Icy Grains Research Articles

Exploring the Role of Ion–Molecule Reactions on Interstellar Icy Grain Surfaces

Exploring the Role of Ion–Molecule Reactions on Interstellar Icy Grain Surfaces

ALMA and GMRT Studies of Dust Continuum Emission and Spectral Lines Toward Oort Cloud Comet C/2022 E3 (ZTF)

Abstract The atomic and molecular compounds of cometary ices serve as valuable knowledge into the chemical and physical properties of the outer solar nebula, where comets are formed. From the cometary atmospheres, the atoms and gas-phase molecules arise mainly in three ways: (i) the outgassing from the nucleus, (ii) the photochemical process, and (iii) the sublimation of icy grains from the nucleus. In this paper, we present the radio and millimeter wavelength observation results of Oort cloud non-periodic comet C/2022 E3 (ZTF) using the Giant Metrewave Radio Telescope (GMRT) band L and the Atacama Large Millimeter/Submillimeter Array (ALMA) band 6. We do not detect continuum emissions and an emission line of atomic hydrogen (HI) at rest frequency 1420 MHz from this comet using the GMRT. Based on ALMA observations, we detect the dust continuum emission and rotational emission lines of methanol (CH3OH) from comet C/2022 E3 (ZTF). From the dust continuum emission, the activity of dust production (Afρ) of comet ZTF is 2280±50 cm. Based on LTE spectral modelling, the column density and excitation temperature of CH3OH towards C/2022 E3 (ZTF) are (4.50±0.25)×10^{14} cm^{−2} and 70±3 K. The integrated emission maps show that CH3OH was emitted from the coma region of the comet. The production rate of CH3OH towards C/2022 E3 (ZTF) is (7.32±0.64)×10^{26} molecules s^{−1}. The fractional abundance of CH3OH with respect to H2O in the coma of the comet is 1.52%. We also compare our derived abundance of CH3OH with the existence modelled value, and we see the observed and modelled values are nearly similar. We claim that CH3OH is formed via the subsequential hydrogenation of formaldehyde (H2CO) on the grain surface of comet C/2022 E3 (ZTF).

Magnetically Aligned Grains as a Ubiquitous Source of Spin-polarized Electrons and Chiral-symmetry-breaking Agents

The unique biosignature of life on Earth is the homochirality of organic compounds, such as amino acids, proteins, and sugars. High-energy spin-polarized (spinup or spindown) electrons (SPEs) from the β decay of radioactive nuclei have been proposed as a source of symmetry breaking, leading to homochirality; however, their exact role is much debated. Here, we propose magnetically aligned dust grains as a new source of SPEs, due to the photoemission of electrons having aligned spins by the Barnett effect. For the interstellar UV radiation field of strength G UV, we find that the SPE emission rate is ΓpeSPE∼10−14GUV electrons per second per H, the fraction of spin-polarized to total photoelectrons is ∼10%, and the SPE yield (photoelectron number per UV photon) can reach ∼1%, using the modern theory of grain alignment. SPEs emitted from aligned grains could play an important role in chiral-induced spin-selectivity-driven reduction chemistry in the icy grain mantles, producing an enantiomer excess of chiral molecules formed on the grain mantle. Finally, we suggest magnetically aligned grains could directly impact the enantio-selectivity through the chiral-induced spin-selective adsorption effect and exchange interaction. We estimate the disalignment of electron spins and depolarization by elastic scattering using the Mott theory and find that these effects are negligible for low-energy SPEs, so that the spins of SPEs remain well aligned during their journey through dust grains and the gas. Our proposed mechanism might explain the chiral asymmetry of prebiotic molecules detected in numerous comets, asteroids, and meteorites.

