our understanding of the chemical evolution of the interstellar medium. To explain grain growth and destruction in warm media, ice mantle formation and sublimation in cold media, and gas line emission spectroscopy, astrochemical models must mimic the gas--solid abundance ratio. Ice-sublimation mechanisms determine the position of snow lines and the nature of gas emitted by and locked inside planetary bodies in star-forming regions. To interpret observations from the interplanetary and extragalactic interstellar mediums, gas phase abundances must be modelled correctly. We provide a collection of thermal desorption data for interstellar ice analogues, aiming to put constraints on the trapping efficiency of water ice, as well as data that can be used to evaluate astrochemical models. We conduct experiments on compact, amorphous H$_2$O films, involving pure ices as well as binary and ternary mixtures. By manipulating parameters in a controlled way, we generate a set of benchmarks to evaluate both the kinetics and thermodynamics in astrochemical models. We conducted temperature-programmed desorption experiments with increasing order of complexity of ice analogues of various chemical compositions and surface coverages using molecular beams in ultrahigh vacuum conditions (1 times 10$^ $ hPa) and low temperatures (10 K). We provide TPD curves of pure ices made of Ar, CO, CO$_2$, NH$_3$, CH$_3$OH, H$_2$O, and NH$_4^+$HCOO$^-$, their binary ice mixtures with compact amorphous H$_2$O, ternary mixtures of H$_2$O : CH$_3$OH : CO, and a water ice made in situ to investigate its trapping mechanisms. Each experiment includes the experimental parameters, ice desorption kinetics for pure species, and the desorption yield (gas--solid ratio) for ice mixtures. From the desorption yields, we find common trends in the trapping of molecules when their abundance is compared to water: compact amorphous water ices are capable of trapping up to 20<!PCT!> of volatiles (Ar, CO, and CO$_2$), sim 3<!PCT!> of CH$_3$OH, and sim 5<!PCT!> NH$_3$ in relation to the water content within the ice matrix; ammonium formate is not trapped in the water ice films, and compact amorphous water ice formed in situ has similar trapping capabilities to a compact amorphous water ice deposited using molecular beams. Deposited or formed in a very compact structure, amorphous water ice of less than 100 layers cannot trap a large fraction of other gases, including CO and CO$_2$. These desorption yields offer insights into the availability of species that can react and form interstellar complex organic molecules during the warm-up phase of ice mantles. Furthermore, in order to be reliable, gas-grain astrochemical models should be able to reproduce the desorption kinetics and desorption yield presented in our benchmark laboratory experiments.

FAUST

Aims. Interstellar complex organic molecules (iCOMs) are species commonly found in the interstellar medium. They are believed to be crucial seed species for the build-up of chemical complexity in star forming regions as well as our own Solar System. Thus, understanding how their abundances evolve during the star formation process and whether it enriches the emerging planetary system is of paramount importance. Methods. We use data from the ALMA Large Program FAUST (Fifty AU STudy of the chemistry in the disk and envelope system of solar protostars) to study the compact line emission towards the [BHB2007] 11 proto-binary system (sources A and B), where a complex structure of filaments connecting the two sources with a larger circumbinary disk has previously been detected. More than 45 methyl formate (CH3OCHO) lines are clearly detected with upper energies in the [123, 366] K range, as well as 8 dimethyl ether transitions (CH3OCH3) in the [93, 191] K range, 1 ketene transition (H2CCO) and 4 formic acid transitions (t-HCOOH). We compute the abundance ratios with respect to CH3OH for CH3OCHO, CH3OCH3, H2CCO, t-HCOOH (as well as an upper limit for CH3CHO) through a radiative transfer analysis. We also report the upper limits on the column densities of nitrogen bearing iCOMs, N(C2H5CN) and N(C2H3CN). Results. The emission from the detected iCOMs and their precursors is compact and encompasses both protostars, which are separated by only 0.2″ (~28 au). The integrated intensities tend to align with the Southern filament, revealed by the high spatial resolution observations of the dust emission at 1.3 mm. A Position-Velocity and 2D analysis are performed on the strongest and uncontaminated CH3OCH3 transition and show three different spatial and velocity regions, two of them being close to 11B (Southern filament) and the third one near 11A. Conclusions. All our observations suggest that the detected methanol, as well as the other iCOMs, are generated by the shocked gas from the incoming filaments streaming towards [BHB2007] 11A and 11B, respectively, making this source one of the few where chemical enrichment of the gas caused by the streaming material is observed.

