Icy Dust Research Articles

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.

Chemistry across dust and gas gaps in protoplanetary disks. Modelling the co-spatial molecular rings in the HD 100546 disk

Nearby extended protoplanetary disks are commonly marked by prominent rings in dust emission, possibly carved by forming planets. High-resolution observations show that both the dust and the gas are structured. These molecular structures may be related to radial and azimuthal density variations in the disk and/or the disk chemistry. The aim of this work is to identify the expected location and intensity of rings seen in molecular line emission in gapped disks while exploring a range of physical conditions across the gap. In particular, we aim to model the molecular rings that are, in contrast with most other gapped disks, co-spatial with the dust rings at sim 20 and sim 200 au in the HD 100546 disk using the thermochemical code DALI. We modelled observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of CO isotopologues C I HCN, CN C2H NO, and HCO+ in the HD 100546 disk. An axisymmetric 3D thermochemical model reproducing the radial profiles of the CO isotopologue observations and the double ring seen in continuum emission was used to make predictions for various emission lines. The effect of the amount of gas in the dust gap, the C/O ratio, an attenuated background UV radiation field, and the flaring index on the radial distribution of different molecules were investigated. The fiducial model of a gapped disk with a gas cavity at $0-15$ au, a dust cavity at $0-20$ au, and a gas and dust gap at $40-175$ au provides a good fit to the continuum and the CO isotopologues in the HD 100546 disk. In particular, the CO isotopologue emission is consistent with a shallow gas gap with no more than a factor of approximately ten drop in gas density at $40-175$ au. Similar to the CO isotopologues, the HCN and HCO+ model predictions reproduce the data within a factor of a few in most disk regions. However, the predictions for the other atom and molecules C I CN C2H and NO, neither match the intensity nor the morphology of the observations. An exploration of the parameter space shows that, in general, the molecular emission rings are only co-spatial with the dust rings if the gas gap between the dust rings is depleted by at least four orders of magnitude in gas or if the C/O ratio of the gas varies as a function of radius. For shallower gaps the decrease in the UV field roughly balances the effect of a higher gas density for UV tracers such as CN C2H and NO. Therefore, the CN C2H and NO radicals are not good tracers of the gas gap depth. In the outer regions of the disk around 300 au, these UV tracers are also sensitive to the background UV field incident on the disk. Reducing the background UV field by a factor of ten removes the extended emission and outer ring seen in CN and C2H respectively, and reduces the ring seen in NO at 300 au. The C/O ratio primarily effects the intensity of the lines without changing the morphology much. The C I HCN, CN, and C2H emission all increase with increasing C/O, whereas the NO emission shows a more complex dependence on the C/O ratio depending on the disk radius. CO isotopologues and HCO+ emission trace gas gaps and gas gap depths in disks. The molecular rings in HCN, CN C2H and NO predicted by thermochemical models do not naturally coincide with those seen in the dust, contrary to what is observed in the HD 100546 disk. This could be indicative of a radially varying C/O ratio in the HD 100546 disk with a C/O above one in a narrow region across the dust rings, together with a shallow gas gap that is depleted by a factor of approximately ten in gas, and a reduced background UV field. The increase in the C/O ratio to approximately greater than one could point to the destruction of some of the CO, the liberation of carbon from ice and grains, or, in the case of the outer ring, it could point to second generation gas originating from the icy dust grains.

The effect of nonlocal disk processes on the volatile CHNOS budgets of planetesimal-forming material