Journey of complex organic molecules: Formation and transport in protoplanetary disks

Complex organic molecules serve as indicators of molecular diversity. Their detection on comets, planets, and moons has prompted inquiries into their origins, particularly the conditions conducive to their formation. One hypothesis suggests that the UV irradiation of icy grains in the protosolar nebula generates significant molecular complexity, a hypothesis supported by experiments on methanol ice irradiation. We investigated the irradiation of methanol ice particles as they migrate through the protosolar nebula. Our objective is to ascertain whether the encountered conditions facilitate the formation of complex organics molecules, and we leverage experimental data in our analysis. We developed a two-dimensional model that describes the transport of pebbles during the evolution of the protosolar nebula, employing a Lagrangian scheme. This model computes the interstellar UV flux received by the particles along their paths, which we compared with experimental values. On average, particles ranging from 1 to 100 mu m in size, released at a local temperature of 20 K, undergo adequate irradiation to attain the same molecular diversity as methanol ice during the experiments within timescales of 25 kyr of protosolar nebula evolution. In contrast, 1 cm sized particles require 911 kyr of irradiation to reach similar molecular diversity, making comparable molecular complexity unlikely. Similarly, particles ranging from 1 to 100 mu m in size, released at a local temperature of 80 K, receive sufficient irradiation after 141 and 359 kyr. In contrast, 1 cm sized particles would require several million years to receive this level of irradiation, which is infeasible since they cross the iceline within approximately 500 kyr. The particles readily receive the irradiation dose necessary to generate the molecular diversity observed in the experiments within the outer regions of the disk. Our model, combined with future irradiation experiments, can provide additional insights into the specific regions where the building blocks of planets form.

Characterization of Monosubstituted Benzene Ices

Aromatic structures are fundamental for key biological molecules such as RNA and metabolites and the abundances of aromatic molecules on young planets are therefore of high interest. Recent detections of benzonitrile and other aromatic compounds in interstellar clouds and comets have revealed a rich aromatic astrochemistry. In the cold phases of star and planet formation, most of these aromatic molecules are likely to reside in icy grain mantles, where they could be observed through IR spectroscopy. We present laboratory IR spectra of benzene and four monosubstituted benzene molecules—toluene, phenol, benzonitrile, and benzaldehyde—to determine their IR ice absorbances in undiluted aromatic ices, and in mixtures with water and CO. We also characterize the aromatic ice desorption rates, and extract binding energies and respective pre-exponential factors using temperature-programmed desorption experiments. We use these to predict at which protostellar and protoplanetary disk temperatures these molecules sublimate into the gas phase. We find that benzene and monosubstituted benzene derivatives are low-volatility with binding energies in the 5220–8390 K (43–70 kJ mol−1) range, which suggests that most of the chemistry of benzene and of functionalized aromatic molecules is to be expected to occur in the ice phase during star and planet formation.

Entrapment in CO2 and H2O Ices: Impact of Ice Matrix Thickness

The volatile inventories of planets depend on the precise partitioning of different volatiles between the gas and solid phases across the planet birth disk. For the case of hyper-volatiles (e.g., CO, N2, and noble gases), the partitioning also depends on how efficiently they are trapped into less volatile ice matrices. The thicknesses of these ice matrices can range from a few molecular layers to macroscopic bodies, and in this study we explore how entrapment efficiency depends on the ice thickness between tens of nanometers and a few micrometers (∼50–3000 monolayers, ML). We carry out a series of temperature-programmed desorption (TPD) experiments on H2O:CO and CO2:CO mixtures with 5:1 and 15:1 matrix-to-CO mixing ratios. Entrapment efficiencies range from 41% to 64%, with higher entrapment efficiencies for the more dilute ices. Surprisingly, we find no significant difference in entrapment across the studied ice thicknesses for either H2O or CO2 ice matrices. Complementary TPD experiments with the additional hyper-volatiles N2 and Ar see a similar trend with ice thickness. We speculate that these results may be due to surface topography such as cracks that lead to hyper-volatile escape from deep ice layers. In either case, these experiments show that entrapment in microscopic ices is relatively insensitive to ice thickness (above ∼50 ML). In protoplanetary disks we therefore expect efficient entrapment in icy grains of a range of grain sizes.

Methane Formation Efficiency on Icy Grains: Role of Adsorption States

Methane (CH4) is one of the major components of the icy mantle of cosmic dust prevalent in cold, dense regions of interstellar media, playing an important role in the synthesis of complex organic molecules and prebiotic molecules. Solid CH4 is considered to be formed via the successive hydrogenation of C atoms accreting onto dust: C + 4H → CH4. However, most astrochemical models assume this reaction on the ice mantles of dust to be barrierless and efficient, without considering the states of adsorption. Recently, we found that C atoms exist in either the physisorbed or chemisorbed state on compact amorphous solid water, which is analogous to an interstellar ice mantle. These distinct adsorption states considerably affect the hydrogenation reactivity of the C atom. Herein, we elucidate the reactivities of physisorbed and chemisorbed C atoms with H atoms via sequential deposition and codeposition processes. The results indicate that only physisorbed C atoms can produce CH4 on ice. Combining this finding with a previous estimate for the fraction of physisorbed C atoms on ice, we determined the upper limit for the conversion of C atoms into CH4 to be 30%.