Grain growth and its chemical impact in the first hydrostatic core phase

The first hydrostatic core (FHSC) phase is a brief stage in the protostellar evolution that is difficult to detect. Its chemical composition determine that of later evolutionary stages. Numerical simulations are the tool of choice to study these objects. Our goal is to characterize the chemical evolution of gas and dust during the formation of the FHSC. Moreover, we are interested in analyzing, for the first time with 3D magnetohydrodynamic (MHD) simulations, the role of grain growth in its chemistry. We postprocessed $2 $ tracer particles from a RAMSES non-ideal MHD simulation using the codes NAUTILUS and SHARK to follow the chemistry and grain growth throughout the simulation. Gas-phase abundances of most of the C, O, N, and S reservoirs in the hot corino at the end of the simulation match the ice-phase abundances from the prestellar phase. Interstellar complex organic molecules (iCOMs) such as methyl formate, acetaldehyde, and formamide are formed during the warm-up process. Grain size in the hot corino $(n_ H )$ increases forty-fold during the last 30 kyr, with negligible effects on its chemical composition. At moderate densities $(10^ <n_ H )$ and cool temperatures $15<T<50$ K, increasing grain sizes delay molecular depletion. At low densities $(n_ H )$, grains do not grow significantly. To assess the need to perform chemo-MHD calculations, we compared our results with a two-step model that reproduces well the abundances of C and O reservoirs, but not the N and S reservoirs. The chemical composition of the FHSC is heavily determined by that of the parent prestellar core. Chemo-MHD computations are needed for an accurate prediction of the abundances of the main N and S elemental reservoirs. The impact of grain growth in moderately dense areas delaying depletion permits the use of abundance ratios as grain growth proxies.

Organic Chemistry in the H2-bearing, CO-rich Interstellar Ice Layer at Temperatures Relevant to Dense Cloud Interiors

Ice chemistry in the dense, cold interstellar medium (ISM) is probably responsible for the formation of interstellar complex organic molecules (COMs). Recent laboratory experiments performed at T ∼ 4 K have shown that irradiation of CO:N2 ice samples analog to the CO-rich interstellar ice layer can contribute to the formation of COMs when H2 molecules are present. We have tested this organic chemistry under a broader range of conditions relevant to the interior of dense clouds by irradiating CO:15N2:H2 ice samples with 2 keV electrons in the 4–15 K temperature range. The H2 ice abundance depended on both, the ice formation temperature and the thermal evolution of the samples. Formation of H-bearing organics such as formaldehyde (H2CO), ketene (C2H2O), and isocyanic acid (H15NCO) was observed upon irradiation of ice samples formed at temperatures up to 10 K, and also in ices formed at 6 K and subsequently warmed up and irradiated at temperatures up to 15 K. These results suggest that a fraction of the H2 molecules in dense cloud interiors might be entrapped in the CO-rich layer of interstellar ice mantles, and that energetic processing of this layer could entail an additional contribution to the formation of COMs in the coldest regions of the ISM.

Formation of Interstellar Complex Organic Molecules on Water-rich Ices Triggered by Atomic Carbon Freezing

The reactivity of interstellar carbon atoms (C) on water-dominated ices is one of the possible ways to form interstellar complex organic molecules (iCOMs). In this work, we report a quantum chemical study of the coupling reaction of C (3P) with an icy water molecule, alongside possible subsequent reactions with the most abundant closed-shell frozen species (NH3, CO, CO2, and H2), atoms (H, N, and O), and molecular radicals (OH, NH2, and CH3). We found that C reacts spontaneously with the water molecule, resulting in the formation of 3C–OH2, a highly reactive species due to its triplet electronic state. While reactions with the closed-shell species do not show any reactivity, reactions with N and O form CN and CO, respectively, the latter ending up in methanol upon subsequent hydrogenation. The reactions with OH, CH3, and NH2 form methanediol, ethanol, and methanimine, respectively, upon subsequent hydrogenation. We also propose an explanation for methane formation observed in experiments through additions of H to C in the presence of ices. The astrochemical implications of this work are: (i) atomic C on water ice is locked into 3C–OH2, making difficult the reactivity of bare C atoms on icy surfaces, contrary to what is assumed in current astrochemical models; and (ii) the extraordinary reactivity of 3C–OH2 provides new routes toward the formation of iCOMs in a nonenergetic way, in particular ethanol, the mother of other iCOMs once it is in the gas phase.