Context. The bulk abundances of CHNOS-bearing species of a planet have a profound effect on its interior structure and evolution. Therefore, it is key to investigate the behavior of the local abundances of these elements in the solid phase in the earliest stages of planet formation, where micrometer-sized dust grows into larger and larger aggregates. However, the physical and chemical processes occurring in planet-forming disks that shape these abundances are highly coupled and nonlocal. Aims. We aim to quantify the effects of the interplay between dynamical processes (turbulent diffusion, dust settling and radial drift), collision processes (coagulation and fragmentation), and the adsorption and desorption of ices on the abundances of CHNOS in local disk solids as a function of position throughout the planet-forming region. Methods. We used SHAMPOO (Stochastic Monomer Processor), which tracks the ice budgets of CHNOS-bearing molecules of a dust monomer as it undergoes nonlocal disk processing in a Class I disk. We used a large set of individual monomer evolutionary trajectories to make inferences about the properties of the local dust populations via a stochastic analysis of 64 000 monomers on a preexisting spatial grid. Results. We find that spatially, monomers can travel larger distances farther out in the disk, leading to a larger spread in positions of origin for a dust population at, for example, r = 50 AU compared to r = 2 AU. However, chemically, the inner disk (r ≲ 10 AU) is more nonlocal due to the closer spacing of ice lines in this disk region. Although to zeroth order the bulk ice mantle composition of icy dust grains remains similar compared to a fully local dust population, the ice mass associated with individual chemical species can change significantly. The largest differences with local dust populations were found near ice lines where the collisional timescale is comparable to the adsorption and desorption timescales. Here, aggregates may become significantly depleted in ice as a consequence of microscopic collisional mixing, a previously unknown effect where monomers are stored away in aggregate interiors through rapid cycles of coagulation and fragmentation. Conclusions. Nonlocal ice processing in a diffusion-dominated, massive, smooth disk has the most significant impact on the inner disk (r ≲ 10 AU). Furthermore, microscopic collisional mixing can have a significant effect on the amounts of ice of individual species immediately behind their respective ice lines. This suggests that ice processing is highly coupled to collisional processing in this disk region, which implies that the interiors of dust aggregates must be considered and not just their surfaces.

Interstellar Detection of O-protonated Carbonyl Sulfide, HOCS+

We present the first detection in space of O-protonated carbonyl sulfide (HOCS+), in the midst of an ultradeep molecular line survey toward the G+0.693-0.027 molecular cloud. From the observation of all K a = 0 transitions ranging from J lo = 2 to J lo = 13 of HOCS+ covered by our survey, we derive a column density of N = (9 ± 2) × 1012 cm−2, translating into a fractional abundance relative to H2 of ∼7 × 10−11. Conversely, the S-protonated HSCO+ isomer remains undetected, and we derive an upper limit to its abundance with respect to H2 of ≤3 × 10−11, a factor of ≥2.3 less abundant than HOCS+. We obtain an HOCS+/OCS ratio of ∼2.5 × 10−3, in good agreement with the prediction of astrochemical models. These models show that one of the main chemical routes to the interstellar formation of HOCS+ is likely the protonation of OCS, which appears to be more efficient at the oxygen end. Also, we find that high values of cosmic-ray ionization rates (10−15–10−14 s−1) are needed to reproduce the observed abundance of HOCS+. In addition, we compare the O/S ratio across different interstellar environments. G+0.693-0.027 appears as the source with the lowest O/S ratio. We find an HOCO+/HOCS+ ratio of ∼31, in accordance with other O/S molecular pairs detected toward this region and also close to the O/S solar value (∼37). This fact indicates that S is not significantly depleted within this cloud due to the action of large-scale shocks, unlike in other sources where S-bearing species remain trapped on icy dust grains.

Nucleation of ice particles below the snow line

An increase in the total surface area of icy dust aggregates was numerically investigated. The temperature of icy dust aggregates in a protoplanetary disk changes through their vertical motion induced by turbulence. The ice becomes vapor if the maximum temperature exceeds the condensation temperature. The vapor molecules recondense when cooling proceeds. Depending on the cooling rate and the amount of the remaining aggregate, the vapor molecules form pure ice particles through homogeneous nucleation or recondense onto the remaining icy aggregates through heterogeneous nucleation. If homogeneous nucleation dominates, the total surface area substantially increases from the original value. The increase in the total surface area was determined for various ice species and vertical temperature distributions. H2O, NH3, and H2S ice particles can be formed through homogeneous nucleation. The total surface area is increased by a factor of up to 400. The increase in the total surface area can affect protoplanetary disk’s surface density and temperature distributions.