Monte Carlo simulation of UV-driven synthesis of complex organic molecules on icy grain surfaces

Context. Complex organic molecules (COMs) have been widely observed in molecular clouds and protostellar environments. One of the formation mechanisms of COMs is radical reactions on the icy grain surface driven by UV irradiation. While many experiments have reported that various COMs (including biomolecules) can be synthesized under such ice conditions, the majority of the reaction processes are unclear. Complementary numerical simulations are necessary to unveil the synthetic process behind the formation of COMs. Aims. In this study, we develop a chemical reaction simulation using a Monte Carlo method. To explore the complex reaction network of COM synthesis, the model was designed to eliminate the need to prepare reaction pathways in advance and to keep computational costs low. This allows for broad parameter surveys and a global investigation of COM synthesis reactions. With this simulation, we investigate the chemical reactions occurring on icy dust surfaces during and after UV irradiation, assuming a protoplanetary disk environment. We aim to reveal the types of organic molecules produced in a disk and the formation mechanisms of COMs, in particular, amino acids and sugars. Methods. The Monte Carlo method we developed here produces reaction sequences by selecting a reaction from all candidate reactions at each calculation step, based on Arrhenius-type weighting. For our purpose, we significantly accelerated the calculation by adopting an approximate estimation of activation energy without expensive quantum chemical calculations. Results. The results show that photodissociation and subsequent radical-radical reactions cause random rearrangement of the covalent bonds in the initial molecules composed of methanol, formaldehyde, ammonia, and water. Consequently, highly complex molecules such as amino acids and sugars were produced in a wide range of the initial conditions. We found that the final abundances of amino acids and sugars have extremely similar dependence on the atomic ratios of the initial molecules, which peak at C/H ~ 0.1–0.3 and O/H ~ 0.3–0.5, although the amino acids abundance is usually more than ten times higher than that of sugars. To understand this dependence, a semi-analytical formula was derived. Additionally, parameter surveys of temperature, photon energy, and other factors have suggested that the decomposition reactions of amino acids and sugars undergo a rapid transition within the threshold of a given parameter.

Binding Energies of N-bearing Species on Interstellar Water Ice Mantles by Quantum Chemical Calculations

Of the about 300 gas-phase molecular species so far detected in the interstellar medium (ISM), mostly via observations of their rotational lines, around 40% contain nitrogen (N) atoms. Likewise, of the less than a dozen interstellar molecules, firmly or likely detected in the solid-state water-dominated icy matrix by means of infrared observations, two bear N. A crucial parameter that regulates whether a species is in the gas or adsorbed on the icy phase is their binding energy (BE) toward the icy grain. Therefore, an accurate quantification of the BE is of paramount importance to properly model the ISM chemistry through numerical models. However, very few BEs are available in the literature, either determined experimentally or theoretically. In the present study, we calculate the BEs of 21 among the most abundant interstellar N-bearing species. We adopted two structural water ice models, representing a crystalline and an amorphous surface, using a reliable cost-effective procedure based on the density functional theory. While on the crystalline surface model only one BE per species is obtained due to the high symmetry of the unit cell, on the amorphous model from 5 to 10 BEs are obtained, due to its richer surface morphological variety. Most of our computed BEs agree with available experimental and other computational values. Finally, we discuss how the newly computed BEs can help estimate which N-bearing species can be frozen at the water snow line and, therefore, incorporated in water-rich ice planetesimals.