Hunting for Complex Organic Molecules in the Interstellar Medium: The Role of Accurate Low-Cost Theoretical Geometries and Rotational Constants.

A new approach to computation at affordable cost of accurate geometrical structures and rotational constants for medium-sized molecules in the gas phase is further improved and applied to a large panel of interstellar complex organic molecules. The most distinctive feature of the new model is the effective inclusion of core-valence correlation and vibrational averaging effects in the framework of density functional theory (DFT). In particular, a double-hybrid functional in conjunction with a quadruple-ζ valence/triple-ζ polarization basis set is employed for geometry optimizations, whereas a cheaper hybrid functional in conjunction with a split-valence basis set is used for the evaluation of vibrational corrections. A thorough benchmark based on a wide range of prototypical systems shows that the new scheme approaches the accuracy of state-of-the-art wave function methods with the computational cost of the standard methods (DFT or MP2) routinely employed in the interpretation of microwave spectra. Since the whole computational workflow involves the postprocessing of the output of standard electronic structure codes by a new freely available web utility, the way is paved for the accurate yet not prohibitively expensive study of medium- to large-sized molecules also by nonspecialists.

Gas-Phase vs. Grain-Surface Formation of Interstellar Complex Organic Molecules: A Comprehensive Quantum-Chemical Study

Several organic chemical compounds (the so-called interstellar complex organic molecules, iCOMs) have been identified in the interstellar medium (ISM). Examples of iCOMs are formamide (HCONH2), acetaldehyde (CH3CHO), methyl formate (CH3OCHO), or formic acid (HCOOH). iCOMs can serve as precursors of other organic molecules of enhanced complexity, and hence they are key species in chemical evolution in the ISM. The formation of iCOMs is still a subject of a vivid debate, in which gas-phase or grain-surface syntheses have been postulated. In this study, we investigate the grain-surface-formation pathways for the four above-mentioned iCOMs by transferring their primary gas-phase synthetic routes onto water ice surfaces. Our objective is twofold: (i) to identify potential grain-surface-reaction mechanisms leading to the formation of these iCOMs, and (ii) to decipher either parallelisms or disparities between the gas-phase and the grain-surface reactions. Results obtained indicate that the presence of the icy surface modifies the energetic features of the reactions compared to the gas-phase scenario, by increasing some of the energy barriers. Therefore, the investigated gas-phase mechanisms seem unlikely to occur on the icy grains, highlighting the distinctiveness between the gas-phase and the grain-surface chemistry.

The Principal Component Analysis Filtering Method for an Unbiased Spectral Survey of Complex Organic Molecules

A variety of interstellar complex organic molecules (COMs) have been detected in various physical conditions. However, in the protostellar and protoplanetary environments, their complex kinematics make line profiles blend together and the line strength of weak lines weaker. In this paper, we utilize the principal component analysis technique to develop a filtering method that can extract COM spectra from the main kinematic component associated with COM emission and increase the signal-to-noise ratio (S/N) of spectra. This filtering method corrects non-Gaussian line profiles caused by the kinematics. For this development, we adopt the ALMA BAND 6 spectral survey data of V883 Ori, an eruptive young star with a Keplerian disk. A filter was, first, created using 34 strong and well-isolated COM lines and then applied to the entire spectral range of the data set. The first principal component (PC1) describes the most common emission structure of the selected lines, which is confined within the water sublimation radius (∼0.″3) in the Keplerian disk of V883 Ori. Using this PC1 filter, we extracted high-S/N kinematics-corrected spectra of V883 Ori over the entire spectral coverage of ∼50 GHz. The PC1-filtering method reduces the noise by a factor of ∼2 compared to the average spectra over the COM emission region. One important advantage of this PC1-filtering method over the previously developed matched-filtering method is the ability to preserve the original integrated intensities of COM lines.