A Constrained Dust Opacity for Models of Dense Clouds and Protostellar Envelopes

Radiative transfer models of dense clouds and protostellar envelopes rely on the availability of appropriate dust opacities, particularly as new JWST data become available. We present the model opacity known as KP5, which fits the (1–1000 μm) extinction curve measured for icy dust in dense molecular clouds by Spitzer and JCMT, and has been used by the community since 2009, but not previously published. The opacity law (absorption, scattering, and Henyey–Greenstein asymmetry parameter) includes H2O, CO, and CO2 ices, and has been shown to be well suited for models of embedded protostars.

New Horizons Venetia Burney Student Dust Counter Observes Higher than Expected Fluxes Approaching 60 au

The NASA New Horizons Venetia Burney Student Dust Counter (SDC) measures dust particle impacts along the spacecraft’s flight path for grains with mass ≥10−12 g, mapping out their spatial density distribution. We present the latest SDC dust density, size distribution, and flux measurements through 55 au and compare them to numerical model predictions. Kuiper Belt objects (KBOs) are thought to be the dominant source of interplanetary dust particles in the outer solar system due to both collisions between KBOs and their continual bombardment by interstellar dust particles. Continued measurements through 55 au show higher than model-predicted dust fluxes as New Horizons approaches the putative outer edge of the Kuiper Belt (KB). We discuss potential explanations for the growing deviation: radiation pressure stretches the dust distribution to further heliocentric distances than its parent body distribution; icy dust grains undergo photosputtering that rapidly increases their response to radiation pressure forces and pushes them further away from the Sun; and the distribution of KBOs may extend much further than existing observations suggest. Ongoing SDC measurements at even larger heliocentric distances will continue to constrain the contributions of dust production in the KB. Continued SDC measurements remain crucial for understanding the Kuiper Belt and the interpretation of dust disks around other stars.

Surviving the heat: multiwavelength analysis of V883 Ori reveals that dust aggregates survive the sublimation of their ice mantles

ABSTRACT Investigating the response of icy dust aggregates to water ice sublimation is essential for understanding the formation and properties of planetesimals in protoplanetary discs. However, their fate remains unclear, as previous studies suggest that aggregates could either survive or completely fall apart to (sub)μm-sized grains. Protoplanetary discs around stars undergoing accretion outbursts represent a unique laboratory to study the ice sublimation process, as the water snowline is pushed outward to regions accessible to current observatories. In this work, we aim to understand the aggregates’ response to ice sublimation by focusing on V883 Ori, a system currently undergoing a powerful accretion outburst. We present new analysis of archival high-resolution ALMA observations of the protoplanetary disc of V883 Ori at 0.88, 1.3, 2.0, and 3.1 mm, and derive new radial spectral index profiles, which we compare with predictions from one-dimensional dust evolution simulations. In the region of V883 Ori where water ice has sublimated, we find lower spectral indices than previously obtained, indicating the presence of cm-sized particles. Coupled with our dust evolution models, we find that the only way to explain their presence is to assume that they formed before the outburst and survived the sublimation process. The resilience of dust aggregates to such intense events leads us to speculate that it may extend to other environments with more gentle heating, such as pebbles drifting through the water snowline in quiescent protoplanetary discs. In that case, it may alter the formation pathway of dry planetesimals interior to the snowline.