Modifications of astrophysical ices induced by cosmic rays

Aims. Astrophysical ices on dust grain mantles in the interstellar medium (ISM) and dense circumstellar envelopes (CSEs) are continuously exposed to galactic cosmic rays (GCRs). In a laboratory setting, we studied the physical and chemical modifications of ice layers induced by energetic heavy ions as GCR analogues. The ice layers used have a molecular composition similar to that of icy grain mantles. Methods. Mixtures of H2O:CO:CH3OH molecules (percentages 73:24:3, 68:30:3, and 58:38:3) were condensed on a substrate at 15 K and irradiated with 40 MeV 58Ni11+ ion beams. Irradiation-induced modifications were followed using the mid-infrared absorption spectroscopy technique. Results. We observed the evolution of infrared bands of CO2, HCO, HCOOH, CH4, H2CO,H2O2, and more complex synthesised molecules. From the molecular column densities, cross-sections and sputtering yields were determined and compared to published results of water and carbon monoxide. Analysis of the chemical modifications reveals that the precursors are easily destroyed when they are in a molecular mixture, while others are desorbed. Conclusions. The main radiolitic modifications induced by GCR irradiations are molecular decomposition and sputtering. Extrapolation to astrophysical radiation conditions shows a strong dependence on the intensity of the GCR distributions at low energies, which allows the analysis of the ice evolution at timescales comparable to those of the ISM and CSE.

A systematic mid-infrared spectroscopic study of thermally processed H2S ices

The positive identification of the molecular components of interstellar icy grain mantles is critically reliant upon the availability of laboratory-generated mid-infrared absorption spectra which can be compared against data acquired by ground- and space-borne telescopes. However, one molecule which remains thus far undetected in interstellar ices is H2S, despite its important roles in astrochemical and geophysical processes. Such a lack of a detection is surprising, particularly in light of its relative abundance in cometary ices which are believed to be the most pristine remnants of pre-solar interstellar ices available for study. In this paper, we present the results of an extensive and quantitative mid-infrared spectroscopic characterisation of H2S ices deposited at 20, 40, and 70 K and thermally processed to sublimation in an ultrahigh-vacuum system. We anticipate our results to be useful in confirming the detection of interstellar H2S ice using high-resolution and high-sensitivity instruments such as the James Webb Space Telescope, as well as in the identification of solid H2S in icy environments in the outer Solar System, such as comets and moons.

Size of icy grains after a heating event in a protoplanetary disk

A heating event in a protoplanetary disk could sublimate H2O ice from icy aggregates. After the heating event, the temperature decreased, and the H2O vapor recondensed to form pure H2O grains through homogeneous nucleation or to form H2O ice layer covering the silicate aggregates left behind during the heating event. As a result, the size of the pure H2O ice grains could change from the commonly assumed value of 0.1μm of the aggregate composing ice-covered grains. Here, we numerically determined the final radii of the grains and the thickness of the H2O ice layer on the aggregates after the cooling. We also estimated the maximum radius of the aggregate consisting of the processed grains. If the size of the silicate aggregate left behind in the heating event was smaller than 0.073μm, continuous collisional growth of the aggregates of the processed grains was possible. If the size was larger than 0.073μm, a short cooling time depending on the heliocentric distance was required to achieve high mechanical strengths of the aggregates for the collisional growth.

Photochemistry of micro-solvated benzonitrile complexes with water/ammonia bordering on the formation of icy grains

Polycyclic aromatic hydrocarbons (PAHs) act as condensation nuclei for water/ammonia molecules forming icy water clusters. In the context of small aromatic rings, we have theoretically examined complexes of benzonitrile with water/ammonia molecules in order to investigate the photochemical kinetics of the icy grain formation. First, we obtain the structure of energetically stable complexes containing different H-bonding patterns and analyze their effects in the infrared (IR) and vacuum ultraviolet (VUV) spectra, before and after ionization. Second, we evaluate the H-bonding reorientation kinetics of the complexes at low temperatures, which indicates that the process involving water occurs faster than the corresponding process involving ammonia. Finally, the thermal rate constants indicate which configurations are the most likely to be ionized under vacuum ultraviolet (VUV) irradiation. Our results are consistent with what has experimentally been observed for related complexes, and are useful as a starting point to investigate the process of the icy grains formation. This study provides insight into possible subsequent photochemical reactions occurring in the interstellar medium (ISM) or in jet-cooled molecular complexes.