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): The Warm-envelope Origin of Hot Corinos

Hot corinos are of great interest due to their richness in interstellar complex organic molecules (COMs) and the consequent potential prebiotic connection to solar-like planetary systems. Recent surveys have reported an increasing number of detected hot corinos in Class 0/I protostars; however, the relationships between their physical properties and the hot-corino signatures remain elusive. In this study, our objective is to establish a general picture of the detectability of hot corinos by identifying the origins of the hot-corino signatures in the sample of young stellar objects (YSOs) obtained from the Atacama Large Millimeter/submillimeter Array Survey of Orion Planck Galactic Cold Clumps project. We apply spectral energy distribution modeling to our sample and identify the physical parameters of the modeled YSOs directly, linking the detection of hot-corino signatures to the envelope properties of the YSOs. Imaging simulations of the methanol emission further support this scenario. We therefore posit that the observed COM emission originates from the warm inner envelopes of the sample YSOs, based on both the warm region size and the envelope density profile. The former is governed by the source luminosity and is additionally affected by the disk and cavity properties, while the latter is related to the evolutionary stages. This scenario provides a framework for detecting hot-corino signatures toward luminous Class 0 YSOs, with fewer detections being observed toward similarly luminous Class I sources.

Revised gas-phase formation network of methyl cyanide: the origin of methyl cyanide and methanol abundance correlation in hot corinos

ABSTRACT Methyl cyanide (CH3CN) 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 CH3CN formation routes, propose two new reactions, and performed new quantum mechanics calculations 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 CH3CN using our astrochemical model. We confirm that the radiative association of CH3+ and HCN, forming CH3CNH+, followed by the electron recombination of CH3CNH+, is the most important CH3CN 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 CH3CN predicted abundances with those measured in cold (∼10 K) and warm (∼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 CH3OH and CH3CN abundances measured in hot corinos.

Quantum mechanical modelling of the grain-surface formation of acetaldehyde on H2O:CO dirty ice surfaces

ABSTRACTAcetaldehyde (CH3CHO) is one of the most detected interstellar complex organic molecules (iCOMs) in the interstellar medium (ISM). These species have a potential biological relevance, as they can be precursors of more complex species from which life could have emerged. The formation of iCOMs in the ISM is a challenge and a matter of debate, whether gas-phase, grain-surface chemistry, or both are needed for their synthesis. In the gas-phase, CH3CHO can be efficiently synthesized from ethanol and/or ethyl radical. On the grain-surfaces, radical–radical recombinations were traditionally invoked. However, several pitfalls have been recently identified, such as the presence of energy barriers and competitive side reactions (i.e. H abstractions). Here, we investigate a new grain-surface reaction pathway for the formation of acetaldehyde, namely the reaction between CH3 and a CO molecule of a dirty water/CO ice followed by hydrogenation of its product, CH3CO. To this end, we carried out ab initio computations of the reaction occurring on an ice composed of 75 per cent water and 25 per cent CO molecules. We found that the CH3 + CO(ice) reaction exhibits barriers difficult to overcome in the ISM, either adopting a Langmuir–Hinshelwood or an Eley–Rideal mechanism. The subsequent hydrogenation step is found to be barrierless, provided that the two reacting species have the correct orientation. Therefore, this pathway seems unlikely to occur in the ISM.

Formation of Complex Organic Molecules on Interstellar CO Ices? Insights from Computational Chemistry Simulations