CoCCoA: Complex Chemistry in hot Cores with ALMA

Context. Complex organic molecules (COMs), especially the oxygen-bearing species, have been observed to be abundant in the gas phase toward low-mass and high-mass protostars. Deep line surveys have been carried out only for a limited number of well-known star-forming regions using the Atacama Large Millimeter/submillimeter Array (ALMA), which has unprecedented resolution and sensitivity, and statistical studies on oxygen-bearing COMs (O-COMs) in a large sample of high-mass protostars using ALMA are still lacking. Aims. We aim to determine the column density ratios of six O-COMs with respect to methanol (CH3OH) in a sample of 14 high-mass protostellar sources in order to investigate their origin through ice and/or gas-phase chemistry. The selected species are: acetalde-hyde (CH3CHO), ethanol (C2H5OH), dimethyl ether (DME; CH3OCH3), methyl formate (MF; CH3OCHO), glycolaldehyde (GA; CH2OHCHO), and ethylene glycol (EG; (CH2OH)2). Methods. We fit the spectra of 14 high-mass sources observed as part of the Complex Chemistry in hot Cores with ALMA (CoCCoA) survey and derived the column densities and excitation temperatures of the six selected O-COMs. We used the minor isotopologue of methanol CH318OH to infer the column density of the main isotopologue CH3OH, of which the lines are generally optically thick. We compared our O-COM ratios with those of five low-mass protostars studied with ALMA from the available literature as well as with the results from experiments and simulations. Results. Although the CoCCoA sources have different morphologies and brightness in their continuum and methanol emission, the O-COM ratios with respect to methanol have very similar values in the high-mass and low-mass samples. The DME and MF have the highest and most constant ratios within one order of magnitude, while the other four species have lower ratios and exhibit larger scatter by one to two orders of magnitude. The ratio between DME and MF is close to one, which agrees well with previous observational findings. Current simulations and experiments can reproduce most observational trends with a few exceptions. For example, they tend to overestimate the abundance of ethanol and GA with respect to methanol. Conclusions. The constant column density ratios of the selected O-COMs among the low- and high-mass sources suggest that these species are formed in similar environments during star formation, probably on icy dust grains in the pre-stellar stages. Where deviations are found, hypotheses exist to explain the differences between observations, simulations, and experiments, such as the involvement of gas-phase chemistry and different emitting areas of molecules.

Interaction of H2S with H atoms on grain surfaces under molecular cloud conditions

Context. Hydrogen sulfide (H2S) is thought to be efficiently formed on grain surfaces through the successive hydrogenation of sulfur atoms. Its non-detection so far in astronomical observations of icy dust mantles thus indicates that effective destruction pathways must play a significant role in its interstellar abundance. While chemical desorption has been shown to remove H2S very efficiently from the solid phase, in line with H2S gas-phase detections, possible ice chemistry triggered by the related HS radical have been largely disregarded so far, despite it being an essential intermediate in the H2S + H reaction scheme. Aims. We aim to thoroughly investigate the fate of H2S upon H-atom impact under molecular cloud conditions, providing a comprehensive analysis combined with detailed quantification of both the chemical desorption and ice chemistry that ensues. Methods. We performed experiments in an ultrahigh vacuum chamber at temperatures between 10 and 16 K in order to investigate the reactions between H2S molecules and H atoms on interstellar ice analogs. The changes in the solid phase during H-atom bombardment were monitored in situ by means of reflection absorption infrared spectroscopy (RAIRS), and desorbed species were complementarily measured with a quadrupole mass spectrometer (QMS). Results. We confirmed the formation of H2S2 via reactions involving H2S + H and quantified its formation cross section under the employed experimental conditions. Additionally, we directly assessed the chemical desorption of H2S by measuring the gas-phase desorption signals with the QMS, providing unambiguous desorption cross sections. Chemical desorption of H2S2 was not observed. The relative decrease of H2S ices by chemical desorption changed from ~85% to ~74% between temperatures of 10 and 16 K, while the decrease as the result of H2S2 formation was enhanced from ~15% to ~26%, suggesting an increasingly relevant sulfur chemistry induced by HS radicals at warmer environments. The astronomical implications are further discussed.