Discovery of Thionylimide, HNSO, in Space: The first N-, S-, and O-bearing Interstellar Molecule

We present the first detection in space of thionylimide (HNSO) toward the Galactic center molecular cloud G + 0.693-0.027, thanks to the superb sensitivity of an ultradeep molecular line survey carried out with the Yebes 40 m and IRAM 30 m telescopes. This molecule is the first species detected in the interstellar medium containing, simultaneously, N, S, and O. We have identified numerous K a = 0, 1, and 2 transitions belonging to HNSO covering from J up = 2 to J up =10, including several completely unblended features. We derive a molecular column density of N = (8 ± 1)×1013 cm−2, yielding a fractional abundance relative to H2 of ∼6 × 10−10, which is about ∼37 and ∼4.8 times less abundant than SO and SO2, respectively. Although there are still many unknowns in the interstellar chemistry of NSO-bearing molecules, we propose that HNSO is likely formed through the reaction of the NSO radical and atomic H on the surface of icy grains, with alternative routes also deserving exploration. Finally, HNSO appears as a promising link between N, S, and O interstellar chemistry, and its discovery paves the route to the detection of a new family of molecules in space.

The GUAPOS project – V: The chemical ingredients of a massive stellar protocluster in the making

ABSTRACT Most stars, including the Sun, are born in rich stellar clusters containing massive stars. Therefore, the study of the chemical reservoir of massive star-forming regions is crucial to understand the basic chemical ingredients available at the dawn of planetary systems. We present a detailed study of the molecular inventory of the hot molecular core G31.41+0.31 from the project GUAPOS (G31.41+0.31 Unbiased ALMA sPectral Observational Survey). We analyse 34 species for the first time plus 20 species analysed in previous GUAPOS works, including oxygen, nitrogen, sulfur, phosphorus, and chlorine species. We compare the abundances derived in G31.41+0.31 with those observed in other chemically-rich sources that represent the initial and last stages of the formation of stars and planets: the hot corino in the Solar-like protostar IRAS 16293-2422 B, and the comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen. The comparative analysis reveals that the chemical feedstock of the two star-forming regions are similar. The abundances of oxygen- and nitrogen-bearing molecules exhibit a good correlation for all pair of sources, including the two comets, suggesting a chemical heritage of these species during the process of star formation, and hence an early phase formation of the molecules. However, sulfur- and phosphorus-bearing species present worse correlations, being more abundant in comets. This suggests that while sulfur- and phosphorus-bearing species are pre-dominantly trapped on the surface of icy grains in the hot close surroundings of protostars, they could be more easily released into gas phase in comets, allowing their cosmic abundances to be almost recovered.

JWST Observations of Young protoStars (JOYS+): Detecting icy complex organic molecules and ions

Context. Complex organic molecules (COMs) are ubiquitously detected in the gas phase and thought to be mostly formed on icy grains. Nevertheless, there have not been any unambiguous detections of COMs larger than CH3OH in ices reported thus far. Exploring this matter in greater detail has now become possible with the unprecedented possibilities offered by the James Webb Space Telescope (JWST) within the infrared (IR) spectral range with its very high sensitivity and spectral resolution in the critical 5–10 µm range, the fingerprint region of oxygen-bearing COMs. Aims. In the JWST Observations of Young protoStars (JOYS+) program, more than 30 protostars are undergoing observation with the Medium Resolution Spectrograph (MRS) of the Mid-IR Instrument (MIRI). The goal of this study is to comprehensively explore the COMs ice signatures in one low- and one high-mass protostar: NGC 1333 IRAS 2A and IRAS 23385+6053, respectively. Methods. We performed global continuum and silicate subtractions of the MIRI-MRS spectra, followed by a local continuum subtraction in optical depth scale in the range around 6.8 and 8.6 µm, the ice COM fingerprint region. We explored different choices for the local continuum and silicate subtraction. Next, we fit the observational data with a large sample of available IR laboratory ice spectra. We used the ENIIGMA fitting tool, a genetic algorithm-based code that not only finds the best fit between the lab data and the observations, but also performs a statistical analysis of the solutions, such as deriving the confidence intervals and quantifying fit degeneracy. Results. We report the best fits for the spectral ranges between 6.8 and 8.6 µm in NGC 1333 IRAS 2A and IRAS 23385+6053, originating from simple molecules and COMs, as well as negative ions. Overall, we find that ten chemical species are needed to reproduce the astronomical data. The strongest feature in this range (7.7 µm) is dominated by CH4, with contributions from SO2 and OCN−. Our results indicate that the 7.2 and 7.4 µm bands are mostly dominated by HCOO−. We also find statistically robust detections of COMs based on multiple bands, most notably, CH3CHO, CH3CH2OH, and CH3OCHO. We also report a likely detection of CH3COOH. Based on the ice column density ratios between CH3CH2OH and CH3CHO of NGC 1333 IRAS 2A and IRAS 23385+6053, we find compelling evidence that these COMs are formed on icy grains. Finally, the derived ice abundances for NGC 1333 IRAS 2A correlate well with those in comet 67P/GC within a factor of 5. Conclusions. Based on the high-quality JWST (MIRI-MRS) spectra, we conclude that COMs are present in interstellar ices, thus providing additional proof for the solid-state origin of these species in star-forming regions. In addition, the good correlation between the ice abundances in comet 67P and NGC 1333 IRAS 2A is fully in line with the idea that cometary COMs may be inherited from the early protostellar phases to a significant extent.