The carbon (3P) atom is a reactive species that, according to laboratory experiments and theoretical calculations, condensates with interstellar ice components. This fact is of uttermost importance for the chemistry in the interstellar medium (ISM) because the condensation reaction is barrierless, and the subsequent species formed are still reactive given their open-shell character. Carbon condensation on CO-rich ices forms the C=C=O (3Σ−) species, which can be easily hydrogenated twice to form ketene (H2CCO). Ketene is very reactive in terrestrial conditions, usually found as an intermediate that is difficult to isolate in chemical synthesis laboratories. These characteristics suggest that ketene can be a good candidate to form interstellar complex organic molecules via a two-step process, i.e., its activation followed by a radical–radical coupling. In this work, reactions between ketene and atomic H and the OH and NH2 radicals on a CO-rich ice model have been explored by means of quantum chemical calculations complemented by kinetic calculations to evaluate if they are favorable in the ISM. Results indicate that the addition of H to ketene (helped by tunneling) to form the acetyl radical (CH3CO) is the most preferred path as the reactions with OH and NH2 possess activation energies (≥9 kJ mol−1) hard to surmount in the ISM conditions unless external processes provide energy to the system. Thus, acetaldehyde (CH3CHO) and, probably, ethanol (CH3CH2OH) formation via further hydrogenations, are the possible unique operating synthetic routes. Moreover, from the computed, relatively large binding energies of OH and NH2 on CO ice, slow diffusion is expected, hampering possible radical–radical couplings with CH3CO. The astrophysical implications of these findings are discussed considering the incoming James Webb Space Telescope observations.

Infrared action spectroscopy as tool for probing gas-phase dynamics: protonated dimethyl ether, (CH3)2OH+, formed by the reaction of CH3OH2 + with CH3OH

Methanol is one of the most abundant interstellar Complex Organic Molecules (iCOMs) and represents a major building block for the synthesis of increasingly complex oxygen-containing molecules. The reaction between protonated methanol and its neutral counterpart, giving protonated dimethyl ether, ( CH 3 ) 2 OH + , along with the ejection of a water molecule, has been proposed as a key reaction in the synthesis of dimethyl ether in space. Here, gas phase vibrational spectra of the ( CH 3 ) 2 OH + reaction product and the [ C 2 H 9 O 2 ] + intermediate complex(es), formed under different pressure and temperature conditions, are presented. The widely tunable free electron laser for infrared experiments, FELIX, was employed to record these vibrational fingerprint spectra using different types of infrared action spectroscopy in the 600-1700 cm − 1 frequency range, complemented with measurements using an OPO/OPA system to cover the O − H stretching region 3400 − 3700 c m − 1 . The formation of protonated dimethyl ether as a product of the reaction is spectroscopically confirmed, providing the first gas-phase vibrational spectrum of this potentially relevant astrochemical ion.

Spiers Memorial Lecture: Astrochemistry at high resolution.

Astrochemistry is the science that studies the chemistry in the Universe, namely the combination of two fields: astronomy and chemistry. It started about fifty years ago and it has progressed in leaps and bounds, often triggered by the advent of new telescopes. From the collection of new interstellar molecule detections, astrochemistry has evolved more and more in the quest to understand how they are formed and thrive in the harsh conditions of the interstellar medium. Collaboration between astronomers and chemists has never been more necessary than today, when new powerful astronomical facilities provide us with ever sharper images of the regions where interstellar molecules are present. This review focuses on the special case of interstellar complex organic molecules (iCOMs), one the most debated astrochemical fields and where the astronomers-chemists collaboration and synergy is indispensable. The review will go through the various phases of the formation of planetary system similar to the solar system, providing the most recent observational picture at each step. The current scenarios of the iCOMs formation will be laid down and the critical chemical processes and quantities involved in each of them will be discussed. The major goal of this review is not only to present the progress but, more importantly, to highlight the many areas of uncertainty. A few specific cases will be discussed to give practical examples of why the huge challenge that represents the formation of iCOMs can only be won if chemists and astronomers work together.

Quantum Chemical Computations of Gas-phase Glycolaldehyde Deuteration and Constraints on Its Formation Route