Resonant infrared irradiation of CO and CH3OH interstellar ices

Context. Solid-phase photo-processes involving icy dust grains greatly affect the chemical evolution of the interstellar medium by leading to the formation of complex organic molecules and by inducing photodesorption. So far, the focus of laboratory studies has mainly been on the impact of energetic ultraviolet (UV) photons on ices, but direct vibrational excitation by infrared (IR) photons is expected to influence the morphology and content of interstellar ices as well. However, little is still known about the mechanisms through which this excess vibrational energy is dissipated, as well as its implications for the structure and ice photochemistry. Aims. In this work, we present a systematic investigation of the behavior of interstellar relevant CO and CH3OH ice analogs following the resonant excitation of vibrational modes using tunable IR radiation. We seek to quantify the IR-induced photodesorption and gain insights into the impact of vibrational energy dissipation on ice morphology. Methods. We utilized an ultrahigh vacuum setup at cryogenic temperatures to grow pure CO and CH3OH ices, as well as mixtures of the two. We exposed the ices to intense, near-monochromatic mid-IR (MIR) free-electron-laser radiation using the LISA end-station at the FELIX free electron laser facility to selectively excite the species. Changes to the ice are monitored by means of reflection-absorption IR spectroscopy combined with quadrupole mass-spectrometry. These methods also allowed us to characterize the photodesorption efficiency. Results. The dissipation of vibrational energy is observed to be highly dependent on the excited mode and the chemical environment of the ice. All amorphous ices undergo some degree of restructuring towards a more organized configuration upon on-resonance irradiation. Moreover, IR-induced photodesorption is observed to occur for both pure CO and CH3OH ices, with interstellar photodesorption efficiencies on the order of 10 molecules cm−2 s−1. This result is comparable to or higher than what is found for UV-induced counterparts. An indirect photodesorption of CO upon vibrational excitation of CH3OH in ice mixtures is also observed to occur, particularly in environments that are rich in methanol. Here, we discuss the astrochemical implications of these IR-induced phenomena.

Collisional evolution of dust and water ice in protoplanetary discs during and after an accretion outburst

ABSTRACTMost protoplanetary discs are thought to undergo violent and frequent accretion outbursts, during which the accretion rate and central luminosity are elevated for several decades. This temporarily increases the disc temperature, leading to the sublimation of ice species as snowlines move outwards. In this paper, we investigate how an FUor-type accretion outburst alters the growth and appearance of dust aggregates at different locations in protoplanetary discs. We develop a model based on the Monte Carlo approach to simulate locally the coagulation and fragmentation of icy dust particles and investigate different designs for their structure and response to sublimation. Our main finding is that the evolution of dust grains located between the quiescent and outburst water snowlines is driven by significant changes in composition and porosity. The time required for the dust population to recover from the outburst and return to a coagulation/fragmentation equilibrium depends on the complex interplay of coagulation physics and outburst properties, and can take up to 4500 yr at 5 au. Pebble-sized particles, the building blocks of planetesimals, are either deprecated in water ice or completely destroyed, respectively, resulting in drier planetesimals or halting their formation altogether. When accretion outbursts are frequent events, the dust can be far from collisional equilibrium for a significant fraction of time, offering opportunities to track past outbursts in discs at millimetre wavelengths. Our results highlight the importance of including accretion outbursts in models of dust coagulation and planet formation.

Collisional Growth and Fragmentation of Dust Aggregates. II. Mass Distribution of Icy Fragments

By performing N-body simulations, we investigated the fundamental processes of collisions between dust aggregates composed of submicron-sized icy dust monomers. We examined the mass distribution of fragments in the collisional outcomes in a wide range of the mass ratio and the collision velocity between colliding dust aggregates. We derived analytic expressions of the mass distribution of large remnants and small fragments by numerical fitting to the simulation results. Our analytic formulae for masses of the large remnants can reproduce the contribution of mass transfer from a large target to a small projectile, which occurs for a mass ratio of ≳3 and is shown in a previous study. We found that the power-law index of the cumulative mass distribution of the small fragments is independent of the mass ratio and only weakly dependent on the collision velocity. On the other hand, the mass fraction of fragments of individual dust monomers decreases with an increasing total mass of colliding aggregates for a fixed mass ratio. This tendency implies that multiple hierarchical disruptive collisions (i.e., collisions between fragments, and collisions between fragments of fragments) are required for producing a large number of individual dust monomers via collisional fragmentation. Our fragment model suggests that the total geometric cross section integrated over the fragments is estimated to be about the same order as the geometric cross section of the target.