Inferring the CO2 Abundance in Comet 45P/Honda-Mrkos-Pajdušáková from [O i] Observations: Implications for the Source of Icy Grains in Cometary Comae

The study of cometary composition is important for understanding our solar system's early evolutionary processes. Carbon dioxide (CO2) is a common hypervolatile in comets that can drive activity but is more difficult to study than other hypervolatiles owing to severe telluric absorption. CO2 can only be directly observed from space-borne assets. Therefore, a proxy is needed to measure CO2 abundances in comets using ground-based observations. The flux ratio of the [O i] λ5577 line to the sum of the [O i] λ6300 and [O i] λ6364 lines (hereafter referred to as the [O i] line ratio) has, with some success, been used in the past as such a proxy. We present an [O i] line ratio analysis of comet 45P/Honda-Mrkos-Pajdušáková (HMP), using data obtained with the Tull Coudé Spectrograph on the 2.7 m Harlan J. Smith Telescope at McDonald Observatory, taken from UT 2017 February 21–23, when the comet was at heliocentric distances of 1.12–1.15 au. HMP is a hyperactive Jupiter-family comet (JFC). Icy grains driven out by CO2 sublimation have been proposed as a driver of hyperactivity, but the CO2 abundance of HMP has not been measured. From our [O i] line ratio measurements, we find a CO2/H2O ratio for HMP of 22.9% ± 1.4%. We compare the CO2/H2O ratios to the active fractions of the nine comets (including HMP) in the literature that have data for both values. We find no correlation. These findings imply that CO2 sublimation driving out icy grains is not the only factor influencing active fractions for cometary nuclei.

Spectroscopic sizing of interstellar icy grains with JWST

Spectroscopic sizing of interstellar icy grains with JWST

Water ice: Temperature-dependent refractive indexes and their astrophysical implications

Context. Interstellar and circumstellar ices are largely composed of frozen water. Therefore, it is important to derive fundamental parameters for H2O ice such as absorption and scattering opacities, for which accurate complex refractive indexes are needed. Aims. The primary goal of the work presented here is to derive ice-grain opacities based on accurate H2O ice complex refractive indexes at low temperatures and to assess the impact this has on the derivation of water ice column densities and porosity in space. Methods. We used the optool code to derive ice-grain scattering and absorption opacity values based on new and previously reported mid-infrared (mid-IR) complex refractive index measurements of H2O ice, primarily in its amorphous form, but not exclusively. Next, we used those opacities in the RADMC-3D code to run a radiative transfer simulation of a protostellar envelope containing H2O ice, which was then used to calculate water ice column densities. Results. We found that the real refractive index in the mid-IR of H2O ice at 30 K is ~14% lower than previously reported in the literature. This has a direct impact on the ice column densities derived from the simulations of embedded protostars. Additionally, we found that ice porosity plays a significant role in the opacity of icy grains and that the H2O libration mode can be used as a diagnostic tool to constrain the porosity level. Finally, the refractive indexes presented here allowed us to estimate a grain size detection limit of 18 μm based on the 3 μm band, whereas the 6 μm band allowed us to trace grain sizes larger than 20 μm. Conclusions. Based on radiative transfer simulations using new mid-IR refractive indexes, we conclude that H2O ice leads to more absorption of infrared light than previously estimated. This implies that the 3 and 6 μm bands remain detectable in icy grains with sizes larger than 10 μm. Finally, we also propose that the H2O ice libration band can be used as a diagnostic tool to constrain the porosity level of the interstellar ice, in addition to the OH dangling bond, which is now routinely used for this purpose.

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.