Despite the detection of numerous interstellar complex organic molecules (iCOMs) for decades, it is still a matter of debate whether they are synthesized in the gas phase or on the icy surface of interstellar grains. In the past, molecular deuteration has been used to constrain the formation paths of small and abundant hydrogenated interstellar species. More recently, the deuteration degree of formamide, one of the most interesting iCOMs, has also been explained with the hypothesis that it is formed by the gas-phase reaction NH2 + H2CO. In this paper, we aim at using molecular deuteration to constrain the formation of another iCOM, glycolaldehyde, which is an important prebiotic species. More specifically, we have performed dedicated electronic structure and kinetic calculations to establish the glycolaldehyde deuteration degree in relation to that of ethanol, which is its possible parent species according to the suggestion of Skouteris et al. We found that the abundance ratio of the species containing one D atom over the all-protium counterpart depends on the produced D isotopomer and varies from 0.9 to 0.5. These theoretical predictions compare extremely well with the monodeuterated isotopomers of glycolaldehyde and that of ethanol measured toward the solar-like protostar IRAS 16293–2422, supporting the hypothesis that glycolaldehyde could be produced in the gas phase for this source. In addition, the present work confirms that the deuterium fractionation of iCOMs cannot be simply anticipated based on the deuterium fractionation of the parent species but necessitates a specific study, as already shown for the case of formamide.

Resolving desorption of complex organic molecules in a hot core

Context. The presence of many interstellar complex organic molecules (COMs) in the gas phase in the vicinity of protostars has long been associated with their formation on icy dust grain surfaces before the onset of protostellar activity, and their subsequent thermal co-desorption with water, the main constituent of the grains’ ice mantles, as the protostar heats its environment to ~100 K. Aims. Using the high angular resolution provided by the Atacama Large Millimetre/submillimetre Array (ALMA), we want to resolve the COM emission in the hot molecular core Sagittarius B2 (N1) and thereby shed light on the desorption process of COMs in hot cores. Methods. We used data taken as part of the 3 mm spectral line survey Re-exploring Molecular Complexity with ALMA (ReMoCA) to investigate the morphology of COM emission in Sagittarius B2 (N1). We also used ALMA continuum data at 1 mm taken from the literature. Spectra of ten COMs (including one isotopologue) were modelled under the assumption of local thermodynamic equilibrium (LTE) and population diagrams were derived for these COMs for positions at various distances to the south and west from the continuum peak. Based on this analysis, we produced resolved COM rotation temperature and column density profiles. H2 column density profiles were derived from dust continuum emission and C18O 1–0 emission and used to derive COM abundance profiles as a function of distance and temperature. These profiles are compared to astrochemical models. Results. Based on the morphology, a rough separation into O- and N-bearing COMs can be done. The temperature profiles span a range of 80–300 K with power-law indices from −0.4 to −0.8, which is in agreement with expectations of protostellar heating of an envelope with optically thick dust. Column density and abundance profiles reflect a similar trend as seen in the morphology. While abundances of N-bearing COMs peak only at the highest temperatures, those of most O-bearing COMs peak at lower temperatures and remain constant or decrease towards higher temperatures. Many abundance profiles show a steep increase at ~100 K. To a great extent, the observed results agree with results of astrochemical models that, besides the co-desorption with water, predict that O-bearing COMs are mainly formed on dust-grain surfaces at low temperatures, while at least some N-bearing COMs and CH3CHO are substantially formed in the gas phase at higher temperatures. Conclusions. Our observational results, in comparison with model predictions, suggest that COMs that are exclusively or, to a great extent, formed on dust grains desorb thermally at ~100 K from the grain surface, likely alongside water. A dependence on the COM binding energy is not evident from our observations. Non-zero abundance values below ~100 K suggest that another desorption process of COMs is at work at these low temperatures: either non-thermal desorption or partial thermal desorption related to the lower binding energies experienced by COMs in the outer, water-poor ice layers. In either case, this is the first time that the transition between two regimes of COM desorption has been resolved in a hot core.

Radical addition and H abstraction reactions in C2H2, C2H4, and C2H6: A gateway for ethyl- and vinyl-bearing molecules in the interstellar medium