Probing the Jet–Torus Interaction in the Radio Galaxy NGC 1052 by Sulfur-bearing Molecules

The radio galaxy NGC 1052 casts absorption features of sulfur-bearing molecules, H2S, SO, SO2, and CS toward the radio continuum emission from the core and jets. Using the Atacama Large Millimeter/submillimeter Array, we have measured the equivalent widths of SO absorption features in multiple transitions and determined the temperatures of 344 ± 43 K and 26 ± 4 K in submillimeter and millimeter wavelengths, respectively. Since submillimeter and millimeter continuum represents the core and jets, the high and low temperatures of the absorbers imply a warm environment in the molecular torus and cooler downstream flows. The high temperature in the torus is consistent with the presence of 22 GHz H2O maser emission, vibrationally excited HCN and HCO+ absorption lines, and sulfur-bearing molecules in the gas phase released from dust. The origin of the sulfur-bearing gas is ascribed to the evaporation of the icy dust component through a jet–torus interaction. Shock heating is the sole plausible mechanism to maintain such a high temperature of gas and dust in the torus. The implication of the jet–torus interaction also supports the collimation of the sub-relativistic jets by the gas pressure of the torus.

CORINOS. I. JWST/MIRI Spectroscopy and Imaging of a Class 0 Protostar IRAS 15398–3359

The origin of complex organic molecules (COMs) in young Class 0 protostars has been one of the major questions in astrochemistry and star formation. While COMs are thought to form on icy dust grains via gas-grain chemistry, observational constraints on their formation pathways have been limited to gas-phase detection. Sensitive mid-infrared spectroscopy with JWST enables unprecedented investigation of COM formation by measuring their ice absorption features. Mid-infrared emission from disks and outflows provide complementary constraints on the protostellar systems. We present an overview of JWST/Mid-Infrared Instrument (MIRI) Medium Resolution Spectroscopy (MRS) and imaging of a young Class 0 protostar, IRAS 15398−3359, and identify several major solid-state absorption features in the 4.9–28 μm wavelength range. These can be attributed to common ice species, such as H2O, CH3OH, NH3, and CH4, and may have contributions from more complex organic species, such as C2H5OH and CH3CHO. In addition to ice features, the MRS spectra show many weaker emission lines at 6–8 μm, which are due to warm CO gas and water vapor, possibly from a young embedded disk previously unseen. Finally, we detect emission lines from [Fe ii], [Ne ii], [S i], and H2, tracing a bipolar jet and outflow cavities. MIRI imaging serendipitously covers the southwestern (blueshifted) outflow lobe of IRAS 15398−3359, showing four shell-like structures similar to the outflows traced by molecular emission at submillimeter wavelengths. This overview analysis highlights the vast potential of JWST/MIRI observations and previews scientific discoveries in the coming years.

Key Parameters Controlling the Photodesorption Yield in Interstellar CO Ice Analogs: The Influence of Ice Deposition Temperature and Thickness

The overabundance of gas molecules in the coldest regions of space points to a nonthermal desorption process. Laboratory simulations show the efficient desorption of CO ice that is exposed to ultraviolet radiation, known as photodesorption, which decreases with increasing ice deposition temperature. However, our understanding of this abnormal phenomenon still remains elusive. In this work, we show that the same phenomenon—and, in particular, a dramatic drop in the photodesorption yield—is observed when the deposition temperature is 19 K and higher. Also, the minimum ice thickness that accounts for a constant photodesorption yield of CO ice is dependent on the deposition temperature, an observation that is reported here for the first time. We propose that the key parameters dominating the absorbed photon energy transfer in CO ice, and contributing to the measured photodesorption yields, are the energy transfer length, the desorption yield contributed by a single ice layer, and the relative effective surface area. These parameters should be incorporated into astrophysical models that simulate the photodesorption of the top CO-rich ice layer in icy dust populations, with a size distribution that is related to the ice thickness.

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.