Context. Recent interstellar detections include a significant number of molecules containing vinyl (C2H3) and ethyl (C2H5) groups in their structure. For several of these molecules, there is no clear experimental or theoretical evidence that supports their formation from simpler precursors. Aims. We carried out a systematic search of viable reactions starting from closed-shell hydrocarbons containing two carbon atoms (ethane, C2H6; ethylene, C2H4; and acetylene, C2H2), with the goal of determining viable chemical routes for the formation of vinyl and ethyl molecules on top of interstellar dust grains. Methods. We used density functional theory calculations in combination with semiclassical instantem theory to derive the rate coefficients for the radical-neutral surface reactions. The effect of a surface was modeled through an implicit surface approach, profiting from the weak interaction between the considered hydrocarbons and the dust surfaces. Results. Our results show that both H and OH radicals are key in converting acetylene and ethylene into more complex radicals that are liable to continue reacting and to form interstellar complex organic molecules. The relevant reactions, for example OH additions, present rate constants above 101 s−1 that are likely competitive with OH diffusion on grains. Similarly, H atom addition to acetylene and ethylene is a very fast process, with rate constants above 104 s−1 in all cases, and is greatly enhanced by quantum tunneling. Hydrogen abstraction reactions are less relevant, but may play a role in specific cases involving the OH radical. Reactions with other radicals NH2 and CH3 are likely to have much less impact on the chemistry of ethyl- and vinyl-bearing molecules. Conclusions. The effective formation at low temperatures of four radicals (C2H3, C2H5, C2H2OH, and C2H4OH) through our proposed mechanism opens the gate for the formation of complex organic molecules, and indicates a potential prevalence of OH-bearing molecules on the grain. Following our suggested reaction pathway, we explain the formation of many of the newly detected molecules, and propose new molecules for detection. Our results reinforce the recent view on the importance of the OH radical in interstellar surface chemistry.

Acetaldehyde binding energies: a coupled experimental and theoretical study

ABSTRACT Acetaldehyde is one of the most common and abundant gaseous interstellar complex organic molecules found in cold and hot regions of the molecular interstellar medium. Its presence in the gas-phase depends on the chemical formation and destruction routes, and its binding energy (BE) governs whether acetaldehyde remains frozen on to the interstellar dust grains or not. In this work, we report a combined study of the acetaldehyde BE obtained via laboratory temperature programmed desorption (TPD) experiments and theoretical quantum chemical computations. BEs have been measured and computed as a pure acetaldehyde ice and mixed with both polycrystalline and amorphous water ice. Both calculations and experiments found a BE distribution on amorphous solid water that covers the 4000–6000 K range when a pre-exponential factor of $1.1\times 10^{18}\, \mathrm{s}^{-1}$ is used for the interpretation of the experiments. We discuss in detail the importance of using a consistent couple of BE and pre-exponential factor values when comparing experiments and computations, as well as when introducing them in astrochemical models. Based on the comparison of the acetaldehyde BEs measured and computed in the present work with those of other species, we predict that acetaldehyde is less volatile than formaldehyde, but much more than water, methanol, ethanol, and formamide. We discuss the astrochemical implications of our findings and how recent astronomical high spatial resolution observations show a chemical differentiation involving acetaldehyde, which can easily explained due to the different BEs of the observed molecules.

Interstellar Complex Organic Molecules in SiO-traced Massive Outflows

The interstellar medium contains dust and gas, in which molecules can proliferate at high densities and in cold conditions. Interstellar complex organic molecules (iCOMs) are C-bearing species that contain at least six atoms. As they are detected in young stellar objects, iCOMs are expected to inhabit early stages of star formation evolution. In this study, we try to determine which iCOMs are present in the outflow component of massive protostars. To do this, we analyzed the morphological extension of blue- and redshifted iCOM emission in a sample of 11 massive protostars employing mapping observations at 1 mm within a ∼1 GHz bandwidth for both the IRAM-30 m and APEX telescopes. We modeled the iCOM emission of the central pointing spectra of our objects using the XCLASS local thermal equilibrium radiative transfer code. We detected the presence of several iCOMs such as CH3OH, 13CH3OH, CH3OCHO, C2H5C15N, and (c-C3H2)CH2. In G034.41+0.24, G327.29-0.58, G328.81+0.63, G333.13-0.43, G340.97-1.02, G351.45+0.66, and G351.77-0.54, the iCOM lines show a faint broad-line profile. Due to the offset peak positions of the blue- and redshifted emission, covering from ∼0.1 to 0.5 pc, these wings are possibly related to movements external to the compact core, such as large-scale low-velocity outflows. We have also established a correlation between the parent iCOM molecule CH3OH and the shock tracer SiO, reinforcing the hypothesis that shock environments provide the conditions to boost the formation of iCOMs via gas-phase reactions.