Monte Carlo simulation of sugar synthesis on icy dust particles intermittently irradiated by UV in a protoplanetary disk

Context. While synthesis of organic molecules in molecular clouds or protoplanetary disks is complex, observations of interstellar grains, analyses of carbonaceous chondrites, and UV photochemistry experiments are rapidly developing and are providing constraints on and clues to the complex organic molecule synthesis in space. This motivates us to construct a theoretical synthesis model. Aims. We developed a new code to simulate global reaction sequences of organic molecules and apply it to sugar synthesis by intermittent UV irradiation on the surface of icy particles in a protoplanetary disk. Here we show the first results of our new simulation. Methods. We applied a Monte Carlo method to select reaction sequences from all possible reactions, using the graph-theoretic matrix model for chemical reactions and modeling reactions on the icy particles during UV irradiation. Results. We obtain results consistent with the organic molecules in carbonaceous chondrites and obtained by experiments, albeit through a different pathway from the conventional formose reactions previously suggested. During UV irradiation, loosely bonded O-rich large molecules are continuously created and destroyed. After UV irradiation is turned off, the ribose abundance rapidly increases through the decomposition of the large molecules via breakage of O−O bonds and replacements of C−OH by C−H to reach O/C = 1 for sugars. The sugar abundance is regulated mostly by the total atomic ratio H/O of starting materials, but not by their specific molecular forms. Deoxyribose is simultaneously synthesized, and most of the molecules end up in complexes with C-rich molecules.

Infrared multiple‐angle incidence resolution spectrometry for vapor‐deposited amorphous water

AbstractInfrared multiple‐angle incidence resolution spectrometry (IR‐MAIRS) is a recently developed spectroscopic technique that combines oblique incidence transmission measurements and chemometrics (multivariate analysis) to obtain both pure in‐plane (IP) and out‐of‐plane (OP) vibration spectra for a thin sample. IR‐MAIRS is established for analyzing the molecular orientation of organic thin films at atmospheric pressure, but it should also be powerful for the structural characterization of vapor‐deposited thin samples prepared in a vacuum. The application of IR‐MAIRS to vapor‐deposited amorphous water is particularly interesting in the fields of physical and interstellar chemistry, because it is a representative model material for interstellar icy dust grains. We recently developed an experimental setup for in situ IR‐MAIRS under low‐temperature, ultrahigh‐vacuum conditions, which thus facilitates measurements of interstellar ice analogs such as vapor‐deposited amorphous water. This review considers the theoretical framework of IR‐MAIRS and our recent experimental results for vapor‐deposited amorphous water. We present spectroscopic signatures for the perpendicular orientation of dangling‐OH bonds for three‐coordinated water molecules at the surface of amorphous water at 90 K. The absolute absorption cross section of the three‐coordinated dangling‐OH bonds is quantitatively measured as 1.0 ± 0.2 × 10−18 cm2 at 3696 cm−1. As IR‐MAIRS can essentially be conducted using only a Fourier‐transform IR spectrometer and an angle‐controllable linear polarizer, it is a useful, low‐cost, and simple spectroscopic technique for studying laboratory analogs of interstellar ices including vapor‐deposited amorphous water.

Acetone–Water Interactions in Crystalline and Amorphous Ice Environments

We present research that systematically examines acetone interacting with various D2O ices of terrestrial and astrophysical interest using time-resolved, in situ reflection absorption infrared spectroscopy (RAIRS). We examine acetone deposited on top of different D2O ice films: high-density, nonporous amorphous (np-ASW), and crystalline (CI) films as well as porous amorphous (p-ASW) with various pore morphologies. Analysis of RAIR spectra changes after acetone exposure, and we find that more hydrogen bonding occurs between acetone and p-ASW ices as compared to acetone and np-ASW or CI ices. Hydrogen bonding quantification occurred by two independent RAIR spectral changes: a greater relative intensity of the 1703 cm-1 feature at low acetone coverage as part of a 14 cm-1 shift in the C═O region and an ∼30% integrated dangling bond area reduction after acetone exposure. Interestingly, when changing the water structure to be more porous (deposited at 70° compared to 30°), there is a further reduction in the amount of hydrogen bonding that occurs. This suggests that there is a lack of access to surface sites with dangling bonds in the pores as initial layers of acetone block the pores and acetone is unable to diffuse within the structure at low temperatures. In general, these results offer a clearer picture of the mechanisms that can occur when small organic hydrocarbons interact with various icy interfaces; a quantitative understanding of these interactions is essential for the accurate modeling of many astrophysical processes occurring on the surface of icy dust particles.